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FRONTISPIECE. 


April 

THE 

M ICROSCOPE 

AND  ITS 

REVELATIONS 


WILLIAM  B.  CARPENTER,  C.B.  M.D.  LLC. 

F.R.S.  F.G.S.  F.L.S. 

CORRESPONDING  MEMBER  OP  THE  INSTITUTE   OF  FRANCE, 
AND  OF  THE  AMERICAN  PHILOSOPHICAL  SOCIETY, 
ETC.,  ETC. 

SIXTH  EDITION 
ILLUSTRATED  BY  TWENTY-SIX  PLATES 
AND   FIVE  HUNDRED    WOOD  ENGBAVINGS 


VOLUME  I. 


NEW  YOEK 
WILLIAM   WOOD    &  COMPANY 

56  &  58  Lafayette  Place 
1883 


PEEFACE. 


The  rapid  increase  which  has  recently  taken  place  in  the  use  of  the 
Microscope, — both  as  an  instrument  of  scientific  research,  and  as  a  means 
of  gratifying  a  laudable  curiosity  and  of  obtaining  a  healthful  recrea- 
tion,— has  naturally  led  to  a  demand  for  information,  both  as  to  the  mode 
of  employing  the  Instrument  and  its  appurtenances,  and  as  to  the  Objects 
for  whose  minute  examination  it  is  most  appropriate.  This  information 
the  Author  has  endeavored  to  supply  in  the  following  Treatise;  in  which 
he  has  aimed  to  combine,  within  a  moderate  compass,  that  information  in 
regard  to  the  use  of  his  Instrument  and  its  Appliances  which  is  most  essen- 
tial to  the  working  Microscopist,  with  such  an  account  of  the  Objects 
best  fitted  for  his  study  as  may  qualify  him  to  comprehend  what  he  ob- 
serves, and  thus  prepare  him  to  benefit  Science  whilst  expanding  and 
refreshing  his  own  mind.  The  sale  of  five  large  Editions  of  this  Manual, 
with  the  many  spontaneous  testimonies  to  its  usefulness  which  the  Author 
has  received  from  persons  previously  unknown  to  him,  justify  the  belief 
that  it  has  not  inadequately  supplied  an  existing  want;  and  in  the  prepa- 
ration of  the  new  Edition  now  called-for,  therefore,  he  has  found  no  rea- 
son to  deviate  from  his  original  plan,  whilst  he  has  endeavored  to  improve 
its  execution  as  to  every  point  which  seemed  capable  of  amended  treat- 
ment. 

In  his  account  of  the  various  forms  of  Microscopes  and  Accessory 
Apparatus,  the  Author  has  not  attempted  to  describe  everything  which 
is  used  in  this  country;  still  less,  to  go  into  minute  details  respecting  the 
construction  of  foreign  instruments.  He  is  satisfied  that  in  nearly  all 
which  relates  both  to  the  mechanical  and  the  optical  arrangements  of  their 
instruments,  the  chief  English  Microscope-makers  are  quite  on  a  level 
with,  if  not  in  advance  of,  their  Continental  rivals;  but,  on  the  other 
hand,  the  latter  have  supplied  instruments  which  are  adequate  to  all  the 
ordinary  purposes  of  scientific  research,  at  a  lower  price  than  such  could 
until  recently  be  obtained  in  this  country.  Several  British  makers,  how- 
ever, are  now  devoting  themselves  to  the  production  of  Microscopes  which 
shall  be  really  good  though  cheap;  and  the  Author  cannot  but  view  with 
great  satisfaction  the  extension  of  the  manufacture  in  this  direction.  In 
the  selection  of  Instruments  for  description  which  it  was  necessary  for 
him  to  make,  he  trusts  that  he  will  be  found  to  have  done  adequate  juS' 


iv 


PREFACE. 


tice  to  those  ^rlio  have  most  claim  to  honorable  distinction.  His  princi- 
ple has  been  to  make  mention  of  such  Makers  as  have  distinguished 
themselves  by  the  introduction  of  any  oieiu  pattern  which  he  regards  as 
deserving  of  special  recommendation;  those  who  have  simply  copied  the 
patterns  of  others  without  essential  modification,  receiving  no  such  recog- 
nition,— not  because  their  instruments  are  inferior^  but  because  they 
are  oiot  original. 

In  treating  of  the  Applications  of  the  Microscope,  the  Author  has 
constantly  endeavored  to  meet  the  wants  of  such  as  come  to  the  study 
of  the  minute  forms  of  Animal  and  Vegetable  life  with  little  or  no  previ- 
ous scientific  prep^iration,  but  desire  to  gain  something  more  than  a 
mere  sight  of  the  objects  to  which  their  observation  may  be  directed. 
Some  of  these  may  perhaps  object  to  the  general  tone  of  his  work  as  too 
highly-pitched,  and  may  think  that  he  might  have  rendered  his  descrip- 
tions simpler  by  employing  fewer  Scientific  terms.  But  he  would  reply 
that  he  has  had  much  opportunity  of  observing  among  the  votaries  of  the 
Microscope  a  desire  for  such  information  as  he  has  attempted  to  convey; 
and  that  the  use  of  scientific  terms  cannot  be  easily  dispensed  with,  since 
there  are  no  others  in  which  the  facts  can  be  readily  expressed.  As  he 
has  made  a  point  of  explaining  these  in  the  places  where  they  are  first 
introduced,  he  cannot  think  that  any  of  his  readers  need  find  much  diflB- 
€ulty  in  apprehending  their  meaning. 

The  proportion  of  space  allotted  to  the  several  departments  has  been 
determined  not  so  much  by  their  Scientific  importance,  as  by  their  special 
interest  to  the  amateur  Microscopist;  and  the  remembrance  of  this  consider- 
ation will  serve  to  account  for  much  that  might  otherwise  appear  either 
defective  or  redundant.  Thus,  the  Author  has  specially  dwelt  on  those 
humble  forms  of  Vegetable  and  Animal  life,  which  the  diligent  collector 
is  most  likely  to  meet  with,  and  which  will  fully  reward  his  most  atten- 
tive scrutiny.  And  he  has  endeavored,  in  his  account  of  them,  to  inter- 
est his  readers  in  the  knowledge  to  be  drawn  from  their  study,  as  to  those 
fundamental  phenomena  of  living  action  which  are  now  universally  admit- 
ted to  constitute  the  basis  of  Physiological  science;  thus  giving  to  the 
portion  of  his  Treatise  which  treats  of  Protophytic  and  Protozoic  organ- 
isms, the  character  of  a  General  Introduction  to  the  study  of  Biology, 
which  will,  he  hopes,  prove  specially  useful  to  such  as  desire  to  follow 
this  study  into  its  higher  walks.  On  the  other  hand,  the  Author  has 
felt  the  necessity  of  limiting  within  a  narrow  compass  his  treatment  of 
various  important  subjects  which  are  fully  discussed  in  Treatises  expressly 
devoted  to  them  (such,  for  example,  as  the  structure  of  Insects,  and  Ver- 
tebrate Histology),  in  order  that  he  might  give  more  space  to  those  on 
which  no  such  sources  of  information  are  readily  accessible.  For  the 
same  reason,  he  has  omitted  all  reference  to  the  Embryonic  Development 
of  Vertebrated  Animals, — a  study  that  is  second  to  none  in  scientific  in- 
terest, but  can  only  be  advantageously  taken  up  by  the  Microscopist  who 


PREFACE. 


V 


lias  been  trained  to  the  pursuit.  And  lie  has  found  himself  obliged  to 
content  himself  with  a  mere  indication  of  the  new  and  important  facts 
now  being  brought  to  our  knowledge  by  Microscopic  inquiry,  in  regard 
to  the  Deposits  at  present  in  progress  on  the  bottom  of  the  Deep  Sea, 
the  Mineral  constitution  of  Sedimentary  and  Igneous  Kocks,  and  other 
branches  of  Micro-Petrological  inquiry,  which  are  throwing  a  flood  of 
new  light  on  the  past  history  of  the  Crust  of  the  Earth. 

It  has  been  the  Author's  object  throughout,  to  guide  the  possessor  of 
of  a  Microscope  to  the  intelligent  study  of  any  department  of  Biology, 
which  his  individual  tastes  may  lead  him  to  follow-out,  and  his  indi- 
vidual circumstances  may  give  him  facilities  for  pursuing.  And  he  has 
particularly  aimed  to  show,  under  each  head,  how  small  is  the  amount 
of  trustworthy  knowledge  already  acquired,  compared  with  that  which 
remains  to  be  attained  by  the  zealous  and  persevering  student.  Being 
satisfied  that  there  is  a  large  quantity  of  valuable  Micro  scope-power 
at  present  running  to  waste  in  this  country, — applied  in  such  desultory 
observations  as  are  of  no  service  whatever  to  Science,  and  of  very  little 
to  the  mind  of  the  observer, — he  will  consider  himself  well  rewarded  for 
the  pains  he  has  bestowed  on  the  production  and  revision  of  this  Manual, 
if  it  should  be  tend  to  direct  this  power  to  more  systematic  labors,  in 
those  fertile  fields  which  only  await  the  diligent  cultivator  to  bear  abun- 
dant fruit. 

In  all  that  concerns  the  worJcing  of  the  Microscope  and  its  appurten- 
ances, the  Author  has  mainly  drawn  upon  his  own  experience,  which 
dates-back  almost  to  the  time  when  Achromatic  Object-glasses  were  first 
constructed  in  this  country.  In  his  last  Edition,  he  felt  himself  obliged 
by  the  demands  which  were  made  by  Official  duties  upon  his  time  and 
attention,  to  seek  the  aid  of  his  friend  Mr.  H.  J.  Slack,  in  the  prepara- 
tion of  the  portion  of  the  work  specially  relating  to  the  Microscope  and 
its  appliances.  But  having  now,  at  last,  the  command  of  his  own  time, 
he  has  preferred  that  this,  like  the  rest  of  the  Treatise,  should  be  the  ex- 
pression of  his  own  matured  views;  and  has  accordingly  taken  much 
trouble  to  acquaint  himself  thoroughly  with  such  recent  advances,  alike 
in  the  theory  and  in  the  practice  of  Microscopy,  as  could  be  most  fittingly 
introduced  into  it. 

Accordingly,  he  has  introduced  at  pp.  156-161  a  concise  account  of 
the  ^  difl:raction-theory  ^  of  Prof .  Abbe,  which  has  now  given  the  com- 
plete rationale  of  the  relation  between  the  ^angular  aperture^  of  Object- 
ives and  their  ^resolving  power.'  And  he  has  followed  this  up  by  a  dis- 
cussion of  the  question  (pp.  161-173)  whether  the  opening-out  of  the 
angular  aperture  to  its  extremest  limits  is  the  end  to  be  specially  aimed- 
at  in  the  construction  of  Objectives  for  the  highest  kinds  of  Biological 
research;  in  other  words,  whether  an  Objective  which  resolves  the  most 
difficult  Diatom  tests,  is  on  that  account  the  one  best  suited  for  follow- 
ing the  life-history  of  Monad,  or  for  studying  the  development  of  a  prob- 


vi 


PREFACE. 


lematical  BaciUus-organism.  Having  the  misfortune  to  differ  in  opinion 
on  this  point  from  certain  American  Microscopists,  who  are  distinguished 
by  their  expertness  in  the  resolution  of  lined  tests  by  Objectives  of  the 
largest  angular  aperture,  and  who  enthusiastically  advocate  the  use  of 
such  Objectives  as  the  only  powers  to  be  trusted  for  Biological  research, 
he  has  requested  his  friend,  Mr.  Dallinger  (than  whom  there  can  be  no 
higher  authority  on  such  a  question),  to  give  him  the  benefit  of  his  ex- 
j)erience  thereon.  And  he  is  authorized  by  Mr.  Dallinger  to  express  his 
entire  concurreiice  in  the  opinion  uniformly  upheld  by  the  Author,  that 
great  'resolving  power'  is  only  exceptionally  needed  in  the  most  difficult 
Biological  investigations;  what  is  especially  required  for  the  study  of  liv- 
ing and  moving  organisms  being  such  crisp  and  clear  definition,  good 
working  distance,  and  considerable  focal  depth,  as  high-jaower  Objectives 
of  the  widest  aperture  cannot  afford.  These  qualities  are  so  admirably 
combined  in  the  '  dry '  l-35th  of  '  moderate  angle '  constructed  to  Mr.  Dal- 
linger's  order  by  Messrs.  Powell  and  Lealand,  that  he  has  been  able  to  do 
work  (of  the  kind  just  specified)  with  this  Objective,  which  it  would  have 
been  simply  impossible  for  him  to  do  with  the  oil-immersion  l-25thof  the 
same  makers,  although  this  far  surpasses  their  l-35tli  in  'resolving' 
power. — When  Prof.  J.  Edwards  Smith,  and  those  who  side  with  him, 
shall  have  produced  Biological  work  of  anything  like  the  same  nature 
and  quality  as  that  of  Mr.  Dallinger,  it  will  be  interesting  to  know  the 
results  of  their  more  extended  experience. 

On  another  point  of  great  practical  importance,  the  Author  has 
thought  it  worth  while  to  avail  himself  of  Mr.  Dallinger's  unrivalled 
experience, — the  utility  of  '  deep  eye-piecing.'  For  he  has  seen  with 
astonishment  that  the  enthusiastic  American  advocates  of  the  widest 
angles  for  Objectives  of  moderate  power,  are  claiming  for  such  objectives 
the  advantage  that  they  may  be  worked-up  to  any  amount  of  amplifica- 
tion by  sufficiently  'deep  eye-piecing;'  solid  eye-joieces  of  half  or  even 
a  quarter  of  an  inch  being  now  spoken-of  as  in  ordinary  use.  He  does 
not  for  a  moment  doubt  that  difficult  lined  tests  may  be  thus  shown;  but 
that  it  is  far  less  trying  to  the  vision,  when  exercised  in  continuous  luork, 
to  gain  the  needed  amplification  by  a  high  Objective  and  shallow  Eye- 
piece, than  by  a  loiv  Objective  (however  wide  its  angle)  and  deep  Eye- 
piece, experience  long  ago  satisfied  him.  Not  having  thus  exercised  his 
eyes,  however,  upon  objects  requiring  the  high  amplifications  used  by  Mr. 
Dallinger,  he  was  fully  prepared  to  submit  his  own  judgment  on  this 
question  to  that  of  a  gentleman  who  has  so  well  earned  his  title  to  pro- 
nounce an  authoritative  verdict  upon  it;  but,  so  far  from  having  in  the 
least  to  give  way,  the  Author  finds  himself  supported  by  Mr.  D.  in  the 
most  emphatic  way.  For  he  learns,  not  only  that  Mr.  D.  's  experience  in  the 
study  of  the  most  difficult  Biological  objects  satisfies  him  of  the  immense  | 
superiority  of  the  highest  Objective  that  admits  of  good  working  distance, 
combined  with  a  low  Eye-piece,  over  the  'strained  amplification '  given 


PREFACE. 


vii 


by  a  4-lOths,  a  l-4th,  or  even  a  l-8tli,  with  deep  eye-pieces;  but  that  Mr. 
D.  is  satisfied  that  if  he  had  tried  to  do  the  work  of  the  last  ten  years  on 
the  latter  plan^  "ha  would  be  now  blind,  instead  of  possessing  as  good 
and  sensitive  a  sight  as  he  had  ten  years  ago/^  As  it  has  been  politely 
suggested  by  an  American  controversialist,  that  the  Author's  inability  to 
appreciate  the  supreme  value  of  wide  aperture  may  be  due  to  the  senile 
deterioration  of  his  vision,  the  Authof  is  happy  to  be  able  to  state  that, — 
thanks  to  his  habit  of  using  shallow  Eye-pieces,  and  of  never  persisting 
in  Microscope-work  when  he  has  felt  visual  fatigue, — his  eyes  are  now  as 
fit  for  Microscopy  as  they  were  when  he  began  so  to  use  them  nearly 
half  a  century  ago. 

He  has  only  to  add  that  he  has  endeavored,  by  a  careful  and  thorough 
revision  of  the  entire  Treatise,  to  render  it  as  serviceable  as  possible  to 
those  for  whom  it  is  specially  intended.  Besides  introducing  a  large 
amount  of  new  matter  into  the  first  four  chapters,  he  has  entirely  re- 
written Chap,  v.,  so  as  to  embody  in  it  an  account  of  those  methods  of 
Hardening,  Staining,  Imbedding,  and  Section-cutting,  which  have  com- 
pletely revolutionized  many  departments  of  Microscopic  investigation, 
in  the  sections  relating  to  the  Protophytic  forms  of  Vegetable  life,  much 
new  matter  has  been  introduced  in  regard  so  the  ScMzomycetes  or  Bacte- 
rium group,  the  Myxomycetes,  and  others  of  those  curious  organisms 
which  occupy  the  border-ground  between  Vegetable  and  Animal  life. 
So,  again,  in  the  section  on  the  Protozoic  forms  of  Animal  life,  large  ad- 
ditions have  been  made  under  the  heads  of  Mojierozoa,  Rhizopoda,  Infu- 
soria (especially  the  flagellate  smd  suctor  ial),  and  Radiolaria;  and  the  sec- 
tion on  Sponges  has  been  entirely  re-written.  Some  important  additions 
have  also  been  made  (Chap.  XXI.)  in  regard  to  the  applications  of  the 
Microscope  to  Geological  inquiry. — In  many  other  instances,  references 
have  been  made  to  the  best  sources  of  information  upon  recent  discov- 
eries of  interest,  which  a  due  regard  to  the  necessary  limits  of  his  book 
made  it  requisite  for  the  Author  to  dismiss  with  a  mere  mention. 

No  fewer  than  fifty  new  Wood-engravings  have  been  added  (for  the 
use  of  eleven  of  which  the  Author  is  indebted  to  the  Council  of  the  Lin- 
naean  Society),  besides  the  reproduction  of  Prof.  Oohn's  beautiful  Plate 
of  Volvox,  which  now  forms  the  Frontispiece. 

To  such  as  feel  inclined  to  take  up  the  use  of  the  Microscope  as  a 
means  of  healthful  and  improving  occupation  for  their  unemployed  hours, 
the  Author  would  offer  this  word  of  encouragement, — that,  notwithstand- 
ing the  number  of  recruits  continually  being  added  to  the  vast  army  of 
Microscopists,  and  the  rapid  extension  of  its  conquests,  the  inexhaustibility 
of  Nature  is  constantly  becoming  more  and  more  apparent;  so  that  no 
apprehension  need  arise  that  the  Microscopist's  researches  can  ever  be 
brought  to  a  stand  for  vmnt  of  an  object! 

London,  May,  1881. 


TABLE  OF  OOIsTTEOTS. 


CHAPTER  I. 


OPTICAL  PRINCIPLES  OP  THE  MICROSCOPE. 


PAGE 

Laws  of  Refraction: — Spherical  and  Chromatic  Aberration,     .       .       .       .  1 

Construction  of  Achromatic  Objectives,   11 

Immersion  Systems,   16 

Simple  Microscope,   18 

Compound  Microscope   22 

Prmciples  of  Binocular  Vision   25 

Stereoscopic  Binocular  Microscopies,   27 

Nachet's,   28 

Wenham's,  .       ,       ,   ^9 

Stephenson's,   81 

Tolles'  Binocular  Eyepiece,   33 

Nachet's  Stereo-pseudoscopic  Binocular,   33 

Special  value  of  Stereoscopic  Binoculars,   35 


CHAPTER  II. 


CONSTRUCTION  OF  THE  MICROSCOPE. 


General  Principles,  *  .  .40 
Simple  Microscopes,    .      .  .43 

Ross's,  43 

Quekett's  Dissecting,  .  .  45 
Siebert  &  Kraft's  Dissecting,  .  46 
Laboratory  Dissecting,     .       .  47 


Beck's  Dissecting  and  Nachet's 

Binocular,  .  .  .  .48 
Field's  Dissecting  and  Mount- 


ing,  49 

Compound  Microscopes,  .  .  51 
Educational  Microscopes,       .      .  53 

Field's,  53 

Crouch's,    .       ...       .  .53 

Parkes's,  53 

Students'  Microscopes,     .      .  .55 

Baker's,  59 

CoUins's,  .  ,  .  .  .59 
Pilhscher's  (International),  .  59 
Ross's  (Zentmayer),  .  .  61 
Wale's  (New  Working),    .       .  61 

Nachet's,  63 

Browning's  (Rotating),  .  .  65 
Crouch's  (Binocular),       .       .  65 


Baker's  (Erecting  ditto),  .       .  65 

Second  Class  Microscopes,  .  .  67 
Powell  and  Lealand's,  .  .  67 
Beck's  (Popular  Binocular),  .  68 
Collins's  (Harley  Binocular),  .  70 
Swift's  (Challenge),  .  .  .  71 
Browning's  Smaller  Stephenson 
Binocular,      .       .       .  .73 

First  Class  Microscopes,  .  .  .73 
Ross's  (Ross  Model),  .  .  .  73 
Ross's  (Jackson-Zentmayer),  .  75 
Powell  and  Lealand's,      ,       .  77 

Beck's,  77 

Beck's  (Improved),   .       .  .80 

Microscopes  for  Special  Purposes  .  80 
Beale's  Pocket  and  Demonstrat- 
ing,  81 

Baker's  Travelling,  .  .  .81 
Swift's  Portable,  .  .  .82 
Nachet's  Chemical,    .       .  .82 

Non-Stereoscopic  Binoculars,  .  .  84 
Powell  and  Lealand's,  .  .  85 
Wenham's,        .      .      •  .85 


X 


TABLE  OF  CONTENTS. 
CHAPTER  III. 


ACCESSORY 


PAGE 

Amplifier,  86 

Draw-tube,  87 

Lister's  Erector,  ,  .  •  .  87 
Micro-Megascope  .  ,  .  ,88 
Nachet's  Erecting  Prism,  .  .  88 
Micro-Spectroscope,  .  .  .89 
Micrometric  Apparatus,  .  .  .92 
Goniometer,  95 


Diaphragm  Eye-piece  and  Indica- 
tor,  95 

Camera  Lucida  and  other  Drawing 


Apparatus,  96 

Nose-piece,  99 

Finders,  99 

Diaphragms,  101 

Achromatic  Condensers,  .  .  .  103 
Webster  Condenser,  .  .  .  103 
Obhque  Illuminators,      .       .       .  104 

Amici's  Prism,  106 

Black-Ground  Illuminators,    •      .  106 


Table  and  Cabinet,  .  .  .  .130 

Daylight  and  Lamps,  .  .  .131 

Position  of  Light,    .  ,  .  .133 

Care  of  the  Eyes,     .  .  .  .134 

Care  of  the  Microscope,  .  ,  .134 

General  Arrangements,  .  .  .  135 

Focal  Adjustment,  .  .  .  .137 

Adjustment  of  Object-Glass,  .  .139 


Materials,  Instruments,  and  ApplU 

ances,  175 

Glass  Slides,  .  .  .  .175 
Thin  Glass,  .  .  .  .176 
Varnishes  and  Cements,  .  .178 
Cells  for  Mounting  Objects,  .  179 
Wooden  Slides  for  Opaque  Ob- 
jects,  183 

Turn-Table,  .  .  .  .184 
Mounting   Plate   and  Water 

Bath,  185 

Slider-Forceps,  Spring-Clip,  and 

Spring-Press,  .       .       .  .185 
Mounting  Instrument,      .  .186 
Dissecting  Apparatus,      .  .186 
Microtomes    and  Section-cut- 
ting,      .      .      .      .  .188 


APPARATUS. 

PAGE 

Wenham's  Reflex  Illuminator,  .  109 
Light  Modifiers,  .  .  .  .110 
Polarizing  Apparatus,  .  .  .111 
Swift's  Combination  Sub-Stage,  .  113 
Side  Illuminators  for  Opaque  Ob- 
jects,  114 

Parabolic  Speculum,       .      .  .116 

Lieberkiihn,  117 

Vertical  Illuminators,  .  .  .118 
Stephenson's  Safety  Stage,  .  .  120 
Stage-Forceps  and  Vice,  .  .  .  120 
Disk-holder  and  Object-holder,  .  121 
Glass  Stage-Plate,  .  .  .  .  122 
Growing  Slides,  ....  122 
Aquatic  Box  and  Cells,  .  .  .  123 
Zoophyte-Trough,    .       ,       ,  .125 

Compressors,  126 

Dipping  Tubes,  .  .  .  .127 
Glass  Syringe,  •  .  •  •  .  128 
Forceps,  128 


Arrangement  for  Transparent  Ob- 
jects,  141 

Arrangement  for  Opaque  Objects,  .  147 
Errors  of  Interpretation,  .  .  150 
Effects  of  Diffraction.  .  .  .154 
Relative  Qualities  of  Objectives,  .  161 
Test-Objects,    .       .       .       .  .165 


Determination  of  Magnifying  Power,  173 


Preparation  and  Mounting  of  Ob- 
jects,  194 

Imbedding  Processes,       .      .  194 
Grinding  and  Polishing  Sec- 
tions of  Hard  Substances      .  196 
Decalcifying  Process,      .       .  201 
Hardening  of  Animal  Substan- 
ces, ......  202 

Staining  Processes,  .  .  .  204 
Chemical  Testing,  .  .  .208 
Preservative  Media,  .  .  .  209 
Mounting  Thin  Sections,  .  .212 
Mounting  in  Canada  Balsam,  .  214 
Mounting  Objects  in  Cells,  .  215 
Labelling  and  Preserving  of  Ob- 
jects,  218 

Collection  of  Objects,      .      .      .  219 


CHAPTER  IV. 

MANAGEMENT  OF  THE  MICROSCOPE. 


CHAPTER  V. 

PREPARATION,  MOUNTING,  AND  COLLECTING  OF  OBJECTS. 


TABLE  OF  CONTENTS. 


xi 


CHAPTER  VI. 

MICROSCOPIC  FORMS  OF  VEGETABLE  LIFE '.—SIMPLER  ALG^. 


Protoplasm— Vegetable  and  Ani- 
mal 222 

Relation  between  Vegetable  and 


Animal  Kingdoms,      .       .       .  223 

Vegetable  Cells  in  general,     .       .  224 

Protophytic  Algce,  ....  229 

Conjugateae,   236 

Volvocineae,   237 

Palmellaceae,   245 

Ulvaceae,   246 

Oscillatoriaceae,      ....  247 


Fungi  differentiated  from  Algae,  .  307 
Schizomycetes,  ....  307 
Fermentative  Action,     .      .  .313 


Alg^,  331 

Hepaticae,  335 

Mosses,  338 

Sphagnaceae,    •      .      .      .  342 


PAGE 

Nostochaceae,  249 

Siphonaceae,  250 

Confervaceae,  254 

OEdogonieae,  257 

Chaetophoraceae,  ....  258 
Batrachospermeae,  ....  258 
Characeae,       .....  259 

Desmidiace^,  269 

Pediastreae,      ....  270 

DlATOMACE.35,  273 


Parasitic  Fungi,  ....  316 
Myxomycetes,  .  .  .  .825 
Lichens,  329 


Ferns,   344 

Equisetaceae,   349 

Rhizocarpeae,   350 

Lycopodiaceae,        ....  350 


CHAPTER  VII. 

PROTOPHYTIC  AND  OTHER  FUNGI.— LICHENS. 


CHAPTER  VIII. 

MICROSCOPIC  STRUCTURE  OF  HIGHER  CRYPTOGAMIA. 


CHAPTER  IX. 


MICROSCOPIC  STUCTURE  OF  PHANEROGAMIC  PLANTS. 

Distinctive  Peculiarities  of  Phan-  Structure     of     Epidermis  and 

erogamia,   352       Leaves,  ,  577 

Elementary  Tissues,       .       .       .  353  Structure  of  Flowers,     .       .       .  382 

Structure  of  Stem  and  Root,  .       .  367  Fertilization. — Seeds,      .      .      .  385 


THE  MICROSCOPE. 


CHAPTER  L 
OPTICAL  PRINCIPLES  OF  THE  MICROSCOPE. 

1.  Laius  of  Refraction: — Splierical  and  Chromatic  Aberration. 

1.  All  Microscopes  in  ordinary  use,  whether  Simple  or  Compound, 
depend  for  their  magnifying  power  on  that  influence  exerted  by  Lenses, 
in  altering  the  course  of  the  rays  of  light  passing  through  them,  which  is 
termed  Refraction.  This  influence  takes  place  in  accordance  with  the 
two  following  laws,  which  are  fully  explained  and  illustrated  in  every 
elementary  treatise  on  Optics: — 

I.  A  ray  of  light  passing  from  a  rarer  into  a  denser  medium,  is 
refracted  towards  a  line  drawn  perpendicularly  to  the  plane  which  divides 
them;  and  vice  versa. 

II.  The  sines  of  the  angles  of  incidence  and  refraction  (that  is,  of  the 
angles  which  the  ray  makes  with  the  perpendicular  hefore  and  after  its 
refraction)  bear  to  one  another  a  constant  ratio  for  each  substance,  which 
is  known  as  its  index  of  refraction. 

Thus  the  ray  E  o  (Fig.  1)  passing  from  Air  into  Water,  will  not  go  on 
to  E,  but  will  be  refracted  towards  the  line  c  c'  drawn  perpendicularly  to 
the  surface  A  b  of  the  water,  so  as  to  take  the  direction  o  w.  If  it  pass 
into  Glass,  it  will  undergo  a  greater  refraction,  so  as  to  take  the  direction 
0  G.  And  if  it  pass  into  Diamond,  the  change  in  its  course  will  be  so 
much  greater,  that  it  will  take  the  direction  o  D.  The  angle  E  o  c  is 
termed  the  ^  angle  of  incidence;'  whilst  the  angles  w  o  c',  G  o  c',  and  r> 
o  c'  are  the  ^angles  of  refraction.'  And  whether  the  angle  of  incidence 
be  large  or  small,  its  sine  e  e'  bears  a  constant  ratio  in  each  case  to  the 
sine  10  or  g  or  d  d\  of  the  angle  of  refraction;  and  this  ratio  is 
what  is  termed  the  ^  index  of  refraction.' 

The  ^  index  of  refraction '  is  determined  for  different  media  by  the 
amount  of  the  refractive  influence  which  they  exert  upon  rays  passing 
into  them,  not  from  air,  but  from  a  vacuum;  and  in  expressing  it,  the 
sine  of  the  angle  of  refraction  is  considered  as  the  unit,  to  which  that  of 
the  angle  of  incidence  bears  a  fixed  relation.  Thus  when  we  say  that  the 
^index  of  refraction'  of  Water  is  1.336,  we  mean  that  the  sine  e  e'  of  the 
angle  of  incidence  e  o  c  of  a  ray  passing  into  water  from  a  vacuum,  is  to 
the  sine  w  lo'  of  the  angle  of  refraction  w  o  c',  as  1.336  to  1,  or  almost 
1 


2 


THE  MICROSCOPE  AND  ITS  REVELATIONS. 


Pig.-  1. 


exactly  as  1^  to  1,  or  as  4  to  3.  So,  again,  the  index  of  refraction  for 
(flint)  Glass,  being  about  1.6,  we  mean  that  the  sine  e  e'  of  the  angle  of 
incidence  of  a  ray  E  o  c  passing  into  Glass  from  a  yacuum,  is  to  the  sine 
of  g  g'  the  angle  of  refraction  G  o  c',  as  1.6  to  1,  or  as  8  to  5.    So  in  the 

case  of  Diamond,  the  sine 
e  e'  is  to  the  sine  d  d'  as 
2.439  to  1,  or  almost  ex- 
actly as  2 J  to  1 ,  or  as  5 
to  2.  Thus,  the  angle  of 
incidence  being  given,  the 
angle  of  refraction  may 
be  always  found  by  divid- 
ing  the  sine  of  the  former 
by  the  ^  index  of  refrac- 
tion,^ which  will  give  the 
sine  of  the  latter.  In  ac- 
cordance with  these  laws, 
a  ray  of  light  passing  from 
one  medium  to  another 
perpendicularly  to  the 
surface  which  divides 
them  undergoes  no  re- 
fraction; and  of  several 
rays  entering  at  different 
angles,  those  nearer  the  perpendicular  are  refracted  less  than  those  more 
inclined  to  the  refracting  surface. — When  a  pencil  of  rays,  however,  im- 
pinges on  the  surface  of  a  denser  medium  (as  when  rays  passing  through 
Air  fall  upon  Water  or  Glass),  some  of  the  incident  rays  are  reflected  from 
that  surface,  instead  of  entering  it  and  undergoing  refraction;  and  the  pro- 
portion of  these  rays  increases  with  the  increase  of  their  obliquity.  Hence 
there  is  a  loss  of  liglit  in  every  case  in  which  pencils  of  rays  are  made  to 
pass  through  lenses  or  prisms:  and  this  diminution  in  the  brightness  of 
the  image  formed  by  refraction  will  bear  a  proportion,  on  the  one  hand, 
to  the  number  of  surfaces  through  which  the  rays  have  had  to  pass;  and, 
on  the  other,  to  the  degree  of  obliquity  of  the  incident  rays,  and  to  the 
difference  of  the  refractive  powers  of  the  two  media.  Hence,  in  the 
passage  of  a  pencil  of  rays  out  of  Glass  into  Air,  and  then  from  Air  into 
Glass  again,  the  loss  of  light  is  much  greater  than  it  is  when  some 
medium  of  higher  refractive  power  than  air  is  interposed  between  the  two 
glass  surfaces;  and  advantage  is  taken  of  this  principle  in  the  construc- 
tion of  Achromatic  objectives  for  the  Microscope,  the  component  lenses 
of  each  pair  or  triplet  (§  14)  being  cemented  together  by  Canada  Balsam; 
as  also  in  the  interposition  of  Water  or  some  other  liquid  between  the 
covering-glass  of  the  object  and  the  front  lens  of  the  objective,  in  the 
*  immersion  lenses^  now  coming  into  general  use  (§  19).  On  the  other 
hand,  advantage  is  taken  of  the  partial  reflection  of  rays  passing  from  air 
into  glass  at  an  oblique  angle  to  the  surface  of  the  latter,  in  the  construc- 
tion of  the  ingenious  (non-stereoscopic)  Binoculars  of  Messrs.  Powell  and 
Lealand  and  of  Mr.  Wenham  (§  81). 

2.  When,  on  the  other  hand,  a  ray,  w  o,  emerges  from  a  dense 
medium  into  a  rare  one,  instead  of  following  the  straight  course,  it  is 
bent  from  the  perpendicular  according  to  the  same  ratio;  and  to  find  the 
course  of  the  emergent  ray,  the  sine  of  the  angle  of  incidence  must  be 
vmltiplied  by  the  ^  index  of  refraction,^  which  will  give  the  sine  of  the 


OPTICAL  PRINCIPLES  OF  THE  MICROSCOPE. 


3 


angle  of  refraction.  And  thus,  when  an  emergent  ray  falls  very  obliquely 
upon  the  surface  of  the  denser  medium^  the  refraction  which  it  would 
sustain  in  passing  forth  into  the  rarer  medium,  tending  as  it  does  to 
deflect  it  still  farther  from  the  perpendicular,  becomes  so  great  that  the  ray 
cannot  pass  out  at  all,  and  is  reflected  back  from  the  plane  which  separates 
the  two  media,  into  the  one  from  which  it  was  emerging.  This  i7iternal 
reflection  will  take  place  whenever  the  product  of  the  sine  of  the  angle  of 
incidence,  multiplied  by  the  index  of  refraction,  exceeds  the  sine  of  90°, 
which  is  the  radius  of  the  circle;  and  therefore  the  ^limiting  angle,' 
beyond  which  an  oblique  ray  suffers  internal  reflection,  varies  for  differ- 
ent substances  in  proportion  to  their  respective  indices  of  refraction. 
Thus,  the  index  of  refraction  of  Water  being  1.336,  no  ray  can  pass  out 
of  it  into  a  vacuum,*  if  its  angle  of  incidence  exceed  48^  28',  since  the 
sine  li  A'  of  that  angle,  H  o  c',  multiplied  by  1.336  equals  the  radius; 
and,  in  like  manner,  the  '  limiting  angle '  for  Flint-glass,  its  index  of 
refraction  being  1.60,  is  38°  41\ — This  fact  imposes  certain  limits  upon 
the  performance  of  microscopic  Lenses,  since  of  the  rays  which  would 
otherwise  pass  out  from  glass  into  air  all  the  more  oblique  are  kept  back; 
whilst,  on  the  other  hand,  it  enables  the  Optician  to  make  most  advan- 
tageous use  of  glass  Prisms  for  the  purpose  of  re-flection,  the  proportion 
of  the  light  which  they  throw  back  being  much  larger  than  that  returned 
from  the  best  polished  metallic  surfaces,  and  the  brilliancy  of  the  reflected 
image  being  consequently  greater.  Such  prisms  are  of  great  value  to  the 
Microscoj)ist  for  particular  purposes,  as  will  hereafter  appear.  (§§  33- 
38.) 

3.  The  Lenses  employed  in  the  construction  of  Microscopes  are  chiefly 
convex ;  those  of  the  opposite  kind,  or  concave,  being  only  used  to  make 
certain  modifications  in  the  course  of  the  rays  passing  through  convex 
lenses,  whereby  their  performance  is  rendered  more  exact  (§§  11,  13). — It 
is  easily  shown  to  be  in  accordance  with  the  laws  of  refraction  already 
cited,  that  when  a  bundle  of  parallel  rays,  passing  through  air,  impinges 
upon  a  spherical  surface  of  glass,  these  rays  will  be  made  to  converge. 
For  the  perpendicular  to  every  point  of  that  surface  is  the  radius  drawn 
from  the  centre  of  the  sphere  to  that  point,  and  prolonged  through  it;  so 
that,  whilst  any  ray  which  coincides  with  the  radial  perpendicular  will  go 
on  without  change  in  its  course  towards  the  centre  of  the  sphere;  every 
ray  which  falls  upon  the  spherical  surface  at  an  inclination  to  its  pi'o- 
longed  radius  undergoes  refraction  in  a  degree  proportionate  (as  already 
explained)  to  that  inclination.  And  the  effect  upon  the  whole  bundle 
will  be  such,  that  its  rays  will  be  caused  to  meet  at  a  point,  called  the 
focns,  some  distance  beyond  the  centre  of  curvature. — This  effect  will  be 
somewhat  modified  by  the  passage  of  the  rays  into  air  again  through  a 
flane  surface  of  glass,  perpendicular  to  the  axial  ray  (Fig.  2);  and  a  lens 
of  this  description,  called  2,  plano-convex  lens,  will  hereafter  be  shown  to 
possess  properties  which  render  it  very  useful  in  the  construction  of 
Microscopes. — But  if,  instead  of  passing  through  a  plane  surface,  the 
rays  re-enter  the  air  through  a  second  convex  surface,  turned  in  the  oppo- 
site direction,  as  in  a  double-convex  lens,  they  will  be  made  to  converge 


^  The  reader  may  easily  make  evident  to  himself  the  internal  reflection  of 
Water,  by  nearly  filling  a  wine-glass  with  water,  and  holding  it  at  a  higher  level 
than  his  eye,  so  that  he  sees  the  surface  of  the  fluid  obliquely  from  beneath: — no 
object  held  above  the  water  will  then  be  visible  through  it,  if  the  eye  be  placed 
beyond  the  limiting  angle;  whilst  the  surface  itself  will  appear  as  if  silvered, 
through  its  reflecting  back  to  the  eye  the  light  which  falls  upon  it  from  beneath. 


4 


THE  MICROSCOPE  AND  ITS  REVELATIONS. 


Fig,  2. 


Parallel  rays,  falling  on  a  plano-convex  lens  of 
grlass,  brought  to  a  focus  at  the  distance  of  the 
diameter  of  its  sphere  of  curvature;  and  con- 
versely, rays  diverging  from  that  point,  ren- 
dered parallel. 


Fig.  3. 


still  more.  This  will  be  readily  comprehended  when  it  is  borne  in  mind 
that  the  contrary  direction  of  the  second  surface,  and  the  contrary  direc- 
tion of  its  refraction  (this  being  fro7n  the  denser  medium  instead  of  i7ito 

it),  antagonize  each  other;  so 
that  the  second  conyex  surface 
exerts  an  influence  on  the  course 
of  the  rays  passing  through  it, 
which  is  almost  exactly  equiva- 
lent to  that  of  the  first.  Hence 
the  focus  of  3j  doicble-conyex  lens 
will  be  at  just  half  the  distance, 
or  (as  commonly  expressed)  will 
be  half  the  length  of  the  focus 
of  a  ^:)Za^^o-conyex  lens  haviug 
the  same  curvature  on  one  side 
(Fig.  3). 

4.  The  distance  of  the  Focus 
from  the  spherical  surface  will 
depend  not  merely  upon  its  de- 
gree of  curvature,  out  also  upon  the  refracting  power  of  the  substance 
of  which  it  may  be  formed;  since  the  lower  the  index  of  refraction,  the 

less  will  the  oblique  rays  be  de- 
flected towards  the  axial  ray,  and 
the  more  rernote  will  be  their  point 
of  meeting;  and  conversely,  the 
greater  the  refractive  index,  the 
more  will  the  oblique  rays  be  de- 
flected towards  the  axial  ray,  and 
the  nearer  will  be  their  point  of 
convergence.  A  lens  made  of  any 
substance  whose  index  of  refrac- 
tion is  1.5,  will  bring  parallel  rays 
to  a  focus  at  the  distance  of  its 
diameter  of  curvature,  after  they 
have  passed  through  one  convex 
surface  (Fig.  2),  and  at  the  dis- 
tance of  its  radius  of  curvature, 
after  they  have  passed  through 
tiuo  convex  surfaces  (Fig.  3);  and  as  this  ratio  almost  exactly  expresses 
the  refractive  power  of  ordinary  crown  or  plate  Glass,  we  may  for  all 
practical  purposes  consider  the  ^  principal  focus '  (as  the  focus  for  parallel 
rays  is  termed)  of  a  douMe-couYex  lens  to  be  at  the  distance  of  its  radius, 
that  is,  in  the  centre  of  curvature,  and  that  of  a  plano-conYex  lens  to  be 
at  the  distance  of  twice  its  radius,  that  is,  at  the  other  end  of  the  diame- 
ter of  its  sphere  of  curvature. 

5.  It  is  evident  from  what  has  preceded,  that  as  a  Double-convex  lens 
brings  parallel  rays  to  a  focus  in  its  centre  of  curvature,  it  will  on  the 
other  hand  cause  those  rays  which  are  diverging  from  that  centre  before 
they  impinge  upon  it,  to  assume  a  parallel  direction  (Fig.  3);  so  that,  if 
a  luminous  body  be  placed  in  the  principal  focus  of  a  double-convex  lens, 
its  divergent  rays,  falling  on  one  surface  of  the  lens,  as  a  cone,  will  pass 
forth  from  its  other  side  as  a  cylinder.  If,  however,  the  rays  which  fall 
upon  a  double-convex  lens  be  diverging  from  the  farther  extremity  of  the 
diameter  of  its  sphere  of  curvature,  they  will  be  brought  to  a  focus  at  an 


Parallel  rays,  falling  on  a  double-convex  lens, 
brought  to  a  focus  in  the  centre  of  its  sphere  of 
curvature :  conversely,  rays  diverging  from  that 
point  rendered  parallel. 


OPTICAL  PKINCIPLES  OF  THE  MICROSCOPE. 


5 


Rays  diverging  from  the  farther  extremity  of 
one  diameter  of  curvature  of  a  double-convex  lens, 
brought  to  a  focus  at  the  same  distance  on  the 
other  side. 


equal  distance  on  the  other  side  of  the  lens  (Fig.  4);  but  the  more  the 
point  of  divergence  is  approximated  to  the  centre  or  principal  focus,  the 
farther  removed  from  the  other 

side  will  be  the  point  of  conver-  Fig.  4, 

gence  (Fig.  5),  until,  the  point 
of  divergence  being  at  the  cen- 
tre, there  is  no  convergence  at 
all,  the  rays  being  merely  render- 
ed parallel  (Fig.  3);  whilst  if 
the  point  of  divergence  be  ie- 
yond  the  diameter  of  the  sphere 
of  curvature,  the  point  of  con- 
vergence will  be  within  it  (Fig. 
5).    The  farther  removed  the 
point  of  divergence,  the  more 
nearly  will  the  rays  approach 
the  parallel  direction:  until,  at 
length,  when  the  object  is  very 
distant,  its  rays  in  eflfect  become 
parallel,  and  are  brought  together  in  the  principal  focus  (Fig.  3).    If,  on 
the  other  hand,  the  point  of  divergence  be  tuitldn  the  princijDal  focus, 
they  will  neither  be  brought  to  converge,  nor  be  rendered  parallel,  but 
will  diverge  in  a  diminished  de- 
gree (Fig.  6).    And  conversely,  irio,^^ 
if  rays  already  converging  fall 
upon  a  double-convex  lens,  they 
will  be  brought  together  at  a 
point  nearer  to  it  than  its  centre 
of  curvature  (Fig.  6). — The  same 
principles  apply  equally  to  a 
plano-convex  lens;  allowance  be- 
in^  made  for  the  double  distance 
of  its  principal  focus.    They  also 
apply  to  a  lens  whose  surfaces 
have  different  curvatures;  the 
principal  focus  of  such  a  lens  be- 
ing found  by  multiplying  the 
radius  of  one  surface  by  the  rad- 
ius of  the  other,  and  dividing  this 
product  by  half  the  sum  of  the 
same  radii. — The  rules  by  which 
the  foci  of  convex  lenses  may  be 
found,  for  rays  of  different  de- 
grees of  convergence  and  diver- 
gence, will  be  found  in  works  on 
Optics. 

6.  The  refracting  influence  of 
concave  lenses  will  evidently  be 
precisely  the  opposite  of  that  of 
convex.    Eays  which  fall  upon 

them  in  a  parallel  direction,  will      ^      ,    ^  .    -u     x,. .  v 

i_-i.-^7.  ././.  Rays  already  converging,  brought  together  by 

be  made  to  diverge  as  ll  from  the  a  douUe-convex  lens  at  a  point  nearer  than  its 

■Drincinal   foons     which    i<^    here  principal  focus;  and  rays  diverging  from  a  point 

n  j-^V  ^^^^^^^  within  its  principal  focus,  still  diverging,  through 

called  the  negative  lOCUS.      ihlS  in  a  diminished  degree. 


Rays  diverging  from  points  more  distant  than 
the  principal  focus  of  a  double-convex  lens  on 
either  side,  brought  to  a  focus  beyond  it ;  the  focus 
of  convergence  being  within  the  diameter  of  cur- 
vature, if  the  focus  of  divergence  be  beyond  it; 
and  vice  versa. 


6 


THE  MICROSCOPE  AND  ITS  REVELATIONS. 


will  be  for  a  plano-concave  lens,  at  the  distance  of  the  diameter  or 
the  sphere  of  curvature;  and  for  a  double-concave,  in  the  centre  of  that 
sphere.  In  the  same  manner,  rays  which  are  converging  to  such  a 
degree,  that,  if  uninterrupted,  they  would  have  met  in  the  principal 
focus,  will  be  rendered  parallel;  if  converging  more  they  will  still  meet, 
but  at  a  greater  distance;  and  if  converging  less,  they  will  diverge  as 
from  a  negative  focus  at  a  greater  distance  than  that  for  parallel  rays. 
If  already  diverging,  they  will  diverge  still  more,  as  from  a  negative 
focus  nearer  than  the  principal  focus;  but  this  negative  focus  will  ap- 
proach the  principal  focus,  in  proportion  as  the  distance  of  the  point 
of  divergence  is  such  that  the  direction  of  the  rays  approaches  the 
parallel. 

7.  If  a  lens  be  convex  on  one  side  and  concave  on  the  other,  forming 
what  is  called  a  me7iisciis,  its  effect  will  depend  upon  the  proportion  be- 
tween the  two  curvatures.  If  they  are  equal,  as  in  a  watch-glass,  scarcely 
any  perceptible  effect  will  be  produced;  if  the  convex  curvature  be  the 
greater,  the  effect  will  be  that  of  a  less  powerful  convex  lens;  and  if  the 
concave  curvature  be  the  more  considerable,  it  will  be  that  of  a  less 
powerful  concave  lens.  The  focus  of  convergence  for  parallel  rays  in  the 
first  case,  and  of  divergence  in  the  second,  may  be  found  by  dividing  the 
product  of  the  two  radii  by  half  their  difference. 

8.  Hitherto  we  have  considered  only  the  effects  of  lenses  either  on  a 
'  bundle '  of  parallel  rays,  or  on  a  ^  pencil  ^  of  rays  issuing  from  a  single 
luminous  point,  and  that  point  situated  in  the  line  of  its  axis.  If  the 
j)oint  be  situated  above  the  line  of  its  axis,  the  focus  will  be  below  it,  and 
vice  versa.  The  surface  of  every  luminous  body  may  be  regarded  as  com- 
prehending an  infinite  number  of  such  points,  from  every  one  of  which  a 
pencils  of  rays  proceeds,  to  be  refracted  in  its  passage  through  a  lens 
according  to  the  laws  already  specified;  so  that  a  complete  but  inverted 
image  or  picture  of  the  object  is  formed  upon  any  surface  placed  in  the 
focus  and  adapted  to  receive  the  rays.  It  will  be  evident  from  what  has 
gone  before,  that  if  the  object  be  placed  at  twice  the  distance  of  the  princi- 
pal focus,  the  image,  being  formed  at  an  equal  distance  on  the  other  side 
of  the  lens  (§  5),  will  be  of  the  same  dimensions  with  the  object  :  whilst, 
on  the  other  hand,  if  the  object  (Fig.  7,  a  i)  be  nearer  the  lens,  the 


object  will  be  more  brilliant  in  the  same  proportion. 

9.  A  knowledge  of  these  general  facts  will  enable  the  learner  to  un- 
derstand the  ordinary  action  of  the  Microscope;  but  the  instrument  is 
subject  to  certain  optical  imperfections,  the  mode  of  remedying  which 
cannot  be  comprehended  without  an  acquaintance  with  their  nature.  One 
of  these  imperfections  results  from  the  unequal  refraction  of  the  rays 


Formation  of  Images  by  Convex  Lenses. 


Fig.  7. 


image  A  B  will  be  farther  from 
it,  and  of  larger  dimensions; 
but  if  the  object  a  b  be  farther 
from  the  lens,  the  image  a  b 
will  be  nearer  to  it,  and  smaller 
than  itself.  Further,  it  is  to 
be  remarked  that  the  larger 
the  image  in  proportion  to 
the  object,  the  less  bright  will 
it  be,  because  the  same  amount 
of  light  has  to  be  spread  over 
a  greater  surface;  whilst  an 
image  that  is  smaller  than  the 


OPTICAL  PRINCIPLES  OF  THE  MICROSCOPE. 


7 


which  pass  through  lenses  whose  curvatures  are  equal  over  their  whole 
surfaces.  If  the  course  of  the  rays  passing  through  an  ordinary  convex 
lens  be  carefully  laid  down  (Pig.  8),  it  will  be  found  that  they  do  not  all 
meet  exactly  in  the  foci  already 
stated;  but  that  the  focus  r  of 
the  rays  ab,  ab,  which  have 
passed  through  the  marginal 
portion  of  the  lens,  is  much 
closer  to  it  than  that  of  the  rays 
al)j  a  by  which  are  nearer  the 
line  of  its  axis.  This  may 
be  shown  experimentally,  by 
^  stopping  out  ^  either  the  cen- 
tral or  the  marginal  portion  of 

the   lens;    for   it  will  then  be  diagram  illustrating  ^pZtencaZ  ^6erra«on. 

found  that  the  rays  which  are  allowed  to  pass  through  the  latter  alone 
form  a  distinct  image  at  f;  whilst  those  which  pass  through  the  former 
alone  form  a  distinct  image  at  /.  Hence,  if  the  whole  aperture  be  in 
use,  and  a  screen  be  held  in  the  focus  f  of  the  marginal  portion  of  the 
lens,  the  rays  which  have  passed  through  its  central  portion  will  be 
stopped  by  it  before  they  have  come  to  a  focus;  whilst,  if  the  screen  be 
carried  back  into  the  focus  f  of  the  latter,  the  rays  which  were  most  dis- 
tant from  the  axis  will  have  previously  met  and  crossed,  so  that  they  will 
come  to  it  in  a  state  of  divergence,  and  will  pass  to  c  and  d.  In  either 
case,  therefore,  the  image  will  have  a  certain  degree  of  indistinctness;  and 
there  is  no  one  point  to  which  all  the  rays  can  be  brought  by  a  single  lens 
of  spherical  curvature.  The  distance  f/,  between  the  focal  points  of  the 
central  and  of  the  peripheral  rays  of  any  lens,  is  termed  its  Spherical 
Aberration. — It  is  obvious  that  the  desired  effect  could  be  produced  by 
such  an  increase  of  the  curvature  round  the  centre  of  the  lens,  and  such 
a  diminution  of  the  curvature  towards  its  circumference,  as  would  make 
the  two  foci  coincident.  And  the  requisite  conditions  may  be  theoreti- 
cally fulfilled  by  a  single  lens,  one  of  whose  surfaces,  instead  of  being 
spherical,  is  a  portion  of  an  ellipsoid  or  hyperboloid  of  certain  proportions. 
But  the  difficulties  in  the  way  of  the  mechanical  execution  of  lenses  of 
this  description  are  such,  that  for  practical  purposes  this  plan  of  construc- 
tion is  altogether  unavailable;  besides  which,  their  performance  would 
only  be  perfectly  accurate  for  parallel  rays. 

10.  Various  means  have  been  devised  for  reducing  the  aberration  of 
lenses  of  spherical  curvature.  In  the  first  place,  it  may  be  kept  down  by 
using  ordinary  lenses  in  the  most  advantageous  manner.  Thus  the  aber- 
ration of  a  Plano-convex  lens  whose  convex  side  is  turned  towards  paral- 
lel rays,  is  only  lyy^-ths  of  its  thickness;  whilst,  if  its  plane  side  be  turned 
towards  them,  the  aberration  is  4^  times  the  thickness  of  the  lens. 
Hence,  when  a  plano-convex  lens  is  used  to  form  an  image  by  bringing 
to  a  focus  parallel  or  slightly-diverging  rays  from  a  distant  object,  its 
convex  surface  should  be  turned  towards  the  object;  but,  when  it  is  used 
to  render  parallel  the  rays  which  are  diverging  from  a  very  near  object, 
its  plane  surface  should  be  turned  towards  the  object.  The  single  lens 
having  the  least  spherical  aberration,  is  a  Double-convex  whose  radii  are 
as  one  to  six:  when  the  flattest  face  of  this  is  turned  towards  parallel 
rays,  the  aberration  is  nearly  3^  times  its  thickness;  but  when  its  most 
convex  side  receives  or  transmits  them,  the  aberration  is  only  Ijfo-ths  of 
its  thickness. — Spherical  Aberration  is  further  diminished  by  reducing 


8 


THE  MICROSCOPE  AND  ITS  REVELATIONS. 


the  aperture  or  working-surface  of  the  lens,  so  as  to  employ  only  the  rays 
that  pass  through  its  central  part,  which,  if  sufficiently  small  in  propor- 
tion to  the  whole  sphere,  will  bring  them  all  to  nearly  the  same  focus. 
Such  a  reduction  is  made  in  the  Object-glasses  of  common  (non-achro- 
matic) Microscopes;  in  which,  Avhatever  be  the  size  of  the  lens  itself,  the 
greater  portion  of  its  surface  is  rendered  inoperative  by  a  stop,  which  is  a 
plate  with  a  circular  aperture  interposed  between  the  lens  and  the  rest  of 
the  instrument.  If  this  aperture  be  gradually  enlarged,  it  will  be  seen 
that,  although  the  image  becomes  more  and  more  illuminated,  it  is  at  the 
same  time  becoming  more  and  more  indistinct;  and  that,  in  order  to  gain 
defining  power,  the  aperture  must  be  reduced  again.  Now,  this  reduction 
is  attended  with  two  great  inconveniences:  in  the  first  place,  the  loss  of 
intensity  of  light,  the  degree  of  which  will  depend  upon  the  quantity 
transmitted  by  the  lens,  and  will  vary  therefore  with  its  aperture;  and, 
secondly,  the  diminution  of  the  Angle  of  Aperture,  that  is,  of  the  angle 
ab  c  (Fig.  10)  made  by  the  most  diverging  of  the  rays  of  the  pencil  issu- 
ing from  any  point  of  an  object,  that  can  enter  the  lens  and  take  part  in 
the  formation  of  an  image  of  it;  on  the  extent  of  which  angle  (as  will  be 
shown  hereafter)  depend  some  of  the  most  important  qualities  of  a  Micro- 
scope. 

11.  The  Spherical  Aberration  may  be  approximately  corrected,  how- 
ever, by  making  use  of  combinations  of  lenses,  so  disposed  that  their  op- 
posite aberrations  shall  correct  each  other,  whilst  magnifying  power  is 
still  gained.  For  it  is  easily  seen  that,  as  the  aberration  of  a  concaye 
lens  is  just  the  opposite  of  that  of  a  convex  lens,  the  aberration  of  a  con- 
vex lens  placed  in  its  most  favorable  position  may  be  corrected  by  that  of 
a  concave  lens  of  much  less  power  in  its  most  unfavorable  position;  so 
that,  although  the  power  of  the  convex  lens  is  weakened,  all  the  rays 
which  pass  through  this  combination  will  be  brought  to  one  focus.  It  is 
thus  that  the  Optician  aims  to  correct  the  Spherical  Aberration,  in  the 
construction  of  those  combinations  of  lenses  which  are  now  employed  as 
Object-glasses  in  all  Compound  Microscopes  that  are  of  any  real  value  as 
instruments  of  observation.  But  this  correction  is  not  always  perfectly 
made:  and  the  want  of  it  becomes  evident  in  the  fog  by  which  the  dis- 
tinctness of  the  image,  and  especially  the  sharpness  of  its  outlines,  is  im- 
paired; and  in  the  eidola,  or  false  images,  on  each  side  of  the  best  focal 
point,  which  impair  the  perfection  of  the  principal  image,  and  can  be 
themselves  brought  into  view  when  proper  means  are  used  for  their  de- 
tection.^ The  skill  of  the  best  constructors  of  Microscopic  objectives  has 
been  of  late  years  successfully  exerted  in  the  removal  of  the  '  residual 
errors '  to  which  these  eidola  were  due;  so  that  objectives  of  the  largest 
angular  aperture  are  now  made  truly  aplanatic,  the  corrections  for  Sphe- 
rical Aberration  being  applied  with  a  perfection  which  was  formerly  sup- 
posed to  be  attainable  only  in  the  case  of  Objectives  of  small  or  moderate 
aperture.  Still,  the  difficulty  (and  the  consequent  cost)  of  producing 
such  objectives,  constitutes  one  out  of  many  reasons  for  the  preference 
of  objectives  of  moderate  aperture,  in  which  the  correction  for  Spherical 
Aberration  can  be  easily  made  complete,  for  all  the  ordinary  purposes  of 
scientific  investigation  (§  17). 

12.  But  spherical  aberration  is  not  the  only  difficulty  with  which  the 
Optician  has  to  contend  in  the  construction  of  Microscopes;  for  one 


^  See  Dr.  Royston  Pigott's  description  of  his  Searcher  for  Aplanatic  Images," 
and  its  uses,  in  the  ''Philos.  Transact."  for  1870,  p.  59. 


OPTICAL  PRINCIPLES  OF  THE  MICROSCOPE. 


9 


equally  serious  arises  from  the  unequal  refrangihiUty  of  the  several  Col- 
ored rays  which  together  make  \\^  White  or  colorless  light,  ^  so  that  they 
are  not  all  brought  to  the  same  focus,  even  by  a  lens  free  from  spherical 
aberration.  It  is  this  difference  in  their  ref rangibility,  which  causes  their 
complete  separation  or  '  dispersion '  by  the  Prism  into  a  sioedrum;  and  it 
manifests  itself,  though  in  a  less  degree,  in  the  image  formed  by  a  convex 
lens.  For  if  parallel  rays  of  white  light  fall  upon  a  convex  surface,  the 
most  refrangible  of  its  component  rays,  namely,  the  violet,  will  be  brought 
to  a  focus  at  a  point  somewhat  nearer  to  the  lens  than  the  princi23al  focus, 
which  is  the  meto  of  the  whole;  and  the  converse  will  be  true  of  the  i^ed 
rays,  which  are  the  least  refrangible,  and  whose  focus  will  therefore  be 
more  distant.  Thus  in  Pig.  9,  the  rays  of  white  light,  A  B,  A."  B^',  Avhich 
fall  on  the  peripheral  j)ortion  of  the  lens,  are  so  far  decomposed,  that  the 
violet  rays  are  brought  to  a  focus  a  c,  and  crossing  there,  diverge  again 
and  j)ass  on  towards  p  f,  whilst 

the  red  rays  are  not  brought  to  nG?.y. 
a  focus  until  D,  crossing  the  di- 
vergent violet  rays  at  e  e.  The 
foci  of  the  intermediate  rays  of 
the  spectrum  (indigo,  blue,  green, 
yellow,  and  orange)  are  interme- 
diate between  these  two  extremes. 
The  distance  c  D  between  the  foci 
of  the  violet  and  of  the  red  rays 
respectively,  is  termed  Chromatic 

Aberration.     If  the  image  be  re-       Diagram  illustrating  Chromatic  Aberration. 

ceivod  upon  a  screen  j)laccd  at  c — 

the  focus  of  the  violet  rays — violet  will  predominate  in  its  own  color,  and  it 
will  be  surrounded  by  a  prismatic  fringe  in  which  blue,  green,  yellow, 
orange,  and  red  may  be  successively  distinguished.  If,  on  the  other 
hand,  the  screen  be  placed  at  d — the  focus  of  the  red  rays — the  image 
will  have  a  predominantly  red  tint,  and  will  be  surrounded  by  a  series  of 
colored  fringes  in  inverted  order,  formed  by  the  other  rays  of  the  spec- 
trum Avhich  have  met  and  crossed.^  The  line  e  e,  which  joins  the  points 
of  intersection  between  the  red  and  the  violet  rays,  marks  the  ^  mean 
focus,^  that  is,  the  situation  in  which  the  colored  fringes  w^ll  be  narrow- 
est, the  ^  dispersion '  of  the  colored  rays  being  the  least.  As  the  axial 
ray  a'  b'  undergoes  no  refraction,  neither  does  it  sustain  any  dispersion; 
and  the  nearer  the  rays  are  to  the  axial  ray,  the  less  dispersion  do  they 
suffer.  Again,  the  more  oblique  the  direction  of  the  rays,  whether  they 
pass  through  the  central  or  the  peripheral  portion  of  the  lens,  the  greater 
will  be  the  refraction  they  undergo,  and  the  greater  also  will  be  their 
dispersion;  and  thus  it  happens  that  when,  by  using  only  the  central  part 
of  a  lens  (§  13),  the  chromatic  aberration  is  reduced  to  its  minimum,  the 
central  part  of  a  picture  may  be  tolerably  free  from  false  colors,  whilst 


^  It  has  been  deemed  better  to  adhere  to  the  ordinary  phraseology,  when 
speaking  of  this  fact,  as  more  generally  intelligible  than  the  language  in  which 
it  might  be  more  scientifically  described,  and  at  the  same  time  leading  to  no  prac- 
tical error. 

2  This  experiment  is  best  tried  with  a  lens  of  long  focus,  of  which  the  central 
part  is  covered  with  an  opaque  stop,  so  that  the  light  passes  only  through  a  peri- 
pheral ring;  since,  if  its  whole  aperture  be  in  use,  the  regular  formation  of  the 
fringes  is  interfered  with  by  the  spherical  aberration,  which  gives  a  different 
focus  to  the  rays  passing  through  each  annular  zone. 


10 


THE  MICROSCOPE  AND  ITS  REVELATIONS. 


its  marginal  portion  shall  exhibit  broad  fringes,  as  is  well  seen  in  the  pic- 
tures exhibited  by  non-achromatic  Oxhydrogen-Microscopes. 

13.  Although  the  Chromatic  aberration  of  a  lens,  like  the  Spherical, 
may  be  diminished  by  the  contraction  of  its  aperture,  so  that  only  its 
central  portion  is  employed,  the  error  cannot  be  got  rid  of  entirely  by  any 
such  reduction,  which,  for  the  reasons  already  mentioned,  is  in  itself 
extremely  undesirable.  Hence  it  is  of  the  first  importance  in  the  con- 
struction of  a  really  efficient  Microscope,  that  the  chromatic  aberration 
of  its  Object-glasses  (in  which  the  principal  dispersion  is  liable  to  occur) 
should  be  entirely  corrected,  so  that  a  large  aperture  may  be  given  to 
these  lenses  without  the  production  of  any  false  colors.  No  such  correc- 
tion can  be  accomplished,  even  theoretically,  in  a  single  lens;  but  it  may 
be  effected  by  the  combination  of  two  or  more,  advantage  being  taken  of 
the  different  relations  which  the  refractive  and  the  dispersive  powers 
bear  to  each  other  in  different  substances.  For  if  we  can  unite  with  a 
convex  lens,  whose  dispersive  power  is  loio  as  compared  to  its  refractive 
power,  a  concave  of  lower  curvature,  whose  dispersive  power  is  relatively 
high,  it  is  obvious  that  the  dispersion  of  the  rays  occasioned  by  the  con- 
vex lens  may  be  effectually  neutralized  by  the  opposite  dispersion  of  the 
concave  (§  6);  whilst  the  refracting  power  of  the  convex  is  only  lowered 
by  the  opposite  refraction  of  the  concave,  in  virtue  of  the  longer  focus  of 
the  latter. — No  difficulty  stands  in  the  way  of  carrying  this  theoretical 
correction  into  practice.  For  the  ^  dispersive^ power  of  jiint'g\n>^^  bears 
so  much  larger  a  ratio  to  its  refractive  power  than  does  that  of  crown- 
glass,  that  a  convex  lens  of  the  former  whose  focal  length  is  7f  inches,  will 
produce  the  same  degree  of  color  as  a  convex  lens  of  crown-glass  whose 
focal  length  is  4J  inches.  Hence  a  concave  lens  of  the  former  material  and 
curvature  will  fully  correct  the  dispersion  of  a  convex  lens  of  the  latter; 
whilst  it  diminishes  its  refractive  power  to  such  an  extent  only  as  to  make 
its  focus  10  inches. — A  perfect  correction  for  Chromatic  Aberration  might 
thus  be  obtained,  if  it  were  not  that  although  the  extreme  rays — violet  and 
red — are  thus  brought  to  the  same  focus,  the  dispersion  of  the  rest  is  not 
equally  compensated;  so  that  what  is  termed  a  secondary  spectrum  is 
produced;  the  images  of  objects,  especially  towards  the  margin  of  the 
field,  being  bordered  on  one  side  with  a  purple  fringe,  and  on  the  other 
with  a  green.  In  the  best  constructed  combinations,  however,  whether 
for  the  Telescope  or  the  Microscope,  the  chromatic  error  is  scarcely 
perceptible;  the  aberrations  of  the  objective  being  so  arranged  as  to  be 
almost  entirely  compensated  by  the  opposite  aberrations  of  the  eye-piece 
(§  ^7). 

14.  It  was  in  the  Telescope  that  the  principle  of  correction  for  Chro- 
matic dispersion,  which  had  been  theoretically  devised  by  Euler  and 
other  mathematicians,  was  first  carried  into  practical  application;  an 
Achromatic  object-glass  having  been  constructed  in  1733  by  Hall,  and  a 
more  perfect  combination  having  been  worked  out  in  1757  by  DoUond, 
whose  system,  known  as  the  ^telescopic  triplet,^  remains  in  use  to  the 
present  time.  This  triplet  consists  of  a  double-concave  lens  of  flint-glass, 
interposed  between  two  double-convex  lenses  of  crown;  such  curves  being 
given  to  their  respective  surfaces,  as  serve  almost  entirely  to  extinguish 
not  only  the  Chromatic,  but  the  Spherical  aberration,  in  the  case  of  rays 
preceding  from  distant  objects,  which  fall  on  the  surface  of  the  object- 
glass  in  a  direction  that  is  virtually  These  rays  form  an  image 
in  the  '  principal  focus  '  of  the  object-glass,  the  size  of  which  varies  with 
its  distance  from  the  lens;  magnifying  power  being  thus  gained  by 


OPTICAL  PRINCIPLES  OF  THE  MICROSCOPE. 


11 


lengthening  the  focus  of  the  objective. — In  the  Microscope,  on  the  other 
hand,  the  conditions  are  altogether  different.  For  the  object-glass 
receives  rays  which  diverge  very  widely  from  a  near  object,  and  the  size 
of  the  image  formed  by  their  convergence  depends  upon  the  propor- 
tionate distances  of  the  object  and  the  image  from  the  lens  (§  8);  mag- 
nifying power  being  thus  gained  by  shortening  the  focus  of  the  object- 
glass.  And  the  chromatic  and  spherical  aberrations  resulting  from  the 
incidence  of  diverging  rays  can  only  be  fairly  corrected  by  a  single- 
triplet  combination,  when  its  focus  is  long  (giving  a  low  magnifying 
power),  and  the  divergence  of  those  rays  moderate,  so  that  the  angle  of 
the  aperture  is  small. 

15.  It  has  only  been  in  comparatively  recent  times  that  the  construc- 
tion of  Achromatic  object-glasses  for  Microscopes  has  been  found  prac- 
ticable; their  extremely  minute  size  having  been  thought  to  forbid  the 
attainment  of  that  accuracy  which  is  necessary  in  the  adjustment  of  the 
several  curvatures,  in  order  that  the  errors  of  each  of  the  separate  lenses 
which  enters  into  the  combination,  may  be  effectually  balanced  by  the 
opposite  errors  of  the  rest.    The  first  successful  attempt  was  made  in 


a  6  c,  its  Angle  of  Aperture. 

this  direction  in  the  year  1823  by  MM.  Selligues  and  Chevalier,  of  Paris; 
the  plan  which  they  adopted  being  the  combination  of  two  or  vaoY^jpairs 
of  lenses,  each  pair  consisting  of  a  double-convex  of  crown-glass  and  a 
plano-concave  of  flint. — In  the  following  year,  Mr.  Tulley,  of  London, 
without  any  knowledge  of  what  had  been  accomplished  in  Paris,  applied 
himself  (at  the  suggestion  of  Dr.  Goring)  to  the  construction  of  Achro- 
matic object-glasses  for  the  Microscope:  and  succeeded  in  producing  a 
single  combination  of  three  lenses,  on  the  telescopic  plan,  the  corrections 
of  which  were  extremely  complete.  This  combination,  however,  was  not 
of  high  power,  nor  of  large  angular  aperture;  and  it  was  found  that 
these  advantages  could  not  be  gained  without  the  addition  of  a  second 
combination. — Prof.  Amici  at  Modena,  also,  who  had  attempted  the 
construction  of  microscopic  object-glasses  as  early  as  1813,  but,  despair- 
ing of  success,  had  turned  his  attention  to  the  application  of  the  reflect- 


fi2  THE  MICROSCOPE  AND  ITS  REVELATIONS. 


PLATE  I. 

2  4 


1  5 

VARIOUS  FORMS  OF  DIATOMACE^. 

Fig.  1.  Actinocyclus  Rolf  sit. 

2.  Asterolampra  concinna. 

3.  HeHopelta  (as  seen  with  black-ground  illumination), 

4.  Asteromphalus  Brookeii. 

5.  Aulacodiscus  Or  eg  anus. 


OPTICAL  PRINCIPLES  OF  THE  MICROSCOPE. 


la 


Plate  IT 


ECHINUS-SPINE  (Original),  and  podura-scale  (after  R.  Beck). 
Fig.  1.  Transverse  section  of  Spine  of  Echinometra  heteropora. 

2.  Markings  on  Scale  of  Fodura,  as  seen  by  transmitted  light  under  a  well-corrected  l-8th  inch 
Objective. 

3.  Partial  obliteration  of  the  markings  by  the  insinuation  of  moisture  between  the  Scale  and  the 
Covering-glass. 

4.  Appearance  of  the  markings,  when  the  Scale  is  illuminated  from  above  the  oblique  light 
falling  at  right  angles  to  them. 

5.  The  same,  when  the  light  falls  on  the  Scale  in  the  direction  of  the  markings. 


14 


THE  MICROSCOPE  AND  ITS  REVELATIONS. 


ing  principle  to  the  Microscope,  resumed  his  original  labors  on  hearing 
of  the  success  of  MM.  Selligues  and  Chevalier;  and,  by  working  on  their 
plan,  he  produced,  in  1827,  an  achromatic  combination  which  surpassed 
anything  of  the  same  kind  that  had  been  previously  executed.  And 
these  were  soon  rivalled  by  the  objectives  produced  in  London  by  Andrew 
Eoss  and  Powell. 

16.  It  was  in  this  country  that  the  next  important  improvements 
originated;  these  being  the  result  of  the  theoretical  investigations  of  Mr. 
J,  J.  Lister,.^  which  led  him  to  the  discovery  of  certain  properties  in 
Achromatic  combinations  that  had  not  been  previously  detected.  Under 
his  guidance,  Mr.  James  Smith,  soon  followed  by  other  Opticians, 
succeeded  in  producing  combinations  far  superior  to  any  which  had  been 
previously  executed,  both  in  extent  of  aperture,  flatness  of  field,  and 
completeness  of  correction;  and  continued  progress  has  been  since  made 
in  the  same  direction  by  the  like  combination  of  theoretical  acumen  with 
manipulative  skill. 

17.  The  enlargement  of  the  Angle  of  Aperture,  and  the  greater  com- 
pleteness of  the  corrections,  first  obtained  by  the  adoption  of  Mr.  Lister's 
princi23les,  soon  rendered  sensible  an  imperfection  in  the  performance  of 
these  lenses  under  certain  circumstances,  which  had  previously  passed 
unnoticed;  and  the  important  discovery  was  made  by  Mr.  A.  Koss  that 
a  very  obvious  difference  exists  in  the  precision  of  the  image,  according 
as  the  object  is  viewed,  with  or  without  a  covering  of  talc  or  thin  glass; 
an  Object-glass  which  is  perfectly  adapted  to  either  of  these  conditions, 
being  sensibly  defective  under  the  other.  The  mode  in  which  this  differ- 
ence arises  is  explained  by  Mr.  Ross^  as  follows: — Let  o  (Fig.  11)  be  any 
point  of  an  object;  o  p  the  axial  ray  of  the  pencil  that  diverges  from  it: 
and  o  T,  0  t',  two  diverging  rays,  the  one  near  to,  the  other  remote  from, 
the  axial  ray.  Now  if  a  G  G  G  represent  the  section  of  a  piece  of  thin 
glass  intervening  between  the  object  and  the  object-glass,  the  rays  o  t 
and  o  t'  will  be  refracted  in  their  passage  through  it,  in  the  directions 
t  r,  t'  r';  and  on  emerging  from  it  again,  they  will  pass  on  towards  E 
and  e'.  Now  if  the  course  of  these  emergent  rays  be  traced  backwards, 
as  by  the  dotted  lines,  the  ray  e  r  will  seem  to  have  issued  from  x,  and 
the  ray  e'  r'  from  Y;  and  the  difference  x  Y,  which  is  called  '  negative 
aberration,'  is  quite  sufficient  to  disturb  the  previous  balance  of  the 
aberrations  of  the  composite  lens  of  the  object-glass.  The  requisite  cor- 
rection may  be  effected,  as  Mr.  Eoss  pointed  out,  by  giving  to  the  front 
pair  (Fig.  10,  1)  of  the  three  of  which  the  Objective  is  composed,  an 
excess  of  ^positive  aberration'  (^.  e,,  by  under-correcting  it,  and  by 
giving  to  the  other  two  pairs  (2,  3)  an  excess  of  '  negative  aberration ' 
(i.  e.,  by  over-correcting  them),  and  by  making  the  distance  between  the 
former  and  the  latter  susceptible  of  alteration  by  means  of  a  screw  collar 
(§  140).  For  when  the  front  pair  is  approximated  most  nearly  to  the 
other  two,  and  its  distance  from  the  object  is  increased,  its  positive 
aberration  is  more  strongly  exerted  upon  the  other  pairs  than  it  is  when 
the  distance  between  the  lenses  is  increased,  and  the  distance  between 
the  front  pair  and  the  object  is  diminished.  Consequently,  if  the  lenses 
have  been  so  adjusted  that  their  correction  is  perfect  for  an  uncovered 
object,  the  approximation  of  the  front  lens  to  the  others  will  give  to  the 
whole  combination  an  excess  of  positive  aberration,  which  will  neutralize 


^See  his  Memoir  in  the    Philosophical  Transactions'*  for  1829. 
Transactions  of  the  Society  of  Arts,"  vol.  li. 


OPTICAL  PRINCIPLES  OF  THE  MICROSCOPE. 


15 


the  negative  aberration  occasioned  by  covering  the  object  with  a  thin 
plate  of  glass. — This  correction  will  obviously  be  more  important  to  the 
perfect  performance  of  the  combination,  the  larger  is  its  angle  of 
aperture;  since  the  wider  the  divergence  of  the  oblique  rays  from  the 
axial  ray,  the  greater  will  be  the  refraction  which  they  will  sustain  in 
passing  through  a  plate  of  glass,  and  the  greater  therefore  will  be  the 
negative  aberration  produced,  which,  if  uncorrected,  will  seriously 
impair  the  distinctness  of  the  image.  It  is  consequently  not  required  for 
loio  powers  whose  angle  of  aperture  is  comparatively  small,  nor  for 
medium  powers,  so  long  as  their  angle  of  aperture  does  not  exceed  50°, 
and  even  objectives  of  \  of  an  inch  focus,  whose  angle  of  aperture  does 
not  exceed  75",  may  be  made  to  perform  very  well  without  adjustment, 
if  their  corrections  be  originally  made  perfect  for  the  average  thickness 
of  glass  used  to  cover  objects  of  the  finer  kind.  And  objectives  of  much 
higher  power  and  larger  angle  of  aperture  (especially  suited  for  Students' 
Microscopes),  are  now  constructed  so  as  to  work  admirably  without 
adjustment,  being  corrected  for  a  standard  thickness — such  as  0.008  or 
0. 006  inch — of  the  glass  covers  supplied  by  their  makers.  Such  non-adjust- 
ing objectives,  when  less  than  \  inch  focus,  are  best  constructed  on  the 
immersion'  system  (§  19). 

18.  For  many  years  the  best  Microscopic  objectives  of  moderate  and 
high  magnifying  power  were  made  by  combining  three  superposed  pairs  of 
increasing  focus  and  diameter  (as  in  Fig.  10),  each  consisting  of  a  double- 
convex  lens  of  crown-glass  partly  achromatized  by  its  own  concave  of 
flint;  the  two  apposed  surfaces  of  each  pair  being  of  the  same  curvature, 
and  cemented  together  by  Canada  balsam.  Various  modifications  of  this 
arrangement,  however,  have  been  introduced  at  various  times  and  by 
various  constructors;  some  proceeding  in  the  direction  of  simplification, 
whilst  others  have  aimed  at  the  greatest  attainable  perfection,  irrespec- 
tive of  complexity  and  constructive  difficulty.  It  is  obvious  that  there 
are  great  practical  advantages  on  the  side  of  any  reduction  in  the  number 
of  component  lenses,  that  is  compatible  with  the  good  performance  of  the 
combination:  liability  to  error,  as  well  in  the  curved  surfaces,  as  in  the 
centering  and  setting  of  each,  l3eing  thereby  diminished,  while  there  is  a 
like  diminution  in  the  loss  of  light  which  occurs  whenever  the  rays  pass 
out  of  one  medium  into  another  (§1).  But,  on  the  other  hand,  it  seems 
certain  that  the  highest  theoretical  perfection  can  be  attained  by  an 
increase  in  the  number  of  component  lenses;  so  that,  if  the  errors  in  work- 
manship are  kept  down  to  the  lowest  possible  point,  the  performance  of 
such  complex  combinations  may  be  made  superior  to  that  of  simpler 
ones. — The  first  important  change  in  the  direction  of  simplification  con- 
sisted in  the  replacement  of  the  fro7if  combination  by  a  single  plano-con- 
vex of  crown.  This  substitution,  which  seems  to  have  been  first  devised 
by  Amici,  has  been  very  generally  adopted;  a  greater  working  distance 
from  the  object  (which  is  very  important  in  the  case  of  the  highest  pow- 
ers) being  attainable  in  this  construction,  than  when  the  front  is  either  a 
doublet  or  a  triplet  combination.  But  most  makers  who  have  used  this 
method  have  added  a  lens  to  the  back  combination,  making  it  a  ^  tele- 
scopic trij)let,'  still  using  a  doublet  in  the  middle;  and  admirable  objec- 
tives on  this  construction  (each  consisting  of  two  flint  concave  and  four 
convex  lenses  of  crown,  with  twelve  surfaces  in  all)  have  been  made  by  the 
best  Opticians — English  and  American,  French  and  German. — A  further 
simplification  has  been  recently  carried  into  effect  by  Mr.  Wenham;  who 


16 


THE  MICROSCOPE  AND  ITS  REVELATIONS. 


has  shown  ^  that  the  whole  color-correction  may  be  effected  in  the  mid- 
dle lens  by  a  double-concave  of  dense  flint  between  two  convex  lenses  of 
crown,  the  back  lens  as  well  as  the  front  being  a  single  plano-convex  of 
crown.  Thus  one  double  concave  lens  of  flint  is  made  to  correct  the 
chromatic  aberrations  of  four  convex  surfaces  of  crown,  the  total  number 
of  surfaces  being  reduced  to  ten.  There  is  a  further  advantage  in  this 
plan  of  construction,  that  no  change  of  the  front  lens  is  needed  to  enable 
the  combination  to  be  used  as  an  '  immersion '  objective  (§  19),  the  requi- 
site adjustment  being  effected  by  the  screw-collar  used  for  cover-correc- 
tion.— There  can  be  no  doubt  that  objectives  of  moderate  angular  aperture 
may  be  made  on  Mr.  Wenham's  system,  so  as  to  combine  great  excellence 
with  comparative  cheapness;  but  it  does  not  seem  equally  suitable  for 
first-class  objectives,  requiring  for  their  greatest  efficiency  the  widest 
attainable  angular  aperture.  These  have  usually  been  made  to  consist  of 
a  front  triplet,  a  middle  doublet,  and  a  back  triplet,  thus  having  eight 
lenses  in  all,  with  sixtee7i  surfaces.  But  the  first-class  constructors  in 
the  United  States  (notably  Messrs.  Tolles,  Spencer,  and  Wales)  have 
added  to  these  a  single  front  plano-convex  of  crown,  by  means  of  which 
a  longer  working  distance  has  been  obtained;  whilst  the  extraordinary 
excellence  of  their  workmanship  (only 'attain  able,  however,  at  a  very  high 
cost)  has  given  to  these  very  complex  combinations  a  perfection  of  per- 
formance, which,  to  say  the  least,  is  unsurpassed  by  that  of  any  objectives 
constructed  for  use  in  the  ordinary  manner,  which  is  now  distinguished 
as  dry. 

19.  It  was  long  since  pointed  out  by  Amici  that  the  introduction  of  a 
drop  of  water  between  the  front  surface  of  the  objective,  and  either  the 
object  itself  or  its  covering-glass,  would  diminish  the  loss  of  light  result- 
ing from  the  passage  of  the  rays  from  the  object  or  its  covering-glass  into 
air,  and  then  from  air  into  the  object-glass.  But  it  is  obvious  that  when 
the  rays  enter  the  object-glass  from  water,  instead  of  from  air,  both  its 
refractive  and  its  dispersive  action  will  be  greatly  changed,  so  as  to  need 
an  important  constructive  modification  to  suit  the  new  condition.  This 
modification  seems  never  to  have  been  successfully  effected  by  Amici 
himself;  and  his  idea  remained  unfruitful  until  it  was  taken  up  by  Hart- 
nack  and  Nachet,  who  showed  that  the  application  of  what  is  now  known 
as  the  Immersion-system  to  objectives  of  high  power  and  large  angular 
aperture  is  attended  with  many  advantages  not  otherwise  attainable. 
For,  as  already  pointed  out  (§  1),  the  loss  of  light  increases  with  the 
obliquity  of  the  incident  rays;  so  that  when  objectives  of  very  wide  angle 
of  aperture  are  used  '  dry,^  the  advantages  of  its  increase  are  in  great 
degree  nullified  by  the  reflection  of  a  large  proportion  of  the  rays  falling 
very  obliqely  upon  the  peripheral  portion  of  the  front  lens.  When,  on 
the  other  hand,  rays  of  the  same  obliquity  enter  the  peripheral  portion  of 
the  lens  from  water,  the  loss  by  reflection  is  greatly  reduced,  and  the  ben- 
efit derivable  from  the  large  aperture  is  proportionally  augmented. 
Again,  the  ^immersion  system'  allows  of  a  greater  working  distance 
between  the  objective  and  the  object,  than  is  otherwise  attainable  with 
the  same  extent  of  angular  aperture;  and  this  is  a  great  advantage,  not 
merely  in  regard  to  convenience  in  manipulation,  but  also  in  giving  a 
greater  range  of  ^  penetration '  or  ^  focal  depth.'  Further,  the  observer 
is  rendered  less  dependent  upon  the  exactness  in  the  correction  for  the 
thickness  of  the  covering-glass,  which  is  needed  where  objectives  of  large 


^    Proceedings  of  Eoyal  Society,"  Vol.  xxi.,  p.  111. 


OPTICAL  PRINCIPLES  OF  THE  MICROSCOPE, 


17 


angle  are  used  ^  dry;'  for  as  the  amount  of  ^  negative  aberration '  (§  17) 
is  far  smaller  when  the  rays  which  emerge  from  the  covering-glass  pass 
into  water  than  when  they  pass  into  air,  variations  in  its  thickness  pro- 
dace  a  much  less  disturbing  effect.  And  thus  it  is  found  practically  that 
Mmmersion  '  objectives  can  be  constructed  with  magnifying  powers  suffi- 
ciently high,  and  angular  apertures  sufficiently  large,  for  all  the  ordinary 
purposes  of  scientific  investigation,  without  any  necessity  for  cover- 
adjustment;  being  originally  adapted  to  give  the  best  results  with  a 
covering-glass  of  suitable  thinness,  and  small  departures  from  this  in 
either  direction  occasioning  very  little  deterioration  in  their  performance. 
For  ^water-immersion^  objectives  of  the  very  largest  aperture,  however, 
to  be  used  upon  the  most  difficult  objects,  exact  cover-correction  is  still 
necessary. — Whilst  immersion '-objectives  constructed  on  the  original 
plan  can  only  be  employed  ^  wet '(that  is  with  the  interposition  of 
water),  Messrs.  Powell  and  Leland — followed  by  other  makers — have  so 
arranged  their  combinations,  that  by  a  change  in  the  front  lens  they  may 
be  used  ^dry,^  as  m  the  ordinary  manner.  And  in  Mr.  Wenham's  sys- 
tem not  even  this  change  is  required,  the  change  from  ^  web' to  Mry,' 
^r\di  vice  versa,  being  accomplished  by  an  alteration  in  the  distance  of  the 
front  lens  from  the  middle  triplet,  made  by  the  screw-collar,  as  in  ordi- 
nary cover-correction. 

20.  The  immersion  system 'has  recently  undergone  a  still  further 
development,  by  the  practical  application  of  a  method  originally  sug- 
gested by  Mr.  Wenham^  (but  never  carried  by  him  into  operation),  and 
iudependently  suggested  by  Mr.  Stephenson^  to  Prof.  Abbe  of  Jena, 
under  whose  scientific  direction  it  has  been  worked  out  by  the  very  able 
German  optician,  Zeiss,  with  complete  success.  This  method  consists  in 
the  replacement  of  the  water  previously  interposed  between  the  covering- 
glass  and  the  front  surface  of  the  objective,  by  a  liquid  having  the  same 
refractive  and  dispersive  power  as  crown-glass;  so  that  the  rays  issuing  at 
any  angle  from  the  upper  plane  surface  of  the  covering-glass,  shall  enter 
the  plane  front  of  the  objective  without  any  change  either  by  refraction 
or  dispersion,  and  without  any  sensible  loss  by  reflection — even  the  most 
oblique  rays  proceeding  in  their  undefiected  course,  until  they  meet  the 
convex  back  surface  of  the  front  lens.  It  is  obvious  that  all  the  advan- 
tages derivable  from  the  system  of  water  immersion  are  obtainable  with 
still  greater  completeness  by  this  system  of  Jiomogeneoiis  immersion,  pro- 
vided that  a  fluid  can  be  found  which  meets  its  requirements.  After  a 
long  course  of  experiments.  Prof.  Abbe  found  that  oil  of  cedar- wood  so 
nearly  corresponds  with  glass,  alike  in  refractive  and  in  dispersive  power, 
that  it  serves  the  purpose  extremely  well,  except  when  it  is  desired  to 
take  special  advantage  of  the  most  divergent  or  marginal  rays,  oil  of  fen- 
nel being  then  preferable.  Objectives  of  yV^^^^  yV'^^^  mch  focal 
length  have  been  constructed  on  this  plan  by  Zeiss;  and  it  appears  cer- 
tain that  by  its  means  a  larger  angle  of  aperture  can  be  effectively  ob- 
tained, than  on  any  other  construction.  Whether  any  tests  can  be  re- 
solved by  its  use,  on  which  other  objectives  fail,  is  a  point  not  yet 
satisfactorily  determined.  But  there  can  be  no  doubt  that  the  system  of 
'homogeneous  immersion'  will  greatly  facilitate  the  use  of  objectives  pos- 
sessing the  largest  angular  aperture,  and  capable  of  affording  the  highest 
magnifying  power,  for  the  ordinary  purpose  of  scientific  research,    it  is 


J  Monthly  Microscopical  Journal,"  Vol.  iii.  (1870),  p,  303. 
2    Journ.  of  Royal  Microsc.  Society,"  Vol.  i.  (1878),  p.  51. 

o 


18 


THE  MICROSCOPE  AND  ITS  REVELATIONS. 


precisely  in  the  case  of  such  objectives  that  the  '  cover-correction  ^  needs 
to  be  most  exact.  And  although  the  practised  microscopist  lias  no  diffi- 
culty in  making  this,  when  the  object  at  which  he  is  looking  (such  as  a 
Diatom,  a  Podura-scale,  or  a  band  of  Nobert's  ruled  lines)  is  Icnown  to 
him,  yet  the  case  is  entirely  different  when  the  object  is  altogether 
unhnoion.  For  in  examining  such  an  object,  he  may  be  able  only  to  sat- 
isfy himself  after  repeated  trials,  involving  much  expenditure  of  time 
and  patience,  as  to  the  cover-correction  which  gives  the  truest  represen- 
tation of  the  object;  whilst,  in  using  a  Miomogeneous '  or  ^oil-immer- 
sion ^  objective,  he  is  able  to  feel  an  absolute  certainty  that,  without  any 
adjustment  at  all,  the  view  which  he  gains  of  an  unknown  object  is  in 
every  respect  at  least  equal  to  that  which  he  can  obtain  from  the  best 
'  dry '  or  '  water-immersion  ^  objective,  most  exactly  adjusted-  for  thick- 
ness of  cover. — This  system  has  been  taken  up  also  by  Messrs.  Powell  and 
Lealand,  who  liave  constructed  admirable  ^oil-immersion'  objectives 
ranging  to  l-25th  inch  focus,  which,  by  a  change  of  the  front  lens,  may 
also  be  used  '  dry.' 

21.  We  are  now  prepared  to  enter  upon  the  application  of  the  Optical 
principles  which  have  been  explained  and  illustrated  in  the  foregoing 
pages,  to  the  construction  of  Microscopes.  These  are  distinguished  as 
Simple  and  Compound;  each  kind  having  its  peculiar  advantages  to  the 
Student  of  Nature.  Their  essential  difference  consists  in  this: — that  in 
the  former,  the  rays  of  light  which  enter  the  eye  of  the  observer  proceed 
directly  from  the  object  itself,  after  having  been  subjected  only  to  a 
change  in  their  course;  whilst  in  the  latter,  an  enlarged  image  of  the 
object  is  formed  by  one  lens,  which  image  is  magnified  to  the  observer  by 
another,  as  if  lie  were  viewing  the  object  itself. — The  Simple  Microscope 
may  consist  of  a  single  lens;  but  (as  will  be  presently  shown)  it  may  be 
formed  of  two,  or  even  three:  these,  however,  being  so  disposed  as  to  pro 
duce  an  action  upon  the  rays  of  light  corresponding  to  that  of  a  single 
lens.  In  the  Compound  Microscope,  on  the  other  hand,  not  less  than 
two  lenses  must  be  employed:  one,  to  form  the  enlarged  image  of  the  ob- 
ject, immediately  over  which  it  is  placed,  and  hence  called  the  object- 
glass;  whilst  the  other  again  magnifies  that  image,  and,  being  interposed 
between  it  and  the  eye  of  the  observer,  is  called  the  eye-glass.  A  perfect 
Object-glass,  as  we  have  seen,  must  consist  of  a  combination  of  lenses; 
and  the  eye-glass  is  best  combined  with  another  lens  interposed  between 
itself  and  the  object-glass,  the  two  together  forming  what  is  termed  an 
eye-piece  (§  27). — These  two  kinds  of  instrument  need  to  be  separately 
considered  in  detail. 

2.  Simple  Microscope. 

22.  In  order  to  gain  a  clear  notion  of  the  mode  in  which  a  Single 
Lens  serves  to  ^magnify'  minute  objects,  it  is  necessary  to  revert  to  the 
phenomena  of  ordinary  Vision.  An  Eye  free  from  any  defect  has  a  con- 
siderable power  of  adjusting  itself  in  such  a  manner  as  to  gain  a  dis- 
tinct view  of  objects  placed  at  extremely  varying  distances;  but  the 
image  formed  upon  the  retina  will  of  course  vary  in  size  with  the  dis- 
tance of  the  object;  and  the  amount  of  detail  perceptible  in  it  will 
follow  the  same  proportion.  To  ordinary  eyes,  however,  there  is  a 
limit  within  which  no  distinct  image  can  be  formed,  on  account  of  the 
too  great  divergence  of  the  rays  of  the  different  pencils  which  then  enter 
the  eye;  since  the  eye  is  usually  adapted  to  receive,  and  to  bring  to  a  focus, 
rays  which  are  parallel  or  but  slightly  divergent.    This  limit  is  vari- 


OPTICAL  PRINCIPLES  OF  THE  MICROSCOPE. 


19 


ous]y  stated  at  from  5  to  10  inches;  but  though  there  are  doubtless 
many  persons  whose  vision  is  good  at  the  shorter  range,  yet  the  longer 
is  probably  the  real  limit  for  persons  of  ordinary  vision:  who,  though 
they  may  see  an  object  much  nearer  the  eye,  discern  little  if  any  more 
of  its  details,  what  is  gained  in  sizie  being  lost  in  distinctness.  Now  the 
utility  of  a  convex  lens  interposed  between  a  near  object  and  the  eye, 
consists  in  its  reducing  the  divergence  of  the  rays  forming  the  several 
pencils  which  issue  from  it;  so  that  they  enter  the  eye  in  a  state  of 
moderate  divergence,  as  if  they  had  issued  from  an  object  beyond  the 
nearest  limit  of  distinct  vision,  a  well-defined  picture  being  thus  formed 
upon  the  retina.  Not  only,  however,  is  the  course  of  the  several  rays 
in  each  pencil  altered  as  regards  the  rest,  but  the  course  of  the  pencils 
themselves  is  changed,  so  that  they  enter  the  eye  under  an  angle  corre- 
sponding with  that  under  which  they  would  have  arrived  from  a  larger 
object  situated  at  a  greater  distance;  and  thus  the  picture  formed  upon 
the  retina  by  any  object  {a  5,  Fig.  12),  corresponds  in  all  respects  with 
one  which  would  have  been  made  by  the  same  object  increased  in  its  di- 
mentions  to  A  b,  and  viewed  at  the  smallest  ordinary  distance  of  distinct 
vision.  A  ^short-sighted^  person,  however,  who  can  only  see  objects  dis- 
tinctly at  a  distance  of  two  or  three  inches,  has  the  same  power  in  his 
eye  alone  by  reason  of  its  great  convexity,  as  that  which  the  person  of 
ordinary  vision  gains  by  the  assistance  of  a  convex  lens  which  shall  enable 
him  to  see  at  the  same  distance  with  equal  distinctness.  It  is  evident, 
therefore,  that  the  magnifying  power  of  a  single  lens,  depending  as  it 
does  upon  the  proportion  between  the  distance  at  which  it  renders  the 
object  visible,  and  the  nearest  distance  of  unaided  distinct  vision,  must 
be  different  to  different  eyes.  It  is  usually  estimated,  however,  by  find- 
ing how  many  times  the  focal 

length  of  the  lens  is  contained  r^'-^^^-^^r 
in  ten  inches;  since,  in  order  to 
render  the  rays  from  the  object  j 
nearly  parallel,   it    must  be 
placed  nearly  in  the  focus  of  | 
the  lens  (Fig.  3);  and  the  pic- 
ture is  referred  by  the  mind  to  j 
an  object  at  the  ordinary  dis- 
tance.    Thus,    if   the  focal 
length  of  a  lens  be  one  inch,  its 
magnifying  power  for  each  di- 
mension will  be  10  times,  and 
consequently   100  superficial; 
while  if  its  focal  distance  be 
only  one-tenth  of  an  inch,  its 
magnifying  power  will  be  100  linear,  or  10,000  superficial. 

23.  But  the  shorter  the  focus  of  the  magnifying  lens,  the  smaller 
must  be  the  diameter  of  the  sphere  of  which  it  forms  part;  and,  unless 
its  aperture  be  proportionateljr  reduced,  the  distinctness  of  the  image ; 
will  be  destroyed  by  the  spherical  and  chromatic  aberrations  (§§  9,  12)  ] 
necessarily  resulting  from  its  high  curvature.  Yet  notwithstanding  the 
loss  of  light  and  other  drawbacks  attendant  on  the  use  of  Single  Lenses 
of  high  power,  they  proved  of  great  value  to  the  older  Microscopists 
(among  whom  Leeuwenhoek  should  be  specially  named),  on  account  of 
their  freedom  from  the  errors  to  which  the  Compound  Microscope  of 
the  old  construction  was  necessarily  subject;  and  the  amount  of  excellent 


Diagram  illustrating  the  action  of  the  Simple  MicrO' 
scope  ;  ab  object;  a  b  its  magnified  image. 


/ 


20  THE  MICROSCOPE  AND  ITS  REVELATIONS.  , 

work  done  by  means  of  them  surprises  every  one  who  studies  the  his- 
tory of  Microscopic  inquiry. — An  important  improvement  on  the  single 
lens  was  introduced  by  Dr.  Wollaston,  who  devised  the  doublet,  still 
known  by  his  name;  which  consists  of  two  plano-convex  lenses,  whose 
focal  lengths  are  in  the  proportion  of  one  to  three,  or  nearly  so,  having 
their  convex  sides  directed  towards  the  eye,  and  the  lens  of  shortest  focal 
length  nearest  the  object.  In  Dr.  W.'s  original  combination,  no  perfo- 
rated diaphragm  (or  ^stop')  was  interposed;  and  the  distance  between 
the  lenses  was  left  to  be  determined  by  experiment  in  each  case.  A 
great  improvement  was  subsequently  made,  however,  by  the  introduction 
of  a  '  stop '  between  the  lenses  and  by  the  division  of  the  power  of  the 
smaller  lens  between  two  (especially  when  a  very  short  focus  is  required),  so 
as  to  form  a  U^iplet,  as  first  suggested  by  Mr.  Holland.^  When  combina- 
tions of  this  kind  are  well  constructed,  both  the  spherical  and  the  chro- 
matic aberrations  are  so  much  reduced,  that  the  angle  of  aperature  may 
be  considerably  enlarged  without  much  sacrifice  of  distinctness;  and 
hence  for  all  save  very  low  powers,  such  ^  doublets '  and  '  triplets '  are 
far  superior  to  single  lenses.  These  combinations  took  the  place  of  sin- 
gle lenses,  among  Microscopists  (in  this  country  at  least)  who  were  pro- 
secuting minute  investigations  in  Anatomy  and  Physiology  prior  to  the 
vast  improvements  effected  in  the  Compound  Microscope  by  the  achro- 
matization  of  its  object-glasses  (§  15);  and,  in  particular,  the  admirable 
researches  of  Dr.  Sharpey,^  on  ciliary  action  in  Animals  (1830-35),  and 
Mr.  Henry  Slack's  beautiful  dissections  of  the  elementary  tissues  of 
Plants,  and  also  his  excellent  observations  on  Vegetable  cyclosis  (1831),^ 
were  made  by  their  means. — The  performance  of  even  the  best  of  these 
forms  of  Simple  microscope,  however,  is  so  far  inferior  to  that  of  a  good 
Compound  microscope,  as  now  constructed,  that  no  one  who  has  the 
command  of  the  latter  form  of  instrument  would  ever  use  the  higher 
powers  of  the  former.  And  as  it  is  for  the  prosecution  of  observations, 
and  for  the  carrying  on  of  dissections,  which  only  require  lotv  powers, 
that  the  Simple  microscope  is  chiefly  needed,  the  WoUaston  doublet  has 
now  almost  gone  out  of  use. 

24.  Another  form  of  Simple  magnifier,  possessing  certain  advantages 
over  the  ordinary  double-convex  lens,  is  that  commonly  known  by  the 
name  of  the  '  Coddington  '  lens.^  The  first  idea  of  it  was  given  by  Dr. 
Wollaston,  who  proposed  to  apply  two  plano-convex  or  hemispherical 
lenses  by  their  plane  sides,  with  a  ^stop'  interposed,  the  central  aperture 
of  which  should  be  equal  to  one-fifth  of  the  focal  length.  The  great  ad- 
vantage of  such  a  lens  is,  that  the  oblique  pencils  pass,  like  the  central 
ones,  at  right  angles  to  the  surface,  so  that  they  are  but  little  subject  to 
aberration.  The  idea  was  further  improved  upon  by  Sir  D.  Brewster, 
who  pointed  out  that  the  same  end  would  be  much  better  answered  by 
taking  a  sphere  of  glass,  and  grinding  a  deep  groove  in  its  equatoriarl 
part,  which  should  be  then  filled  with  opaque  matter,  so  as  to  limit  the 
central  aperture.    Such  a  lens  gives  a  large  field  of  view,  admits  a  con- 


^    Transactions  of  the  Society  of  Arts,"  Vol.  xlix. 

2  See  his  article  Cilia  in  the  Cyclopaedia  of  Anatomy  and  Physiology,"  and 
the  references  under  that  head  in  the  Index  to  the  present  work. 

2  See  his  Memoir,  with  two  beautiful  Plates,  in  the  ' '  Transactions  of  the  Soci- 
ety of  Arts,"  Vol.  xhx.,  pp.  0,  7. 

^  This  name,  however,  is  most  inappropriate;  since  Mr.  Coddington  neither 
was,  nor  ever  claimed  to  be,  the  inventor  of  the  mode  of  construction  by  which 
this  lens  is  distinguished. 


OPTICAL  PRINCIPLES  OF  THE  MICROSCOPE. 


21 


siderable  amount  of  light,  and  is  equally  good  in  all  directions;  but  its 
power  of  definition  is  by  no  means  equal  to  that  of  an  achromatic  lens,  or 
even  of  a  doublet.  This  form  is  chiefly  useful,  therefore,  as  a  Hand- 
magnifier,  in  which  neither  hign  power  nor  perfect  definition  is  required; 
its  peculiar  qualities  rendering  it  superior  to  an  ordinary  lens,  for  the 
class  of  objects  for  which  a  hand-magnifier  of  medium  power  is  required. 
Many  of  the  magnifiers  sold  as  ^  Coddington '  lenses,  however,  are  not 
really  portions  of  spheres,  but  are  manufactured  out  of  ordinary  double- 
convex  lenses,  and  are  therefore  destitute  of  the  special  advantages  of  the 
real  ^  Coddington.' — The  ^  Stanhope^  lens  somewhat  resembles  the  pre- 
ceding in  appearance,  but  differs  from  it  essentially  in  properties.  It  is 
nothing  more  than  a  double-convex  lens,  having  two  surfaces  of  unequal 
curvatures,  separated  from  each  other  by  a  considerable  thickness  of 
glass;  the  distances  of  the  two  surfaces  from  each  other  being  so  adjusted, 
that  when  the  most  convex  is  turned  towards  the  eye,  minute  objects 
placed  on  the  other  surface  shall  be  in  the  focus  of  the  lens.  This  is  an 
easy  mode  of  applying  a  rather  high  magnifying  power  to  scales  of  but- 
terflies' wings,  and  the  other  similar  flat  and  minute  objects,  which  will 
readily  adhere  to  the  surface  of  glass;  and  it  also  serves  to  detect  the 
presence  of  the  larger  animalcules  or  of  crystals  in  minute  drops  of 
fluid,  to  exhibit  the  ^ eels'  in  paste  or  vinegar,  etc.,  etc. — A  modified 
form  of  the  ^Stanhope'  lens,  in  which  the  surface  remote  from  the  eye 
is  plane  instead  of  convex,  has  been  brought  out  in  France  under  the 
name  of  '  Stanhoscope,'  and  has  been  especially  applied  to  the  enlarge- 
ment of  minute  pictures  photographed  on  its  plane  surface  in  the  focus 
of  its  convex  surface.  A  good  '  Stanhoscope,'  magnifying  from  100  to 
150  diameters,  is  a  very  convenient  form  of  hand-magnifier  for  the  recog- 
nition of  Diatoms,  Infusoria,  etc. ;  all  that  is  required  being  to  place  a 
minute  drop  of  the  liquid  to  be  examined  on  the  plane  surface  of  the  lens, 
and  then  to  hold  it  up  to  the  light.  ^ 

25.  For  the  ordinary  purposes  of  Microscopic  dissection,  single  lenses 
of  from  3  to  1  inch  focus  answer  very  well.  But  when  higher  powers 
are  required,  and  when  the  use  of  even  the  lower  powers  is  continued  for 
any  length  of  time,  great  advantage  is  derived  from  the  employment  of 
Achromatic  combinations  now  made  expressly  for  this  purpose  by  several 
Opticians.  The  writer  has  worked  most  satisfactorily  for  several  years 
with  the  ^platyscopic  lens,'  magnifying  about  15  diameters,  made  by  Mr. 
Browning,  who  makes  similar  combinations  of  20  and  30  diameters. 
And  he  can  speak  equally  favorably  of  the  ^  Steinheil  doublets '  (con- 
structed by  the  eminent  Munich  optician  of  that  name,  and  introduced 
into  this  country  by  Messrs.  Murray  and  Heath),  of  which  there  are  six, 
ranging  from  2f  inches  to  f  inch  focus.  The  Browning  and  the  Stein- 
heil combinations  give  much  more  light  than  single  lenses,  with  much 
better  definition,  a  very  flat  field,  longer  working  distance  (which  is  very 
important  in  minute  dissection),  and,  as  a  consequence,  greater  '  focal 
depth'  or  ^penetration' — i.  e,  a  clearer  view  of  those  parts  of  the  object 
which  lie  above  or  below  the  exact  local  plane.  And  only  those  who, 
like  the  writer,  have  carried  on  a  piece  of  minute  and  difficult  dissection 
through  several  consecutive  hours,  can  appreciate  the  advantage  in  com- 
fort and  in  diminished  fatigue  of  eye  y  which  is  gained  by  the  substitution 


1  See  Quart.  Journ.  of  Microsc.  Science,"  Vol.  vi.,  N.S.  (1866),  p.  263.—Of 
the  Stanhoscopes  sold  by  Toy-dealers  at  a  very  low  price,  only  a  part  are  really 
serviceable;  care  is  requisite,  therefore,  in  the  selection. 


22 


THE  MICROSCOPE  AND  ITS  REVELATIONS. 


of  one  of  these  Achromatic  combinations  for  a  single  lens  of  equivalent 
focus,  even  where  the  use  of  the  former  reveals  no  detail  that  is  not  dis- 
cernible by  the  latter. 

3.  Compound  Microscope. 

26.  The  Compound  Microscope,  in  its  most  simple  form,  consists  of 
only  two  lenses,  the  object-glass  and  the  eye-glass.  The  former,  c  d  (Fig. 
13),  receives  the  light-rays  direct  from  the  object,  A  b,  brought  into  near 
proximity  to  it,  and  forms  an  enlarged  but  inverted  and  reversed'  image, 
a'b',  at  a  greater  distance  on  the  other  side  (§  8);  whilst  the  latter,  L  m, 
receives  the  rays  which  are  diverging  from  this  image,  as  if  they  pro- 
ceeded from  an  object  actually  occupying  its  position  and  enlarged  to  its 
dimensions,  and  brings  these  to  the  eye  at  E,  so  altering  their  course  as  to 
make  that  image  appear  far  larger  to  the  eye,  precisely  as  in  the  case  of 
the  Simple  microscope  (§  22). — It  is  obvious  that,  in  the  use  of  the  very 
same  lenses,  a  considerable  variety  of  magnifying  power  may  be  obtained, 
by  merely  altering  their  position  in  regard  to  each  other  and  to  the  ob- 
ject: for  if  the  eye-glass  be  carried  farther  from  the  object-glass,  whilst 
the  object  is  approximated  nearer  to  the  latter,  the  image  a'  b'  will  be 
formed  at  a  greater  distance  from  it,  and  its  dimensions  will  consequently 
be  augmented;  whilst,  on  the  other  hand,  if  the  eye-glass  be  brought 
nearer  to  the  object-glass,  and  the  object  removed  farther  from  it,  the 
distance  of  the  image  will  be  a  much  smaller  multiple  of  the  distance  of 
the  object,  and  its  dimensions  proportionately  diminished.  We  shall 
hereafter  see  that  this  mode  of  varying  the  magnifying  power  of  Com- 
pound Microscopes  may  be  turned  to  good  account  in  more  than  one 
mode  (§§  83,  84);  but  there  are  limits  to  the  use  which  can  be  advantage- 
ously made  of  it. — The  amplification  may  also  be  varied  by  altering  the 
magnifying  power  of  the  Eye-glass;  but  here,  too,  there  are  limits  to  the 
increase;  since  defects  of  the  object-glass  which  are  not  perceptible  when 
its  image  is  but  moderately  enlarged,  are  brought  into  injurious  promi- 
nence when  the  imperfect  image  is  amplified  to  a  much  greater  extent. 
In  practice,  it  is  generally  found  much  better  to  vary  the  power  by  em- 
ploying object-glasses  of  different  foci:  an  object-glass  of  long  focus  form- 
ing an  image  which  is  not  at  many  times  the  distance  of  the  object  from 
the  other  side  of  the  lens,  and  which,  therefore,  is  not  of  many  times  its 
dimension;  whilst  an  object-glass  of  short  focus  requires  that  the  object 
should  be  so  nearly  approximated  to  it,  that  the  distance  of  the  image  is 
a  much  higher  multiple  of  the  object,  and  its  dimensions  are  proportion- 
ably  larger. — In  whatever  mode  increased  amplification  may  be  obtained, 
two  things  must  always  result  from  the  change:  the  proportion  of  the 
surface  of  the  object  of  which  an  image  can  be  formed  must  be  dimin- 
ished; and  the  quantity  of  light  spread  over  that  image  must  be  propor- 
tionably  lessened. 

27.  In  addition  to  the  two  lenses  of  which  the  Compound  Microscope 
essentially  consists,  it  is  found  advantageous  to  introduce  another  (r  r. 
Fig.  14),  between  the  object-glass  and  the  image  formed  by  it;  the  pur- 
pose of  this  lens  being  to  change  the  course  of  the  rays  in  such  a  manner, 
that  the  image  may  be  formed  of  dimensions  not  too  great  for  the  whole 
of  it  to  come  within  the  range  of  the  Eye-glass.  As  it  thus  allows  more 
of  the  object  to  be  seen  at  once,  it  has  been  called  the  field-glass;  but  it 
is  now  usually  considered  as  belonging  to  the  ocular  end  of  the  instru- 
ment— the  eye-glass  and  the  field-glass  being  termed  the  Eye-piece,  Ya- 


OPTICAL  PRINCIPLES  OF  THE  MICROSCOPE. 


23 


rioi-is  forms  of  this  Eye-  Krei  13.  ^.14. 

piece  have  been  proposed 
by  different  Opticians; 
and  one  or  another  will 
bo  preferred,  according  to 
the  purpose  for  which  it 
may  be  required.  That 
which  it  is  most  advan- 
tageous to  employ  with 
Achromatic  object-glass- 
es, to  the  performance  of 
which  it  is  desired  to  give 
the  greatest  possible  ef- 
fect, is  termed  the  Hiiy- 
ghenian;  having  been  em- 
ployed by  Huyghens  for 
his  telescopes,  although 
without  the  knowledge  of 
all  the  advantages  which 
its  best  construction  ren- 
ders it  capable  of  afford- 
ing. It  consists  of  two 
plano-convex  lenses  (e  e 
and  F  F,  Fig.  14),  with 
their  plane  sides  towards 
the  eye;  these  are  placed 
at  a  distance  equal  to  half 
the  sum  of  their  focal 
length;  or,  to  speak  with 
more  precision,  at  half 
the  sum  of  the  focal 
length  of  the  eye  glass, 
and  of  the  distance  from 
the  field-glass  at  which 
an  image  of  the  object- 

2rlaSS  would  be  formed  ^ja^ram  of  simplest  form 
?     . ,        A    ^   J       9  T     or  Compound  Microscope, 

by  it.  A  ^stop'  or  dia- 
phragm, B  B,  must  be  placed  between  the  two  lenses,  in  the  visual 
focus  of  the  Eye-glass,  which  is,  of  course,  the  position  wherein  the 
image  of  the  object  will  be  formed  by  the  rays  brought  into  conver- 
gence by  their  passage  through  the  field-glass.  —  Huyghens  devised 
this  arrangement  merely  to  diminish  the  Spherical  aberration;  but  it 
was  subsequently  shown  by  Boscovich  that  the  Chromatic  dispersion 
was  also  in  great  part  corrected  by  it.  Since  the  introduction  of 
Achromatic  object-glasses  for  Compound  Microscopes,  it  has  been 
further  shown  that  nearly  all  error  may  be  avoided  by  a  slight  over- 
correction of  these;  so  that  the  blue  and  red  rays  may  be  caused  to  enter 
the  eye  in  a  parallel  direction  (though  not  actually  co-incident),  and 
thus  to  produce  a  colorless  image.  Thus  let  iS"  M  K  (Fig.  15)  represent 
the  two  extreme  rays  of  three  pencils,  which,  without  the  field-glass, 
would  form  a  blue  image  convex  to  the  eye-glass  at  b  b,  and  a  red  one  at 
R  r;  then,  by  the  intervention  of  the  field-glass,  a  blue  image,  concave  to 
the  eye-glass,  is  formed  at  b^  b',  and  a  red  one  at  r'  r'.  As  the  focus  of 
the  Eye-glass  is  shorter  for  blue  rays  than  for  red  rays  by  just  the  diffcr- 


Diagram  of  complete  Corn,' 
pound  Microscope. 


24 


THE  MICROSCOPE  AND  ITS  REVELATIONS. 


ence  in  the  place  of  these  images,  their  rays,  after  refraction  by  it,  enter 
the  eye  in  a  parallel  direction,  and  produce  a  picture  free  from  false  color. 
If  the  object-glass  had  been  rendered  perfectly  achromatic,  the  bhie  rays, 
after  passing  through  the  field-glass,  would  have  been  brought  to  a  focus 
at  i,  and  the  red  at  r;  so  that  an  error  would  be  produced,  which  would 
have  been  increased  instead  of  being  corrected  by  the  eye-glass.  Another 

advantage  of  a  well-constructed  Huyglien- 
^^^^^^^J^^^^^^^^  ian  eye-piece  is,  that  the  image  produced 
^^^^^^HHH^^^^^B  by  the  meeting  of  the  rays  after  passing 
^^^^HBHIHH^^^^H  through  the  field-glass,  is  by  it  rendered 
^^^^^^^^^^W^^^^H  concave  towards  the  eye-glass,  instead  of 
H|^H^^^^n^H|H  convex,  so  that  every  part  of  it  may  be  in 
^^^^B^SS^H^^^^H  focus  at  the  same  time,  and  the  field  of 
HH|^H|^H^H^HHH  view  thereby  rendered  fiat.^  —  Two  or 
^^|^HH^H^H|^^H^B  more  Huyghenian  Eye-pieces,  of  differ- 
^^M^B^^mIH^H  magnifying  powers,  known  as  A, 

^^HHH^H|^H||B^H  C,  etc.,  are  usually  supplied  with  a  Com- 
^^^H^^HI^^HnH^H  pound  Microscope.  The  utility  of  the 
^^HM^^H|^^HH|^^H  higher  powers  will  mainly  depend  upon 
^^^^^^^^H^^^^^^H  the  excellence  of  the  Objectives;  for  when 
^HH^^^^H|^^^fl|^B  an  Achromatic  combination  of  small 
^^HH^^HI^HHlj^H  aperture,  which  is  sufficiently  well  cor- 
^^HI^^BI^^HH^^H  rected  to  perform  very  tolerably  with  a 
^KBtHEUKKEtltl^  '  ^  '  shallow  ^  eye-piece,  is  used  with 
„    .     „  _    ,    .     „     .       an  eye-piece  of  hisfher  maffnifyinsr  power 

Section  of  Huygheman  Eye-piece      ,  a         £  c  ;\  y  \ 

adanted  to  over-corrected  Achroma-  (commonly  spokeu  ot  as  a  ^  deeper  onc), 
tic  Objectives.  image  may  lose  more  in  brightness  and 

in  definition  than  is  gained  by  its  amplification;  whilst  the  image  given  by 
an  Objective  of  large  angular  aperture  and  very  perfect  correction,  shall 
sustain  so  little  loss  of  light  or  of  definition  by  '  deep  eye-piecing,^  that  the 
increase  of  magnifying  power  shall  be  almost  clear  gain.  Hence  the  modes 
in  which  different  Objectives  of  the  same  power,  whose  performance  with 
shallow  eye-pieces  is  nearly  the  same,  are  respectively  affected  by  deep 
eye-pieces,  afford  a  good  test  of  their  respective  merits;  since  any  defect 
in  the  corrections  is  sure  to  be  brought  out  by  the  higher  amplification 
of  the  image,  whilst  a  deficiency  of  aperture  is  manifested  by  the  want  of 
light. — The  working  Microscopist  will  generally  find  the  A  eye-piece 
most  suitable,  B  being  occasionally  employed  Avhen  a  greater  power  is 
required  to  separate  details,  whilst  0  and  other  still  deeper  are  useful  for 
the  purpose  of  testing  the  goodness  of  Objectives,  or  for  special  investi- 
gations requiring  the  highest  amplification  with  Objectives  of  the  finest 
quality.  When  great  penetration  or  '  focal  depth  ^  is  required,  low  Ob- 
jectives and  deep  Eye-pieces  will  often  be  found  convenient. 

28.  For  viewing  large  flat  objects,  such  as  transverse  sections  of 
Wood  (Chap,  ix.)  or  of  Echinus-spines  (Plate  ii.  Fig.  1),  under  low 
magnifying  powers,  the  Eye-piece  known  as  Kellner^s  may  be  employed 
with  advantage.  In  this  construction,  the  field-glass,  which  is  a  double- 
convex  lens,  is  placed  in  the  focus  of  the  eye-glass,  without  the  interposi- 
tion of  a  diaphragm;  and  the  eye-glass  is  an  achromatic  combination  of  a 


^  Those  who  desire  to  gain  more  information  upon  this  subject  than  they  can 
from  the  above  notice  of  ic,  may  be  referred  to  Mr.  Varley's  investigation  of  the 
properties  of  the  Huyghenian  Eye-piece,  in  the  51st  volume  of  the  "  Transactions 
of  the  Society  of  Arts;"  and  to  tlie  article    Microscope,"  by  Mr.  Ross,  in  the 
Penny  Cyclopaedia,"  reprinted,  with  additions,  in  the  "English  Cyclopaedia." 


OPTICAL  PRINCIPLES  OF  THE  MICROSCOPE. 


25 


plano-concave  of  flint  with  a  double-conyex  of  crown,  which  is  slightly 
under-corrected,  so  as  to  neutralize  the  over-correction  given  to  the  Ob- 
jectives for  use  with  Huyghenian  eye-pieces  (§  27).  A  flat  well-illumi- 
nated field  of  as  much  as  fourteen  inches  in  diameter  may  thus  be  ob- 
tained with  very  little  loss  of  light;  but,  on  the  other  hand,  there  is  a 
certain  impairment  of  defining  power,  which  renders  the  Kellner  eye- 
piece unsuitable  for  objects  presenting  minute  structural  details;  and  it 
is  an  additional  objection,  that  the  smallest  speck  or  smear  upon  the  sur- 
face of  the  field  glass  is  made  so  unpleasantly  obvious,  that  the  most  care- 
ful cleansing  of  that  surface  is  required  every  time  that  this  Eye-piece  is 
used.  Hence  it  is  better  fitted  for  the  occasional  display  of  objects  of  the 
character  already  specified  than  for  the  ordinary  wants  of  the  working 
Microscopist. 

29.  A  solid  Eye-piece  made  on  the  principle  of  the  '  Stanhope'  lens 
(§  24)  is  sometimes  used  in  place  of  the  ordinary  Huyghenian,  when  high 
magnifying  power  is  required  for  testing  the  performance  of  Objectives. 
The  lower  surface,  which  has  the  lesser  convexity,  serves  as  a  Afield-glass;' 
whilst  the  image  formed  by  this  is  magnified  by  the  highly  convex  upper 
surface  to  which  the  eye  is  applied;  the  advantage  supposed  to  be  derived 
from  this  construction  lying  in  the  abolition  of  the  plane  surfaces  of 
the  two  lenses  of  the  ordinary  eye-piece.  A  '  positive '  or  Eamsden's  Eye- 
piece— in  which  the  field  glass,  whose  convex  side  is  turned  upwards,  is 
placed  so  much  nearer  the  eye-glass  that  the  image  formed  by  the  Objec- 
tive lies  below  instead  of  above  it, — was  formerly  used  for  the  purpose  of 
Micrometry;  a  divided  glass  being  fitted  in  the  exact  plane  occupied  by 
the  image,  so  that  its  scale  and  that  image  are  both  magnified  together 
by  the  lenses  interposed  between  them  and  the  eye.  The  same  end,  how- 
ever, may  be  so  readily  attained  with  the  Huyghenian  eye-piece  (§  91), 
that  no  essential  advantage  is  gained  by  the  use  of  that  of  Eamsden,  the 
field  of  which  is  distinct  only  in  its  centre. 

4.  Stereoscopic  Binocular  Microscope. 

30.  The  admirable  invention  of  the  Stereoscope  by  Professor  Wheat- 
stone,  has  led  to  a  general  appreciation  of  the  value  of  the  conjoint  use  of 
both  eyes  in  conveying  to  the  mind  a  notion  of  the  solid  forms  of  objects, 
such  as  the  use  of  either  eye  singly  does  not  generate  with  the  like  certainty 
or  effectiveness.  And  after  several  attempts,  which  were  attended  with 
various  degrees  of  success,  the  principle  of  the  Steroscope  has  now  been 
applied  to  the  Microscope,  with  an  advantage  which  those  only  can  truly 
estimate,  who  (like  the  Author)  have  been  for  some  time  accustomed  to 
work  with  the  Stereoscopic  Binocular^  upon  objects  that  are  peculiarly 
adapted  to  its  powers.  As  the  result  of  this  application  cannot  be  rightly 
understood  without  some  knowledge  of  one  of  the  fundamental  principles 
of  Binocular  vision,  a  brief  account  of  this  will  be  here  introduced. — All 
vision  depends  in  the  first  instance  on  the  formation  of  a  picture  of  the 
object  upon  the  retina  of  the  Eye,  just  as  the  Camera  Obscura  forms  a 
picture  upon  the  ground  glass  placed  in  the  focus  of  its  lens.  But  the 
two  images  that  are  formed  by  the  two  eyes  respectively,  of  any  solid 
object  that  is  placed  at  no  great  distance  in  front  of  them,  are  far  from 


^  It  has  become  necessary  to  distinguish  the  Binocular  Microscope  which  gives 
true  Stereoscopic  effects  by  the  combination  of  two  dissimilar  picture,  from  a 
Binpcular  which  simply  enables  us  to  look  with  both  eyes  at  images  which  are 
essentially  identical  (§  81). 


26 


THE  MICROSCOPE  AND  ITS  REVELATIONS. 


being  identical;  the  perspective  projection  of  the  object  varying  with  the 
point  of  view  from  which  it  is  seen.  Of  this  the  reader  may  easily  con- 
vince himself,  by  holding  up  a  thin  book  in  such  a  position  that  its  back 
shall  be  at  a  moderate  distance  in  front  of  the  nose,  and  by  looking  at  the 
book,  first  with  one  eye  and  then  with  the  other;  for  he  will  find  that  the 
two  views  he  thus  obtains  are  essentially  different,  so  that  if  he  were  to 
represent  the  book  as  he  actually  sees  it  with  each  eye,  the  two  pictures 
would  by  no  means  correspond.  Yet  on  looking  at  the  object  with  the  two 
eyes  conjointly,  there  is  no  confusion  between  the  images,  nor  does  the  mind 
dwell  on  either  of  them  singly;  but  from  the  blending  of  the  two  a  con- 
ception is  gained  of  a  solid  projecting  body,  such  as  could  only  be  other- 
wise acquired  by  the  sense  of  Touch.  Now  if,  instead  of  looking  at  the 
solid  object  itself,  we  look  with  the  right  and  left  eyes  repectively  at  pic- 
tures of  the  object,  corresponding  to  those  which  would  be  formed  by  it 
on  the  retinae  of  the  two  eyes  if  it  were  placed  at  a  moderate  distance  in 
front  of  them,  and  these  visual  pictures  are  brought  into  concidence,  the 
same  conception  of  a  solid  projecting  form  is  generated  in  the  mind,  as  if 
the  object  itself  were  there.    The  Stereoscope — whether  in  the  forms 


originally  devised  by  Prof.  Wheatstone,  or  in  the  popular  modification 
long  subsequently  introduced  by  Sir  D.  Brewster — simply  serves  to  bring 
to  the  two  eyes,  either  by  reflection  from  mirrors,  or  by  refraction  through 
prisms  or  lenses,  the  two  dissimilar  pictures  which  would  accurately 
represent  the  solid  object  as  seen  by  the  two  eyes  respectively;  these  being 
thrown  on  the  two  retinae  in  the  precise  positions  they  would  have  occu- 

f ied  if  formed  there  direct  from  the  solid  Object,  of  which  the  mental 
mage  (if  the  pictures  have  been  correctly  taken)  is  the  precise  counter- 
part.^ Thus  in  Fig.  16  the  upper  pair  of  pictures  (a,  b),  when  combined 
in  the  Stereoscope,^  suggest  the  idea  of  a  projecting  truncated  Pyramid, 


^  Although  it  is  a  comparatively  easy  matter  to  draw  in  outline  two  different 
perspective  projections  of  a  Geometrical  Solid,  such  as  those  which  are  repre- 
sented in  Fig.  16,  it  would  have  been  quite  impossible  to  delineate  landscapes, 
buildings,  figures,  etc.,  with  the  same  precision;  and  the  Stereoscope  would  never 
have  obtained  the  appreciation  it  now  enjoys,  but  for  the  ready  means  supplied 
by  Photography  of  obtaining  simultaneous  pictures,  perfect  in  their  perspective, 
and  truthful  in  their  lights  and  shades,  from  two  different  points  of  view  so 
selected  as  to  give  an  effective  Stereoscopic  combination. 

*  This  combination  may  be  made  without  the  Stereoscope,  by  looking  at  these 


OPTICAL  PRINCIPLES  OF  THE  MICROSCOPE. 


27 


with  the  small  square  in  the  centre,  and  the  four  sides  sloping  equally 
away  from  it;  whilst  the  combination  of  the  lower  pair,  c,  D  (which  are 
identical  with  the  upper,  but  are  transferred  to  o})posite  sides),  no  less 
vividly  brings  to  the  mind  the  visual  conception  of  a  receding  Pyramid, 
still  with  the  small  square  in  the  centre,  but  the  four  sides  sloping  equally 
towards  it. 

31.  Thus  we  see  that  by  simply  crossing  the  picture  in  the  Stereos- 
cope, so  as  to  bring  before  each  eye  the  picture  taken  for  the  other,  a 
^conversion  of  relief^  is  produced  in  the  resulting  solid  image;  the  pro- 
jecting parts  being  made  to  recede,  and  the  receding  parts  brought  into 
relief.  In  like  manner,  when  several  objects  are  combined  in  the  same 
crossed  pictures,  their  apparent  relative  distances  are  reversed;  the  remo- 
ter being  brought  nearer,  and  the  nearer  carried  backwards;  so  that  (for 
example)  a  Stereoscopic  photograph  representing  a  man  standing  in  front 
of  a  mass  of  ice,  shall,  by  the  crossing  of  the  picture,  make  the  figure 
appear  as  if  imbedded  in  the  ice.  A  like  conversion  of  relief  may  also  be 
made  m  the  case  of  actual  solid  objects  by  the  use  of  the  Pseudoscope; 
an  instrument  devised  by  Prof.  Wheatstone,  which  has  the  effect  of  re- 
versing the  perspective  projections  of  objects  seen  through  it  by  the  two 
eyes  respectively;  so  that  the  interior  or  a  basin  or  jelly-mould  is  made  to 
appear  as  a  projecting  solid,  while  the  exterior  is  made  to  appear  hollow. 
Hence  it  is  now  customary  to  speak  of  stereoscopic  Vision  as  that  m  which 
the  conception  of  the  true  natural  relief  of  an  object  is  called-up  in  the 
mind,  by  the  normal  combination  of  the  two  perspective  projections 
formed  of  it  by  the  right  and  left  eyes  respectively;  whilst  hj pseudosco- 
pic  vision,  we  mean  that  •  conversion  of  relief^  which  is  produced  by  the 
combination  of  two  reversed  perspective  projections,  whether  these  be 
obtained  directly  from  the  object  (as  by  the  Pseudoscope),  or  from 
^crossed  ^  pictures  (as  in  the  Stereoscope).  It  is  by  no  means  every  solid 
object,  however,  or  every  pair  of  stereoscopic  j)ictures,  which  can  become 
the  subject  of  this  conversion.  The  degree  of  facility  with  which  the 
^  converted '  form  can  be  apprehended  by  the  Mind,  appears  to  have  great 
influence  on  the  readiness  with  which  the  change  is  produced.  And 
while  there  are  some  objects — the  interior  of  a  plaster  mask  of  a  face,  for 
example — which  can  always  be  ^ converted^  (or  turned  inside-out)  at 
once,  there  are  others  which  resist  such  conversion  with  more  or  less  of 
persistence.* 

32.  Now  it  is  easily  shown  theoretically,  that  the  picture  of  any  pro- 
jecting object  seen  through  the  Microscope  with  only  the  right-hsmd 
half  of  an  objective  having  an  even  moderate  angle  of  aperture,  must 
differ  sensibly  from  the  picture  of  the  same  object  received  through  the 
fe/'^-hand  of  the  same  objective;  and  further,  that  the  difference  between 
such  pictures  must  increase  with  the  angular  aperture  of  the  objective. 
This  difference  may  be  practically  made  apparent  by  adapting  a  ^  stop ' 
to  the  objective,  in  such  a  manner  as  to  cover  either  the  right  or  the  left 
half  of  its  aperture;  and  by  then  carefully  tracing  the  outline  of  the  ob- 
ject as  seen  through  each  half.  But  it  is  more  satisfactorily  brought  into 
view  by  taking  two  Photographic  pictures  of  the  object,  one  through  each 
lateral  half  of  the  objective;  for  these  pictures  when  properly  paired  in 
the  Stereoscope,  give  a  magnified  image  in  relief y  bringing  out  on  a  large 

figures  with  the  axis  of  the  eyes  brought  into  convergence  upon  a  somewhat 
nearer  point,  so  that  A  is  made  to  fall  on  B,  and  c  on  D. 

1  For  a  fuller  discussion  of  this  subject,  see  the  Author's  Mental  Physiology," 
§§  168-170. 


28 


THE  MICROSCOPE  AND  ITS  REVELATIONS. 


scale  the  solid  form  of  the  object  from  which  they  were  taken.  What  is 
needed,  therefore,  to  give  the  true  Stereoscopic  power  to  the  Microscope, 
is  a  means  of  so  bisecting  the  cone  of  rays  transmitted  by  the  objective, 
that  of  its  two  lateral  halves  one  shall  be  transmitted  to  the  right  and  the 
other  to  the  left  eye.  If,  however,  the  image  thus  formed  by  the  right 
half  of  the  objective  of  a  Compound  Microscope  were  seen  by  the  right 
eye,  and  that  formed  by  the  left  half  were  seen  by  the  left  eye,  the  result- 
ant conception  would  be  not  stereoscopic  hut 2)seudosco2nc;  the  projecting 
parts  being  made  to  appear  receding,  and  vice  versa.  The  reason  of  this 
is  that  as  the  Microscope  itself  reverses  the  picture  (§  26),  the  rays  pro- 
ceeding through  the  right  and  the  left  hand  halves  of  the  objective  must 
be  made  to  cross  to  the  left  and  the  right  eyes  respectively,  in  order  to 
correspond  with  the  direct  view  of  the  object  from  the  two  sides;  for  if 
this  second  reversal  does  not  take  place,  the  effect  of  the  first  reversal  of 
the  images  produced  by  the  Microscope  exactly  corresponds  with  that 
produced  by  the  ^crossing '  of  the  pictures  in  the  Stereoscope,  or  by  that 
reversal  of  the  two  perspective  projections  formed  direct  from  the  object, 
which  is  effected  by  the  Pseudoscope  (§  31).  It  was  from  a  want  of  due 
appreciation  of  this  principle  (the  truth  of  which  can  now  be  practically 
demonstrated,  §  38),  that  the  earlier  "attempts  at  producing  a  Stereosco- 
pic Binocular  Microscope  tended  rather  to  produe  a  ^pseudoscopic  con- 
version '  of  the  objects  viewed  by  it,  than  to  represent  them  in  their  true 
relief 


Arrangement  of  Prisms  in  Nachet's  Stereoscopic  Nachet's  Stereoscopic  Binocular. 

Binocular  Microscope. 

33.  NacheVs  Stereoscopic  Binocular, — The  first  really  satisfactory 
solution  of  the  problem  was  that  worked  out  by  MM.  Nachet;  whose 
original  Binocular  was  constructed  on  the  method  shown  in  Fig.  17. 
The  cone  of  rays  issuing  from  the  back  lens  cf  the  objective  masts 
the  flat  surface  of  a  prism  {p)  placed  above  it,  who  section  is  an  equilat- 


OPTICAL  PRINCIPLES  OF  THE  MICROSCOPE. 


29 


eral  triangle;  and  is  divided  by  reflection  within  this  prism  into  two  lat- 
eral halves,  which  cross  each  other  in  its  interior.  The  rays  a  b  that 
form  the  right  half  of  the  cone,  impinging  very  obliquely  on  the  internal 
face  of  the  prism,  suffer  total  reflection  (§  2),  emerging  through  its  left 
side  perpendicularly  to  its  surface,  and  therefore  undergoing  no  refrac- 
tion; whilst  the  rays  a'  V  forming  the  left  half  of  the  cone,  are  reflected 
in  like  manner  towards  the  right.  Each  of  these  pencils  is  received  by 
a  lateral  prism,  which  again  changes  its  direction,  so  as  to  render  it  par- 
allel to  its  original  course;  and  thus  the  two  halves  a  b  and  a'  V  of  the 
original  pencil  are  completely  separated  from  each  other,  the  former 
being  received  into  the  left-hand  body  of  the  Microscope  (Fig.  18),  and 
the  latter  into  its  right-hand  body.  These  two  bodies  are  parallel;  and, 
by  means  of  an  adjusting  screw  at  their  base,  which  alters  the  distance 
between  the  central  and  the  lateral  prisms,  they  can  be  separated  from 
or  approximated  towards  each  other,  so  that  the  distance  between  their 
axes  can  be  brought  into  exact  coincidence  with  the  distance  between  the 
axes  of  the  eyes  of  the  individual  observer. — This  instrument  gives  true 
Stereoscopic  projection  to  the  conjoint  image  formed  by  the  mental  fu- 
sion of  the  two  distinct  pictures;  and  with  low  powers  of  moderate  angu- 
lar aperture  its  performance  is  highly  satisfactory.  There  are,  however, 
certain  drawbacks  to  its  general  utility.  First,  every  ray  of  each  jDencil 
suffers  tii^o  reflections,  and  has  to  pass  through /tv^/r  surfaces;  this  neces- 
sarily involves  a  considerable  loss  of  light,  with  a  further  liability  to  the 
impairment  of  the  image  by  the  smallest  want  of  exactness  in  the  form 
of  either  of  the  prisms.  Second,  the  mechanical  arrangements  requisite 
for  varying  the  distance  of  the  bodies,  involve  an  additional  liability  to 
derangement  in  the  adjustment  of  the  prisms.  Third,  the  instrument 
can  only  be  used  for  its  own  special  purpose;  so  that  the  observer  must 
also  be  provided  with  an  ordinary  single-bodied  Microscope,  for  the  ex- 
amination of  objects  unsuited  to  the  powers  of  his  Binocular.  Fourth, 
the  parallelism  of  the  bodies  involves  parallelism  of  the  axes  of  the  ob- 
server's eyes,  the  maintenance  of  which  for  any  length  of  time  is  fatiguing. 

34.  Wenhani^s  Stereoscopic  Binocular. — All  these  objections  are  over- 
come in  the  admirable  arrangement  devised  by  the  ingenuity  of  Mr. 
Wenham;  in  whose  Binocular  the  cone  of  rays  proceeding  upwards  from 
the  objective  is  divided  by  the  interposition  of  a  prism  of  the  peculiar 
form  shown  in  Fig.  19,  so  placed  in  the  tube  which  carries  the  objec- 
tive (Figs.  20,  21  a)  as  only  to  interrupt  one  half,  a  c,  of  the  cone, 
the  other  half,  a  b,  going  on  continuously  to  the  eye-piece  of  the  prin- 
cipal or  right-hand  body  R,  in  the  axis  of  which  the  objective  is  placed. 
The  interrupted  half  of  the  cone  (Fig.  19,  a),  on  its  entrance  into  the 
prism,  is  scarcely  subjected  to  any  refraction,  since  its  axial  ray  is  per- 
pendicular to  the  surface  it  meets;  but  within  the  prism  it  is  subjected 
to  two  reflections  at  b  and  c,  which  send  it  forth  again  obliquely  in  the 
line  d  towards  the  eye-piece  of  the  secondary,  or  left  hand  body  (Fig.  20, 
L);  and  since  at  its  emergence  its  axial  ray  is  again  perpendicular 
to  the  surface  of  the  glass,  it  suffers  no  more  refraction  on  passing  out 
of  the  prism  than  on  entering  it.  By  this  arrangement,  the  image 
received  by  the  right  eye  is  formed  by  the  rays  which  have  passed  through 
the  left  half  of  the  objective,  and  have  come  on  without  any  interruption 
whatever;  whilst  the  image  received  by  the  left  eye  is  formed  by  the 
rays  which  have  passed  through  the  right  half  of  the  objective,  and  have 
been  subjected  to  two  reflections  within  the  prism,  passing  through  only 
two  surfaces  of  glass.    The  adjustment  for  the  variation  of  distance 


30 


THE  MICROSCOPE  AND  ITS  REVELATIONS. 


between  the  axes  of  the  eyes  in  different  individuals,  is  made  by  draw 
ing-out  or  pushing-in  the  eye- pieces,  which  are  moved  consentaneously 

Fig.  20  Fig. 


Wenham's  Prism.  Wenham's  Stereoscopic  Binocular  Microscope. 


by  means  of  a  milled-head,  as  shown  in  Fig.  21. — Jfow,  although  it  may 
be  objected  to  Mr.  Wenham's  method  (1),  that  as  the  rays  which  pass 
through  the  prism  and  are  obliquely  reflected  into  the  secondary  body, 
traverse  a  longer  distance  than  those  which  pass-on  uninterrupedly  into 
the  principal  body,  the  picture  formed  by  them  will  be  somewhat  larger 
than  that  v/hich  is  formed  by  the  other  set;  and  (2),  that  the  picture 
formed  by  the  rays  which  have  been  subjected  to  the  action  of  the  prism 
must  be  inferior  in  distinctness  to  that  formed  by  the  uninterrupted  half 
of  the  cone  of  rays, — these  objections  are  found  to  have  no  practicable 
weight.  For  it  is  well  known  to  those  who  have  experimented  upon  the 
phenomena  of  Stereoscopic  vision  (1),  that  a  slight  difference  in  the  size 
of  the  two  pictures  is  no  bar  to  their  perfect  combination;  and  (2),  that 
if  one  of  the  pictures  be  good,  the  full  effect  of  relief  is  given  to  the 
image,  even  though  the  other  picture  be  faint  and  imperfect,  provided 
that  the  outlines  of  latter  are  sufficiently  distinct  to  represent  its  perspec- 
tive projection.  Hence  if,  instead  of  the  two  equally  half-good  pictures 
which  are  obtainable  by  MM.  Nachet's  original  construction,  we  had  in 
Mr.  Wenham's  one  good  and  one  indifferent  picture,  the  latter  would  be 
decidedly  preferable.  But,  in  point  of  fact,  the  deterioration  of  the  sec- 
Olid  picture  in  Mr.  Wenham's  arrangement  is  less  considerable  than  that 
of  hoth  pictures  in  the  original  arrangement  of  MM.  Nachet;  so  that  the 
optical  performance  of  the  Wenhani  Binocular  is  in  every  way  superior. 
It  has,  in  addition,  these  further  advantages  over  the  preceding, — First, 
the  greater  comfort  in  using  it  (especially  for  some  length  of  time  to- 
gether), which  results  from  the  convergence  of  the  axes  of  the  eyes  at 
their  usual  angle  for  moderately-near  objects;  second,  that  this  Binocu- 
lar arrangement  does  not  necessitate  a  special  instrument,  but  may  be 


OPTICAL  PRINCIPLIS  OF  THE  MICROSCOPE. 


31 


applied  to  any  Microscope  which  is  capable  of  carrying  the  weight  of 
the  secondary  body;  the  prism  being  so  fixed  in  a  movable  frame  that  it 
may  in  a  moment  be  taken  out  of  tlie  tube  or  replaced  therein,  so  that 
when  it  has  been  removed,  the  principal  body  acts  in  every  respect  as  an 
ordinary  Microscope,  the  entire  cone  of  rays  ])assing  uninterruptedly  into 
it;  and  thirds  that  the  simplicity  of  its  construction  renders  its  derange- 
ment almost  impossible.^ 

35.  Stephenson'^ s  Binocular. — A  new  form  of  Stereoscopic  Binocular 
has  been  recently  introduced  by  Mr.  Stephenson,^  which  has  certain  ad- 
vantages over  both  the  preceding. — The  cone  of  rays  passing  upwards 
from  the  object-glass,  meets  a  pair  of  prisms  (a  a,  Fig.  22)  fixed  in  the 
tube  of  the  microscope  immediately  above  the  posterior  combination  of 
the  objective,  so  as  to  catch  the  light-rays  on  their  emergence  from  it; 
these  it  divides  into  two  halves,  each  of  which  is  subjected  to  internal 
reflection  from  the  inner  side  of  the  prism 
through  which  it  passes;  and  the  slight  separa- 
tion of  the  two  prisms  at  their  upper  end,  gives 
to  the  two  pencils  B  B,  a  divergence  which  car- 
ries them  through  two  obliquely-placed  bodies 
to  their  respective  eye-pieces.  By  this  internal 
reflection,  a  lateral  reversal  is  produced,  which 
antagonizes  the  lateral  reversal  of  the  Micro- 
scopic image;  so  that  each  eye  receives  the  image 
formed  by  its  own  half  of  the  objective,  in  the 
position  required  for  the  production  of  Stereo- 
scopic relief  by  the  mental  combination  of  the 
two.  As  the  cone  of  rays  is  equally  divided  by 
the  two  prisms,  and  its  two  halves  are  similarly 
acted-on,  the  two  picture  are  equally  illuminat- 
ed, and  of  the  same  size;  while  the  close  ap- 
proximation of  the  prisms  to  the  back  lens  of  the 
objective  enables  even  higher  powers  to  be  used 
Avith  very  little  loss  of  light  or  of  definition, 

provided  that  the  angles  and  surfaces  of  the  prisms  Stephenson's  Binocular  Prisms. 

are  worked  with  exactness.  And  as  the  two  bodies  can  be  made  to  con- 
verge at  a  smaller  angle  than  in  the  Wenham  arrangement,  the  observer 
looks  through  them  with  more  comfort.  But  Mr. 
Stephenson's  ingenious  arrangement — which  was  first 
worked-out  practically  by  the  late  Thomas  Eoss,  and 
has  since  been  very  successfully  constructed  by  Brown- 
ing— is  liable  to  the  great  drawback  of  not  being  con- 
vertible (like  Mr.  Wenham's)  into  an  ordinary  Mono- 
cular, by  the  withdrawal  of  a  prism;  so  that  the  use 
of  this  form  of  it  will  be  probably  restricted  to  those 
who  desire  to  work  stereoscopically  with  high  powers, 
— In  order  to  avoid  slight  errors  arising  from  the  im- 
pinging of  the  central  ray  of  the  cone,  at  its  emergence 
from  the  objective,  against  the  double  edge  of  the 


^  The  Author  cannot  allow  this  opportunity  to  pass  without  expressing  his  sense 
of  the  liberality  with  which  Mr.  Wenham  freely  presented  to  the  Public  this  im- 
portant invention,  by  which  there  can  be  no  doubt  that  he  might  have  largely 
profited  if  he  had  chosen  to  retain  the  exclusive  right  to  it. 

2  "Monthly  Microscopical  Journal,"  Vol.  iv.  (1870),  p.  61,  and  Vol.  vii.  (1872), 
p.  167. 


32 


THE  MICROSCOPE  AND   ITS  REVELATIONS. 


prism  combination,  Mr.  Stephenson  has  devised  a  special  form  of  sub- 
stage  Condenser  (also  made  by  Mr.  Browning),  which  causes  the  illumi- 
nating rays  to  issue  from  the  object  in  two  separate  pencils,  which  will 
strike  the  surfaces  of  the  two  prisms.  This  consists  of  two  deep  cylin- 
drical lenses  A  and  B,  whose  focal  lengths  are  as  2.3  to  1,  having  their 
curved  faced  opposed  to  each  other,  as  shown  in  section  at  c;  the  larger 
and  less  convex  being  placed  with  its  plane  side  downwards,  so  as  to  re- 
ceive light  from  the  mirror,  or  (which  is  preferable)  direct  from  a  lamp. 
Under  this  combination  slides  a  movable  stop,  with  two  circular  open- 
ings, as  shown  in  Fig.  24.    The  lamp  being  placed  in  front  of  the  instru- 


Double  Stop  for  Stephenson  Binocular.  Stephenson's  Erecting  Prism. 


ment,'the  two  apertures  admit  similar  pencils  of  light  from  it;  so  that 
each  eye  receives  a  completely  equal  illumination^  and  no  confusion  can 
occur  from  the  impinging  of  the  rays  on  the  lower  edges  of  the  prisms. 
With  this  arrangement  the  Podura-markings  are  shown  as  figured  by  the 
late  Eichard  Beck  (Plate  ii.,  fig.  2);  while  the  curvatures  of  the  scale 
come  out  with  the  distinctness  peculiar  to  Binocular  vision. 

36.  But  one  of  the  greatest  advantages  attendant  on  Mr.  Stephenson's 
construction,  is  its  capability  of  being  combined  with  an  erecting  arrange- 
ment; which  renders  it  applicable  to  purposes  for  which  the  Wenham 
Binocular  cannot  be  conveniently  used.  By  the  interposition  of  a  plane 
silvered  mirror,  or  (still  better)  of  a  reflecting  prism  (Fig.  25),  above  the 
tube  containing  the  binocular  prisms,  each  half  of  the  cone  of  rays  is 
so  deflected,  that  its  image  is  reversed  vertically  ;  the  rays  entering  the 
prism  through  the  surface  c  B,  being  reflected  by  the  surface  A  B,  so  as 
to  pass  out  again  by  the  surface  A  c  in  the  direction  of  the  dotted  lines. 
Thus  the  right  and  the  left  half  cones  are  directed  respectively  into  the 
right  and  the  left  bodies,  which  are  inclined  at  a  convenient  angle,  as  shown 
in  Fig.  26;  so  that — the  stage  being  horizontal — the  observer  can  look  at 
his  object  at  the  inclination  which  he  finds  most  comfortable.  The 
angle  to  which  the  prism  is  worked  can  be  varied  to  suit  individual 
requirements;  but  if  it  should  be  desired  to  use  the  instrument  with 
Polarized  light,  it  will  be  found  advantageous  that  the  reflection  from  the 
surface  A  b  should  be  at  the  polarizing  angle  of  56f since,  by  substi- 
tuting for  the  silvered  mirror  or  prism  a  highly  polished  mirror  of  black 
glass,  this  will  then  act  as  an  analyzer,  with  some  decided  advantages  over 
the  Nicol  prism,  except  in  being  incapable  of  rotation. — The  great  value 


OPTICAL  PRINCIPLES  OF  THE  MICKOSCOPE. 


33 


of  the  Erecting  Binocular  consists  in  its  Dia.  2tt 

applicability  to  the  picking  out  of  very 
minute  objects,  such  as  Diatoms^  Polycyst-  ^ 
ina,  or  Foraminifera;  and  to  the  pro-* 
secution  of  minute  dissections,  especially 
when  these  have  to  be  carried  on  in  fluid. 
No  one  who  has  only  thus  worked  monocii- 
larly,  can  appreciate  the  guidance  derivable 
from  iinocular  vision,  when  once  the  habit 
of  working  with  it  has  been  formed. 

37.  Tolles^  Binocular  Eyepiece, — An  in- 
genious Eye-piece  has  been  constructed  by 
Mr.  Tolles  (Boston,  U.  S.),  which,  fitted 
into  the  body  of  a  Monocular  Microscope, 
converts  it  into  an  Erecting  Stereoscopic 
Binocular.    This  conversion  is  effected  by        ^      ,  ^,  ^. 

.,•  «  i  p        '     ^     Stephenson's  Erecting  Binocular, 

the  interposition  oi  a  system  oi  prisms 

similar  to  that  originally  devised  by  MM.  Nachet  (Eig.  17),  but  made 
on  a  larger  scale,  between  an  ^erector  (resembling  that  used  in  the 
eye-piece  of  a  day  telescope)  and  a  pair  of  ordinary  Huyghenian  eye- 
pieces; the  central  ov  dividing  prism  being  placed  at  or  near  the  plane  of 
the  secondary  image  formed  by  the  erector,  w^hile  the  two  eye-pieces  are 
placed  immediately  above  the  two  lateral  prisms;  and  the  combination 
thus  making  that  division  in  the  pencils  forming  the  secondary  image, 
which  in  the  Nachet  Binocular  it  makes  in  the  pencils  emerging  from  the 
objective. — As  all  the  image-forming  rays  have  to  pass  through  the  two 
surfaces  of  four  lenses  and  two  prisms,  besides  sustaining  two  internal 
reflections  in  the  latter,  it  is  surprising  that  Prof.  H.  L.  Smith — while 
admitting  a  loss  of  light — should  feel  able  to  speak  of  the  definition  of  this 
instrument  as  not  inferior  to  that  of  either  the  Wenham  or  the  Nachet 
Binocular.  It  is  obviously  a  great  advantage  that  this  Eye-pieoe  can  be 
used  with  any  microscope,  and  with  Objectives  of  high  power;  but  as  its 
effectiveness  must  depend  upon  extraordinary  accuracy  of  workmanship, 
its  cost  must  necessarily  be  great.  ^ 

38.  Nachefs  Stereo-pseitdoscopic  Binocular. — An  ingenious  modifi- 
cation of  Mr.  Wenham's  arrangement  has  been  introduced  by  MM. 
^'achet;  which  has  the  attribute  altogether  peculiar  to  itself,  of  giving  to 
the  image  either  its  true  Stereoscopic  projection,  or  a  Pseudoscopic  ^  con- 
version of  relief,'  at  the  will  of  the  observer.  This  is  accomplished  by 
the  use  of  two  prisms,  one  of  them  (Eig.  27,  a)  placed  over  the  cone  of 
rays  proceeding  upwards  from  the  objective,  and  the  other  (b)  at  the 
base  of  the  secondary  or  additional  body,  which  is  here  placed  on  the 
right  (Eig.  28).  The  prism  A  has  its  upper  and  lower  surfaces  parallel; 
one  of  its  lateral  faces  is  inclined  at  an  angle  of  45°,  whilst  the  other  is 
vertical.  When  this  is  placed  in  the  position  1,  so  that  its  inclined  surface 
lies  over  the  left  half  {I)  of  the  cone  of  rays,  these  rays,  entering  the 
prism  perpendicularly  (or  nearly  so)  to  its  inferior  plane  surface,  under- 
go total  reflection  at  its  oblique  face,  and  being  thus  turned  into  the 
horizontal  direction,  emerge  through  the  vertical  surface  at  right  angles 
to  it.  They  then  enter  the  vertical  face  of  the  other  prism  b;  and,  after 
suffering  reflection  within  it,  are  transmitted  upwards  into  the  right-hmd 


*  See    American  Journal  of  Science."  vol.  xxxviii.  (1864),  p.  Ill,  and  vol. 
xxxix.  (1865),  p.  212;  and  "Monthly  Microsc.  Journal,"  vol.  vi.  (1871),  p.  45. 
3 


34: 


THE  MICROSCOPE  AND  ITS  REVCLATIONS. 


body  r',  passing  out  of  the  prism  perpendicularly  to  the  plane  of  emersion, 
which  has  such  an  inclination  that  the  right-hand  or  secondary  body  (r, 
Fig.  28)  may  diverge  from  the  left  or  principal  body  at  a  suitable  angle. 
On  the  other  hand,  the  right  half  (r)  of  the  cone  of  rays  passes  upwards, 
without  essential  interruption,  through  the  two  parallel  surfaces  of  the 
prism  A,  into  the  left-hand  body  {V),  and  is  thus  crossed  by  the  other  in 
the  interior  of  the  prism.  But  if  the  prism  A  be  pushed  over  towards  the 
right  (by  pressing  the  button  a,  Fig.  28),  so  as  to  leave  the  left  half  of 
the  objective  uncovered  (as  shown  in  Fig.  27,  2),  that  half  \V)  of  the 


Arrangement  of  Prisms  in  Nachet's  Stereo-pseudoscopic  Binocular:—!,  for  Stereoscopic;  2,  fop 

Pseudoscopic  effect. 

cone  of  rays  will  go  on  without  any  interruption  into  the  left'\\w.^  body 
(?'),  whilst  the  right  half  (r  r')  will  be  reflected  by  the  oblique  face  of  the 
prism  into  the  horizontal  direction,  will  emerge  at  its  vertical  face,  and, 
being  received  by  the  second  prism  B,  will  be  directed  by  it  into  the 
right-h.^mdi  body  (r'). — Now,  in  the  first  position,  the  two  Ixalves  of  the 
cone  of  rays  being  made  to  cross  into  the  opposite  bodies,  true  Stereo- 
scopic relief  is  given  to  the  image  formed  by  their  recombination,  just  as 
in  the  arrangements  previously  described.  But  when,  in  the  second 
position,  each  half  of  the  cone  passes  into  the  body  of  its  own  side,  so 
that  the  reveasal  of  the  images  produced  by  the  Microscope  itself  (§  26) 
is  no  longer  corrected  by  the  crossing  of  the  two  pencils  separated  by  the 
prism  A,  a  Pseudoscopic  effect,  or  ^conversion  of  relief,^  is  produced,  the 
projections  of  the  surface  of  the  object  being  represented  as  hollows,  and 
its  concavities  being  turned  into  convexities.  The  suddenness  with 
which  this  conversion  is  brought  about,  without  any  alteration  in  the 
position  either  of  the  object  or  of  the  observer,  is  a  phenomenon  which 
no  intelligent  person  can  witness  without  interest;  whilst  it  has  a  very 
special  value  for  those  who  study  the  Physiology  and  Psychology  of 
Binocular  vision.^ — As  originally  constructed,  the  adjustment  for  dis- 


'  The  result  of  the  numerous  appHcations  which  the  Author  has  made  of  this 
instrument  to  a  great  variety  of  Microscopic  objects  has  led  to  a  confirmation  of 
the  principle  of  Pseudoscopic  vision,  stated  at  the  conclusion  of  §  31. — Where,  as 
in  the  case  of  the  saucer-like  disks  of  the  Arachnoidiscus  (Plate  xii.),  the  real  and 
the  converted  forms  are  equally  familiar,  the  *  conversion '  either  of  the  convex 
exterior  or  the  concave  interior  is  made  both  suddenly  and  completely.  In  more 
complex  and  less  familiar  forms,  on  the  other  hand,  the  conversion  frequently 
requires  time;  being  often  partial  in  the  first  instance,  and  only  gradually  becom- 
ing complete.  And  there  are  some  objects  which  resist  conversion  altogether,  the 
only  effect  being  a  confusion  of  the  two  images. 


OPTICAL  PRINCIPLES  OF  THE  MICROSCOPE. 


35 


tance  between  the  eyes  was  made  by  giving  a  horizontal  traversing 
motion  to  the  prism  b  and  the  secondary  body  placed  above  it,  by  means 
of  a  screw  action.  But  this  method  was  open  to  the  two  objections  that 
the  focal  distance  of  the  secondary  body  was  thereby 
altered,  and  that  the  traversing  fittings  were  liable 
to  become  loose  by  wear.  To  meet  these,  M.  Nachet 
devised  the  construction  represented  in  Fig.  28;  in 
which  the  adjustment  of  the  distance  between  the 
eye-pieces  is  effected  by  altering  the  angle  of  conver- 
gence between  the  bodies.  This  is  done  by  turning 
the  screw  v,  which  is  furnished  with  two  threads  of 
different  speeds,  whereby  an  inclination  is  given  to 
the  prism  equal  to  half  the  angular  displacement 
of  the  tube;  an  arrangement  necessitated  by  the  fact 
that  the  displacement  of  the  rays  reflected  by  a  rotat- 
ing surface  is  double  the  angle  described  by  that 
surface.^ — As  an  ordinary  working  instrument,  how- 
ever, this  improved  Nachet  Binocular  can  scarcely 
be  equal  to  that  of  Wenham  or  Stephenson;  whilst 
it  must  be  regarded  as  inferior  to  the  former  in  the 
following  particulars :  First,  that  as  the  uninterrupted 
half  of  the  cone  of  rays  (when  the  interposed  prism 
is  adjusted  for  Stereoscopic  vision)  has  to  pass  through 
the  hvo  plane  surfaces  of  the  prism,  a  certain  loss 
of  light  and  deterioration  of  the  picture  are  neces- 
sarily involved;  whilst,  as  the  interrupted  half  of  the  cone  of  rays 
has  to  pass  through  four  surfaces,  the  picture  formed  by  it  is  yet 
more  unfavorably  affected;  second ,  that  as  power  of  motion  must  be 
given  to  loth  prisms — to  A,  for  the  reversal  of  the  images,  and  to  b  for 
the  adjustment  of  the  distance  between  the  two  bodies — there  is  a  greater 
liability  to  derangement.^  It  does  not  give  the  equal  illumination  of  Mr. 
Stephenson's,  is  less  free  from  optical  error,  and  cannot,  like  his,  be 
used  with  high  powers. 

39.  The  Stereoscopic  Binocular  is  put  to  its  most  advantageouse  use, 
when  applied  either  to  opaque  objects  of  whose  solid  forms  we  are 
desirous  of  gaining  an  exact  appreciation,  or  to  transparent  objects 
which  have  such  a  thickness  as  to  make  the  accurate  distinction  between 
their  nearer  and  their  more  remote  planes  a  matter  of  importance. 
That  its  best  and  truest  effects  can  only  be  obtained  by  objectives  not 
exceeding  40^  of  angular  aperture,  may  be  shown  both  theoretically  and 
practically.  Taking  the  average  distance  between  the  pupils  of  the  two 
eyes  as  the  base  of  a  triangle,  and  any  point  of  an  object  placed  at  the 
ordinary  reading  distance  as  its  apex,  the  vertical  angle  inclosed  between 
its  two  sides  will  be  from  12°  to  15°;  which,  in  other  words,  is  the  angle 
of  divergence  between  the  rays  preceding  from  any  point  of  an  object  at 


Nachet's  Stereo-pseudo- 
scopic  Microscope. 


1  Monthly  Microscopical  Journal,"  Vol.  i.  (1869),  p.  31. 

2  M.  Nachet's  arrangement,  like  Mr.  Wenham's,  can  be  adapted  to  any  existing 
Microscope  ;  and  it  seems  peculiarly  suitable  to  those  of  French  or  German  con- 
struction, in  which  the  body  is  much  shorter  than  in  the  ordinary  English  models. 
For  in  the  application  of  the  Wenham  arrangement  to  a  short  Microscope,  the 
requisite  distance  between  the  eye-glasses  of  its  two  bodies  can  only  be  obtained 
by  making  those  bodies  converge  at  an  angle  so  wide  as  to  produce  great  discom- 
fort in  the  use  of  the  instrument,  from  the  necessity  of  maintaining  an  unusual 
degree  of  convergence  between  the  axes  of  the  eyes. 


36 


THE  MICROSCOPE  AND  ITS  REVELATIONS. 


the  ordinary  reading  distance  to  the  two  eyes  respectively.  This  angle, 
therefore,  represents  that  at  which  the  two  pictures  of  an  object  should 
be  taken  in  the  Photographic  Camera,  in  order  to  produce  the  effect  of 
ordinary  binocular  vision  without  exaggeration;  and  it  is  the  one  which 
is  adopted  by  Portrait-photographers,  who  have  found  by  experience  that 
a  smaller  angle  makes  the  image  formed  by  the  combinations  of  the 
pictures  appear  too  flat ^  whilst  a  larger  angle  exaggerates  its  projection. 
Now,  in  applying  this  principle  to  the  Microscope,  we  have  to  treat  the 
two  lateral  halves  (l,  e,  Fig.  29)  of  the  objective  as  the  two  separate 
lenses  of  a  double  portrait-camera;  and  to  consider  at  what  angle  each 
half  should  be  entered  by  the  rays  passing  through  it  to  form  its  picture.' 


To  any  one  acquainted  with  the  principles  of  Optics,  it  must  be  obvious 
that  the  picture  formed  by  each  half  of  the  objective  must  be  (so  to 
speak)  an  average  or  general  resultant  of  the  dissimilar  pictures  formed 
by  its  different  parts.  Thus,  if  we  could  divide  the  lateral  halves  or  semi- 
lenses  L,  R,  of  the  objective  by  vertical  lines  into  the  three  bands  ale 
and  a'  V  and  could  stop  off  the  two  corresponding  bands  on  either 
side,  so  as  only  to  allow  the  light  to  pass  through  the  remaining  pair,  we 
should  find  that  the  two  j)ictures  we  should  receive  of  the  object  would 
vary  sensibly,  according  as  they  are  formed  by  the  bands  a  a',  h  J',  or  c  c\ 


^  The  writer  has  been  surprised  to  find  that  the  advantages  of  the  Stereoscopic 
Binocular  have  been  treated  by  certain  Microscopists  of  eminence  as  altogether 
chimerical;  no  real  difference  (they  assert)  being  discernible  between  the  right-hand 
and  the  left-hand  pictures. — This  assertion  is  obviously  placed  upon  the  limita- 
tion of  the  use  of  the  instrument  to  thin  transparent  objects.  It  is  where  the 
surface  is  uneven  (as  is  the  case  with  most  Opaque  objects^  or  where  a  Trans- 
parent object  shows  different  structures  in  different  planes  of  its  thickness  (as  in 
injected  preparations),  that  the  special  value  of  the  Binocular  shows  itself.  The 
dissimilarity  of  the  two  pictures  of  such  objects  received  through  the  two 
halves  of  the  objective,  was  long  since  demonstrated  by  Mr,  Wenham,  who,  by 
covering  with  a  diaphragm,  first  the  right  and  then  the  left  half  of  an  objective 
of  2-3ds  inch  focus  and  28°  aperture,  and  carefully  drawing  the  two  images  thus 
obtained,  found  them  to  be  such  as  would  combine  stereoscopically,  so  as  to  bring 
out  the  object  in  reUef.  See  "  Transact,  of  Microsc.  Soc,"  N,  S.,  Vol.  ii.  (1854), 
p.  1. 


OPTICAL  PRINCIPLES  OF  THE  MICROSCOPE. 


37 


For,  supposing  the  pictures  taken  through  the  bands  h  V  to  be  suffi- 
ciently dissimilar  in  their  prospective  projections,  to  give,  Avhen  com- 
bined in  the  Microscope,  a  sufficient  but  unexaggerated  Stereoscopic 
relief,  those  taken  through  the  bands  a  a'  on  either  side  of  the  centre 
would  be  no  more  dissimilar  than  two  portraits  taken  at  a  very  small 
angle  between  the  cameras,  and  their  combination  would  very  inade- 
quately bring  out  the  effect  of  relief;  whilst,  on  the  other  hand,  the  two 
j)ictures  taken  through  the  extreme  lateral  bands  c  g%  would  differ  as 
widely  as  portraits  taken  at  too  great  an  angle  of  divergence  between  the 
cameras,  and  their  combination  would  exaggerate  the  actual  relief  of  the 
object.  Now,  in  each  of  the  lateral  halves,  a  spot  v  v'  may  be  found  by 
mathematical  computation,*  which  may  be  designated  the  visual  centre  of  ^ 
the  whole  Semi-lens;  that  is,  the  spot  which,  if  all  the  rest  of  the  semi- 
lens  were  stopped-off,  would  form  a  picture  most  nearly  corresponding 
to  that  given  iDy  the  whole  of  it.  This  having  been  determined,  it  is 
easy  to  ascertain  what  should  be  the  angle  of  aperture  (op  Fig.  30)  of 
the  entire  lens,  in  order  that  the  angles  v  p  v'  between  the  '  visual 
centres'  of  its  two  halves  should  be  15^.  The  investigation  of  this 
question  having  been  kindly  undertaken  for  the  author  by  his  friend  Dr. 
Hirst,  the  conclusion  at  which  he  arrived  was  that  the  angle  of  aperture 
of  the  entire  lens  should  be  about  36.6"^.  This,  which  he  gave  as  an 
approximate  result  only  (the  requisite  data  for  a  complete  Mathematical 
solution  of  the  question  not  having  yet  been  obtained),  harmonizes  most 
remarkably  with  the  results  of  experimental  observations  made  upon 
opaque  objects  of  knoiun  shape,  with  Objectives  of  different  angular 
apertures;  so  that  the  Stereoscoj)ic  images  produced  by  the  several 
objectives  may  be  compared,  not  only  with  each  other,  but  with  the 
actual  forms  which  they  ought  to  present.  No  better  objects  can  be 
selected  for  this  purpose  than  those  which  are  perfectly  spherical ;  such 
as  various  globular  forms  of  the  Polycystina  (Plate  xix.),  or  the  Pollen- 
grains  of  the  Malvacece  and  many  other  Flowering-plants.  When  either 
of  these  is  placed  under  a  Stereoscopic  Binocular,  provided  with  an 
objective  of  half-inch  or  4-lOths  inch  focus  having  an  angular  aperture 
of  80°  or  90°,  the  effect  of  projection  is  so  greatly  exaggerated,  that  the 
side  next  the  eye,  instead  of  resembling  a  hemisphere,  looks  like  the 
small  end  of  an  egg.  If,  then,  the  aperture  of  such  an  objective  be 
reduced  to  60°  by  a  diaphragm  placed  behind  its  back  lens,  the  exaggera- 
tion is  diminished,  though  not  removed;  the  hemispherical  circle  now 
looking  like  the  large  end  'of  an  egg.  But  if  the  aperture  be  further 
reduced  to  40°  by  the  same  means,  it  is  at  once  seen  that  the  hemi- 
spheres turned  towards  the  eye  are  truly  represented;  the  effect  of 
spherical  projection  being  quite  adequate,  without  being  in  the  least  exag- 
gerated. Hence  it  may  be  confidently  affirmed — alike  on  theoretical  and 
on  practical  grounds — that  when  an  objective  of  wider  angle  than  40°  is 
used  with  the  Stereoscopic  Binocular,  the  object  viewed  by  it  is  repre- 
sented in  exaggerated  relief,  so  that  its  apparent  form  must  be  more  or 
less  distorted.^ — There  are  other  substantial  reasons,  moreover,  why 


^  This  position  has  been  contested  by  observers  who  have  used  high  powers 
binocularly  with  transparent  objects,  and  who,  in  their  zeal  for  large  angles  of 
aperture,  affirm  that  no  exaggeration  of  Stereoscopic  effect  is  produced  by  the 
combination  of  the  two  pictures  thus  obtained.  But  it  seems  to  be  forgotten 
that  such  objects  cannot  afford  the  actual  measure  of  Stereoscopic  effect,  which 
is  given  by  opaque  objects  of  known  form — as  above  described.  And,  so  far  as  tihe 
Author's  experience  extends,  every  competent  observer  who  makes  use  of  a  good 


38 


THE  MICROSCOPE  AND  ITS  REVELA.TIONS. 


Objectives  of  limited  angle  of  aperture  should  be  preferred  (save  in 
particular  cases)  for  use  with  the  Stereoscopic  Binocular.  ^  As  the  special 
value  of  this  instrument  is  to  convey  to  the  mind  a  notion  of  the  solid 
forms  of  objects,  and  of  the  relations  of  their  parts  to  each  other,  not 
merely  on  the  same,  but  on  different  planes,  it  is  obvious  that  those 
Objectives  are  most  suitable  to  produce  this  effect,  which  possess  the 
greatest  amount  of  penetration  or  focal  depth  ;  that  is,  which  most  dis- 
tinctly show,  not  merely  what  is  precisely  in  the  focal  plane,  but  what  lies 
nearer  to  or  more  remote  from  the  objective.  Now,  as  will  be  explained 
hereafter  (§  158,  ii.),  increase  of  the  angle  of  apei'ture  is  necessarily 
attended  with  diminution  of  ^penetrating'  power;  »o  that  an  objective 
,  of  60°  or  80°  of  aperture,  though  exhibiting  minute  surface-details  which 
an  objective  of  40°  cannot  show,  is  much  inferior  to  it  in  suitability  to 
convey  a  true  conception  of  the  general  form  of  any  object,  the  parts  of 
which  project  considerably  above  the  focal  plane  or  recede  below  it. 

40.  In  concluding  these  general  observations  upon  the  use  of  the  Ste- 
reoscopic Binocular,  the  Author  would  draw  attention  to  two  important 
advantages  he  has  found  it  to  possess;  his  own  experience  on  these  points 
being  fully  confirmed  by  that  of  others. — In  the^r^^  place,  the  penetrat- 
ing poioer  or  focal  depth  of  the  Binocular  is  greatly  superior  to  that  of  the 
Monocular  microscope;  so  that  an  object  whose  surface  presents  consider- 
able inequalities  is  very  much  more  distinctly  seen  with  the  former  than 
with  the  latter.  The  difference  may  in  part  be  attributed  to  the  practical 
reduction  in  the  angle  of  aperture  of  the  Objective,  which  is  produced  by 
the  division  of  the  cone  of  rays  transmitted  through  it  into  two  halves; 
so  that  the  picture  received  through  each  half  of  an  Objective  of  60^  is 
formed  by  rays  diverging  at  an  angle  of  only  30°.  But  that  this  optical 
explanation  does  not  go  far  to  account  for  the  fact,  is  easily  proved  by  the 
simple  experiment  of  looking  at  the  object  in  the  first  instance  through 
each  eye  separately  (the  prism  being  in  place),  and  then  with  both  eyes 
together;  the  distinctness  of  the  parts  which  lie  above  and  beneath  the 
focal  plane  being  found  to  be  much  greater  when  the  two  pictures  are 
combined,  than  it  is  in  either  of  them  separately.  In  the  absence  of  any 
adequate  optical  explanation  of  the  greater  range  of  focal  depth  thus 
shown  to  be  possessed,  by  the  Stereoscopic  Binocular,  the  Author  is 
inclined  to  attribute  it  to  an  allowance  for  the  relative  distances  of  the 
parts,  which  seems  to  be  unconsciously  made  by  the  mi7id  of  the  ob- 
server, when  the  solid  image  is  shaped  out  in  it  by  the  combination  of 
the  two  pictures. — This  seems  the  more  likely  from  the  second  fact  to  be 


half-inch  Objective  of  40°  aperture — resembling  the  one  first  constructed  to  his 
order  by  Messrs.  Powell  and  Lealand,  and  now  procurable  from  several  excellent 
rnakers— in  the  study  of  Folycystina,  the  smaller  Foraminifera,  or  the  larger 
discoidal  Diatoms,  viewed  as  opaque  objects,  soon  becomes  sensible  of  its  advan- 
tage over  Objectives  of  the  same  power  but  of  larger  angular  aperture,  in  giving 
(1)  unexaggerated  relief,  (2)  much  greater  focal  depth,  and  (3)  such  a  working 
distance  as  enables  side-illumination  to  be  conveniently  used.  Having  lately  had 
occasion  to  give  much  attention  to  the  structure  and  development  of  Isthmia 
(Chap.  VII.),  the  writer  has  found  great  advantage  from  the  use  of  a  l-4th 
objective,  constructed  by  Zeiss,  of  what  will  be  considered  by  many  the  absurdly 
low  angle  of  40°;  the  truth  of  the  conception  it  gives  of  the  solid  forms  of  the 
frustules  (when  viewed  as  opaque  objects),  which  is  capable  of  easy  verification, 
being  in  striking  contrast  with  the  violent  exaggeration  of  relief  which  is  pro- 
duced when  the  same  objects  are  similarly  viewed  through  a  l-4th  inch  of  90°  or 
120°  aperture.  Doubtless  the  elementary  structure  of  the  frustule  can  only  be 
properly  studied  by  an  Objective  of  large  angle;  but  this  is  an  altogether  different 
inquiry. 


OPTICAL  PRINCIPLES  OF  THE  MICROSCOPE. 


39 


now  me'iitioned:  namely,  that  when  the  Binocular  is  employed  upon 
objects  suited  to  its  powers,  the  prolonged  use  of  it  is  attended  with  very 
much  less  fatigue  than  is  that  of  the  Monocular  Microscope.  This,  again, 
may  be  in  some  degree  attributed  to  the  division  of  the  work  between  the 
two  eyes;  but  the  Author  is  satisfied  that,  unless  there  is  a  feeling  of  dis- 
comfort m  the  eye  itself,  the  sense  of  fatigue  is  rather  mental  than  visual, 
and  that  it  proceeds  from  the  constructive  effort  which  the  observer  has 
to  make,  who  aims  at  realizing  the  solid  form  of  the  object  he  is  examin- 
ing, by  an  interpretation  based  on  the  flat  picture  of  it  presented  by  his 
vision,  aided  only  by  the  use  of  the  focal  adjustment,  which  enables  him 
to  determine  what  are  its  near  and  what  its  remote  parts,  and  to  form  an 
estimate  of  their  difference  of  distance.  Now,  a  great  part  of  this  con- 
structive effort  is  saved  by  the  use  of  the  Binocular;  which  at  once  brings 
before  the  Mind's  eye  the  solid  image  of  the  object,  and  thus  gives  to  the 
observer  a  conception  of  its  form  usually  more  complete  and  accurate 
than  he  could  derive  from  any  amount  of  study  of  a  Monocular  picture.* 


'  It  has  happened  to  the  Author  to  be  frequently  called  on  to  explain  the  ad- 
vantages of  the  Binocular  to  Continental  (especially  German)  savans,  who  had  not 
been  previously  acquainted  with  the  instrument.  And  he  has  been  struck  with 
finding  that  when  he  exhibited  to  them  objects  with  which  they  had  already 
become  familiar  by  careful  study,  and  of  whose  solid  forms  they  had  attained  an 
accurate  conception,  they  perceived  no  advantage  in  the  Stereoscopic  combina- 
tion, seeiiig  such  objects  with  it  (visually)  just  as  they  had  been  previously  accus- 
tomed to  see  them  (mentally)  without  it.  But  wlien  he  has  exhibited  to  them 
suitable  objects  with  which  they  had  not  been  previously  familiarized,  and  has 
caused  them  to  look  at  these  in  the  first  instance  monocularly,  and  then  stereo- 
scopically,  he  has  never  failed  to  satisfy  them  of  the  value  of  the  latter  method, 
except  when  some  visual  imperfection  has  prevented  them  from  properly  appre- 
ciating it.  He  may  mention  that  he  has  found  the  wing  ©f  the  little  Moth  known 
as  Zenzera  CEsculi,  which  has  an  undulating  surface,  whereon  the  scales  are  set 
at  various  angles,  instead  of  having  the  usual  imbricated  arrangement,  a  pecu- 
liarly appropriate  object  for  this  demonstration.  The  general  inequality  of  its 
surface,  and  the  individual  obliquities  of  its  scales,  being  at  once  shown  by  the 
Binocular,  with  a  force  and  completeness  which  could  not  be  attained  by  the  most 
prolonged  and  careful  Monocular  study. 


THE  MICKOSCOFE  AND  ITS  REVELATIONS. 


OHAPTEE  II. 

CONSTRUCTION  OF  THE  MICROSCOPE. 

41.  The  ojjtical  jDrinciples  whereon  the  operation  of  the  Microscope 
depends  having  now  been  explained,  we  have  next  to  consider  the  mechani- 
cal provisions  whereby  they  are  brought  to  bear  upon  the  different 
purposes  which  the  instrument  is  destined  to  serve.  And  first,  it  will  be 
desirable  to  state  those  general  principles  which  have  now  received  the 
sanction  of  universal  experience,  in  regard  to  the  best  arrangement  of  its 
constituent  parts. — Every  cornplete  Microscope,  whether  Simple  or  Com- 
pound, must  possess,  in  addition  to  the  lens  or  combination  of  lenses 
which  affords  its  magnifying  power,  a  stage  whereon  the  Object  may 
securely  rest,  a  concave  mirror  for  the  illumination  of  transparent  objects 
from  beneath,  and  a  condensing -lens  for  the  illumination  of  opaque  objects 
from  above. 

I.  Now,  in  whatever  mode  these  may  be  connected  with  each  other, 
it  is  essential  that  the  Optical  part  and  the  Stage  should  he  so  disposed,  as 
either  to  he  altogether  free  from  tendency  to  vibratio7i,  or  to  vibrate  together; 
since  it  is  obvious  that  any  movement  of  one,  in  which  the  other  does  not 
partake,  will  be  augmented  to  the  eye  of  the  observer  in  proportion  to  the 
magnifying  power  employed.  In  a  badly-constructed  instrument,  even 
though  placed  upon  a  steady  table  resting  upon  the  firm  floor  of  a  well- 
built  house,  when  high  powers  are  used,  the  object  is  seen  to  oscillate  so 
rapidly  at  the  slightest  tremor — such  as  that  caused  by  a  person  walking 
across  the  room,  or  by  a  carriage  rolling-by  in  the  street — as  to  be  fre- 
quently almost  indistinguishable:  whereas  in  a  well-constructed  instru- 
ment, scarcely  any  perceptible  effect  will  be  produced  by  even  greater  dis- 
turbances. Hence,  in  the  choice  of  a  Microscope,  it  should  always  be 
subjected  to  this  test,  and  should  be  unhesitatingly  rejected  if  the  result 
be  unfavorable.  If  the  instrument  should  be  found  free  from  fault  when 
thus  tested  with  high  powers,  its  steadiness  with  low  powers  may  be 
assumed;  but,  on  the  other  hand,  though  a  Microscope  may  give  an  image 
free  from  perceptible  tremor  when  the  lower  powers  only  are  employed,  it 
may  be  quite  unfit  for  use  with  the  higher. — The  Author  has  found  no 
test  for  steadiness  so  crucial  as  the  vibration  of  a  paddle-steamer  going  at 
full  speed  against  a  head-sea;  and  the  result  of  his  comparison  between 
the  two  principal  ^models'  generally  used  in  this  country  will  be  stated 
hereafter  (§  49). 

II.  The  next  requisite  is  a  capahility  of  accurate  adjust^nent  to  every 
variety  of  focal  distance,  without  movement  of  the  object.  It  is  a  principle 
univorsally  recognized  in  the  construction  of  good  Microscopes,  that  the 
stafj'3  whereon  the  object  is  placed  should  be  a  fixture;  the  movement  by 
which  the  focus  is  to  be  adj  3ted  being  given  to  the  optical  portion. 
This  movement  should  be  such  as  to  allow  free  range  from  a  minute  fraction 


CONSTRUCTION  OF  THE  MICK06COPE. 


41 


of  an  inch  to  three  or  four  inches,  with  equal  j)ower  of  obtaining  a  delicate 
adjustment  at  any  part.  It  should  also  be  so  accurate,  that  the  optic 
axis  of  the  instrument  should  not  be  in  the  least  altered  by  any  move- 
ment in  a  yertical  direction;  so  that  if  an  object  be  brought  into  the 
centre  of  the  field  with  a  low  power,  and  a  high  power  be  then  substi- 
tuted, the  object  should  be  found  in  the  centre  of  its  field,  notwithstand- 
ing the  great  alteration  in  the  focus.  In  this  way  much  time  may  often 
be  saved  by  employing  a  low  power  as  a  finder  for  an  object  to  be 
examined  by  a  higher  one;  and  when  an  object  is  being  viewed  by  a 
succession  of  powers,  little  or  no  readjustment  of  its  place  on  the  stage 
should  be  required.  For  the  Simple  Microscope,  in  which  it  is  seldom 
advantageous  to  use  lenses  of  shorter  focus  than  l-4th  inch  (save  where 
^ doublets^  are  employed,  §  23),  a  rack-and-pinion  adjustment,  if  it  be 
made  to  work  both  tightly  and  smoothly,  answers  sufficiently  well;  and 
this  is  quite  adequate  also  for  the  focal  adjustment  of  the  Compound  body, 
when  objectives  of  low  power  only  are  employed.  But  for  any  lenses 
whose  focus  is  less  than  half-an-inch,  a  '  fine  adjustment,'  or  '  slow  motion,' 
by  means  of  a  screw-movement  operating  either  on  the  object-glass  alone 
or  on  the  entire  body,  is  of  great  value;  and  for  the  highest  powers  it  is 
quite  indispensable.  In  some  Microscopes,  indeed,  which  are  provided 
with  a  ^fine  adjustment,'  the  rack-and-pinion  movement  is  dispensed 
with,  the  '  coarse  adjustment '  being  given  by  merely  sliding  the  body  up 
and  down  in  the  socket  which  grasps  it;  but  this  plan  is  only  admissible 
where,  for  the  sake  of  extreme  cheapness  or  portability,  the  instrument 
has  to  be  reduced  to  the  form  of  utmost  simplicity. 

III.  Scarcely  less  important  than  the  preceding  requisite,  in  the  case 
of  the  Compound  Microscope,  especially  with  the  long  body  of  the 
ordinary  English  model,  is  the  capability  of  being  placed  in  either  a 
vertical  or  a  horizontal  position^  or  at  any  angle  with  the  horizon,  without 
deranging  the  adjustment  of  its  parts  to  each  other,  and  without  placing 
the  eye-piece  in  such  a  position  as  to  be  inconvenient  to  the  observer. 
It  is  certainly  a  matter  of  surprise,  that  some  Microscopists,  especially  on 
the  Continent,  should  still  forego  the  advantages  of  the  inclined  position, 
these  being  attainable  by  a  very  small  addition  to  the  cost  of  the  instru- 
ment; but  the  inconvenience  of  the  vertical  arrangement  is  much  less 
when  the  body  of  the  microscope  is  short,  as  in  the  ordinary  Continental 
model;  and  there  are  many  cases  in  which  it  is  absolutely  necessary  that 
the  stage  should  be  horizontal.  This  position,  however,  can  at  any  time 
be  given  to  the  stage  of  the  inclining  Microscope,  by  bringing  the  optic 
axis  of  the  instrument  into  the  vertical  direction.  And  even  with  the 
stage  horizontal,  a  convenient  inclination  may  be  given  to  the  visual  axis, 
not  merely  by  such  modifications  in  general  construction  as  constitute 
the  special  features  of  the  erecting  Binocular  of  Mr.  Stephenson  (§  36) 
or  the  Inverted  Microscope  of  Dr.  Lawrence  Smith  (§  80),  but  by  the 
application  to  the  ordinary  vertical  body  of  the  erecting  eye-piece  of  M. 
Natchet  (§  86). — In  ordinary  cases  an  inclination  of  the  body  at  the  angle 
of  about  55°  to  the  horizon  will  usually  be  found  most  convenient  for 
unconstrained  observation;  and  the  instrument  should  be  so  constructed, 
as,  when  thus  inclined,  to  give  to  the  stage  such  an  elevation  above  the 
table,  that,  when  the  hands  are  employed  at  it,  the  arms  may  rest  con- 
veniently upon  the  table.  In  this  manner  a  degree  of  support  is  attained, 
which  gives  such  free  play  to  the  muscles  of  the  hands,  that  movements 
of  the  greatest  nicety  may  be  executed  by  them;  and  the  fatigue  of  long- 
continued  observation  is  greatly  diminished.    Such  minutiae  may  appear 


42 


THE  MICROSCOPE  AKD  ITS  EEYELATIONS. 


too  trivial  to  deserve  mention;  but  no  practised  Microscopist  will  be  slow 
to  acknowledge  tlieir  value. — For  otlier  purposes,  again,  ifc  is  requisite 
that  the  Microscope  should  be  placed  horizontally,  as  when  the  Camera 
Lucida  is  used  for  drawing  or  measuring.  It  ought,  therefore,  to  be 
made  capable  of  every  such  variety  of  position;  and  the  Stage  must  of 
course  be  provided  with  some  means  of  holding  the  object,  when  it  is 
itself  placed  in  a  position  so  inclined  that  the  object  would  slip  down 
unless  sustained. 

lY.  The  last  principle  on  which  we  shall  here  dwell,  as  essential  to  the 
value  of  a  Microscope  designed  for  ordinary  work,  is  Simplicity  in  the 
construction  and  adjustment  of  every  part.  Many  ingenious  mechanical 
devices  have  been  invented  and  executed,  for  the  purpose  of  overcoming 
difficulties  which  are  in  themselves  really  trivial.  A  moderate  amount 
of  dexterity  in  the  use  of  the  hands  is  sufficient  to  render  most  of  these 
superfluous;  and  without  such  dexterity,  no  one  even  with  the  most  com- 
plete mechanical  facilities,  will  ever  become  a  good  Microscopist.  Among 
the  conveniences  of  simplicity,  the  practised  Microscopist  will  not  fail  to 
recognize  the  saving  of  time  effected  by  being  able  quickly  to  set  up  and 
put  away  his  instrument.  Where  a  number  of  parts  are  to  be  screwed 
together  before  it  can  be  brought  into  use,  interesting  objects  (as  well  as 
time)  are  not  unfrequently  lost;  and  the  same  cause  will  often  occasion 
the  instrument  to  be  left  exposed  to  the  air  and  dust,  to  its  great  detri- 
ment, because  time  is  required  to  put  it  away;  so  that  a  slight  advantage 
on  the  side  of  simplicity  of  arrangement  often  causes  an  inferior  instru- 
ment to  be  preferred  by  the  working  Microscopist  to  a  superior  one.  Yet 
there  is,  of  course,  a  limit  to  this  simplification;  and  no  arrangement  can 
be  objected-to  on  this  score,  which  gives  advantages  in  the  examination 
of  difficult  objects,  or  in  the  determination  of  doubtful  questions,  such  as 
no  simpler  means  can  afford. — The  meaning  of  this  distinction  will  be- 
come apparent,  if  it  be  applied  to  the  cases  of  the  Mechanical  Stage  and 
the  Achromatic  Condenser,  For  although  the  Mechanical  Stage  many 
be  considered  a  valuable  aid  in  observation,  as  facilitating  the  finding  of 
a  minute  object,  or  the  examimation  of  the  entire  surface  of  a  large  one, 
yet  it  adds  nothing  to  the  clearness  of  our  view  of  either;  and  its  place 
may  in  great  degree  be  supplied  by  the  fingers  of  a  good  manipulator. 
On  the  other  hand,  the  use  of  the  Achromatic  Condenser  not  only  con- 
tributes very  materially,  but  is  absolutely  indispensable,  to  the  formation 
of  a  perfect  image,  in  the  case  of  many  objects  of  a  difficult  class:  the  want 
of  it  cannot  be  compensated  by  the  most  dexterous  use  of  the  ordinary 
appliances;  and  consequently,  although  it  may  fairly  be  considered  super- 
fluous as  regards  a  large  proportion  of  the  purposes  to  which  the  Microscope 
is  directed,  whether  for  investigation  or  for  display,  yet  as  regards  the 
particular  objects  just  alluded  to,  it  must  be  considered  as  no  less  neces- 
sary a  part  of  the  instrument  than  the  Achromatic  Objective  itself. 
Where  expense  is  not  an  object,  the  Microscope  should  doubtless  be  fitted 
with  both  these  valuable  accessories;  where,  on  the  other  hand,  the  cost 
is  so  limited  that  only  07^6  can  be  afforded,  that  one  should  be  selected 
which  will  make  the  instrument  most  useful  for  the  purposes  to  which  it 
is  likely  to  be  applied. 


In  the  account  now  to  be  given  of  the  principal  forms  of  Microscope 
readily  procurable  in  this  country,  it  will  be  the  Author's  object,  not  so 
much  to  enumerate  and  describe  the  various  patterns  which  the  several 


CONSTRUCTION  OF  THE  MICROSCOPE. 


4:3 


Makers  of  the  instrument  have  produced;  as,  by  selecting  from  among 
them  those  examples  which  it  seems  to  him  most  desirable  to  make  known, 
and  by  specifying  the  peculiar  advantages  which  each  of  these  presents, 
to  guide  his  readers  in  the  choice  of  the  kind  of  microscope  best  suited 
on  the  one  hand,  to  the  class  of  investigations  they  may  be  desirous  of 
following  out,  and,  on  the  other,  to  their  pecuniary  ability.  He  is 
anxious,  however,  that  he  should  not  be  supposed  to  mark  any  preference 
for  the  particular  instruments  he  has  selected,  over  those  constructed  upon 
the  same  general  plan  by  other  Makers.  To  have  enumerated  them  all, 
would  obviously  be  quite  incompatible  with  the  plan  of  his  Treatise;  but 
he  has  considered  it  fair  (save  in  one  or  two  special  cases)  to  give  the 
preference  to  those  Makers  who  have  worked  out  their  own  plans  of  con- 
struction, and  have  thus  furnished  (to  say  the  least)  the  general  designs 
which  have  been  adopted  with  more  or  less  of  modification  by  others. 

SIMPLE  MICROSCOPES. 

42.  Under  this  head,  the  common  Hand-Magnifier  or  pocket  lens  first 
claims  our  attention;  being  in  reality  a  Simple  Microscope,  although  not 
commonly  accounted  as  such.  Although  this  little  instrument  is  in  every 
one's  hands,  and  is  indispensable  to  the  Naturalist — furnishing  him  with 
the  means  of  at  once  making  such  preliminary  examinations  as  often 
afford  him  most  important  guidance — yet  there  are  comparatively  few 
who  know  how  to  handle  it  to  the  best  advantage.  The  chief  difficulty 
lies  in  the  steady  fixation  of  it  at  the  requisite  distance  from  the  object; 
especially  when  the  lens  employed  is  of  such  short  focus,  that  the  slight- 
est want  of  exactness  in  this  adjustment  produces  evident  indistinctness 
of  the  image.  By  carefully  resting  the  hand  which  carries  the  glass, 
however,  against  that  which  carries  the  object,  so  that  both,  whenever 
they  move,  shall  move  together,  the  observer,  after  a  little  practice,  will  be 
able  to  employ  even  high  powers  with  comparative  facility.  The  lenses 
most  generally  serviceable  for  Hand-magnifiers  range  in  focal  length 
from  two  inches  to  half  an  inch;  and  a  combination  of  two  or  three  such 
in  the  same  handle,  with  an  intervening  perforated  plate  of  tortoiseshell 
(which  serves  as  a  diaphragm  when  they  are  used  together),  will  be  found 
very  useful.  When  such  a  magnifying  power  is  desired,  as  would  require 
a  lens  of  a  quarter  of  an  inch  focus,  it  is  best  obtained  by  the  substitution 
of  a  '  Coddington'  (§  24),  or,  still  better,  of  the  Browning  or  the  Stein- 
heil  Doublet  (§  25),  for  the  ordinary  double-convex  lens.  The  handle 
of  the  magnifier  may  be  pierced  with  a  hole  at  the  end,  most  distant 
from  the  joint  by  which  the  lenses  are  attached  to  it;  and  through  this 
may  be  passed  a  wire,  which,  being  fitted  vertically  into  a  stand  or 
foot,  serves  for  the  support  of  the  magnifying  lenses  in  a  horizontal  posi- 
tion, at  any  height  at  which  it  may  be  covenient  to  fix  them.  Such  a 
little  apparatus  is  a  rudimentary  form  (so  to  speak)  of  what  is  commonly 
understood  as  a  Simple  Microscope;  the  term  being  usually  applied  to 
those  instruments  in  which  the  magnifying  powers  are  supported  other- 
wise than  in  the  hand,  or  in  which,  if  the  whole  apparatus  be  supported 
by  the  hand,  the  lenses  have  a  fixed  bearing  upon  the  object. 

43.  Rosses  Simple  Microscope, — This  instrument  holds  an  intermedi- 
ate place  between  the  Hand -magnifier  and  the  complete  microscope  ;  be- 
ing, in  fact,  nothing  more  than  a  lens  supported  in  such  a  manner  as 
to  be  capable  of  being  readily  fixed  in  a  variety  of  positions  suitable  for 
dissecting  and  for  other  manipulations.    It  consists  of  a  circular  brass 


44 


THE  MICROSCOPE  AND  ITS  REVELATIONS. 


foot,  •wherein  is  screwed  a  short  tubular  pillar  (Fig.  31),  which  is  ^  sprung ' 
at  its  upper  end,  so  as  to  grasp  a  second  tube,  also  ^  sprung,^  by  the  draw- 
ing-out of  which  the  pillar  may  be  elongated  to  about  3  inches.  This 
carries  at  its  upper  end  a  jointed  socket,  through  which  a  square  bar 
about  3^  inches  long  slides  rather  sfciffly;  and  one  end  of  this  bar  carries 
another  joint,  to  which  is  attached  a  ring  for  holding  the  lenses.  By 
lengthening  or  shortening  the  pillar,  by  varying  the  angle  which  the 
square  bar  makes  with  its  summit,  and  by  sliding  that  bar  through  the 
socket,  almost  any  position  and  elevation  may  be  given  to  the  lens,  that 
can  be  required  for  the  purposes  to  which  it  may  be  most  usefully  ap- 
plied ;  care  being  taken  in  all  instances,  that  the  ring  which  carries  the 
lens  should  (by  means  of  its  joint)  be  placed  horizontally.  At  A  is  seen 
the  position  which  adapts  it  best  for  picking  out  minute  shells,  or  for 
other  similar  manipulations ;  the  sand  or  dredgings  to  be  examined  being 
spread  upon  a  piece  of  black  paper,  and  raised  upon  a  book,  a  box,  or 
some  other  support,  to  such  a  height  that  when  the  lens  is  adjusted 

Fig.  31. 

A  S 


Ross's  Simple  Microscope. 


thereto,  tne  eye  may  De  applied  to  it  continuously  without  unnecessary 
fatigue.  It  will  be  found  advantageous  that  the  foot  of  the  microscope 
should  not  stand  upon  the  paper  over  which  the  objects  are  spread,  as  it 
is  desirable  to  shake  this  from  time  to  time  in  order  to  bring  a  fresh  por- 
tion of  the  matters  to  be  examined  into  view ;  and  generally  speaking, 
it  will  be  found  convenient  to  place  it  on  the  opposite  side  of  the  object, 
rather  than  on  the  same  side  with  the  observer.  At  B  is  shown  the  po- 
sition in  which  it  may  be  most  conveniently  set  for  the  dissection  of  ob- 
jects contained  in  a  plate  or  trough,  the  sides  of  which,  being  higher 
than  the  lens,  would  prevent  the  use  of  any  magnifier  mounted  on  a 
Horizontal  arm. — The  powers  usually  supplied  with  this  instrument  are 
one  of  an  inch  focus,  and  a  second  of  either  a  half  or  a  quarter  of  an  inch. 
By  unscrewing  the  pillar,  the  whole  is  made  to  pack  into  a  small  flat  case, 
the  extreme  portability  of  which  is  a  great  recommendation.  Although 
the  uses  of  this  little  instrument  are  greatly  limited  by  its  want  of  stage, 


CONSTRUCTION  OF  THE  MICROSCOPE, 


45 


mirror,  etc.,  yet,  for  the  class  of  purposes  to  wliicli  it  is  suited,  it  has 
advantages  over  perhaps  every  other  form  that  has  been  devised. 

44.  QueJcetfs  Dissecting  Micro- 
scope,— By  the  Scientific  investiga- 
tor who  desi;.^es  a  large  flat  stage, 
combined  with  portability,  the  ar- 
rangement devised  by  Mr.  John 
Quekett  (Fig.  32)  will^be  found  ex- 
tremely convenient.  The  Stage, 
whicli  constitutes  the  principal  part 
of  the  apparatus,  is  a  plate  of  brass 
(bronzed')  nearly  six  inches  square, 
screwed  to  a  piece  of  mahogany  of 
the  same  size,  and  about  5-8ths  of  an 
inch  thick;  underneath  this  is  a  fold- 
ing flap  four  inches  broad,  attached 
on  each  side  by  hinges;  and  the  two 
flaps  are  so  shaped,  that,  when  folded 
together,  one  lies  closely  upon  the 
other,  as  shown  at  b,  Fig.  32,  whilst, 
when  opened,  as  shown  at  A,  they 
give  a  firm  support  to  the  stage  at 
a  convenient  height.^  At  the  back 
of  the  stage-plate  is  a  round  hole, 
through  which  a  tubular  stem  works 
vertically  with  a  rack-and-pinion 
movement,  carrying  at  its  summit 
the  horizontal  Arm  for  the  magnify- 


ing powers ;  and 


into  the  underside  '^''''''''l^l^Zl^i.f^'Z^l^r^tl. 


of  the  stage-plate  there  screws  a  stem 
which  carries  the  mirror-frame.  From  this  frame  the  mirror  may  be  re- 
moved, and  its  place  supplied  by  a  convex  lens,  which  serves  as  a  condenser 
for  opaque  objects,  its  stem  being  then  fitted  into  a  hole  in  the  stage,  at 
one  side  or  in  front  of  its  central  perforation.  The  instrument  is  usually 
furnished  with  three  magnifiers — namely,  an  inch  and  a  half -inch  ordinary 
lenses,  and  a  quarter-inch  Coddington;  and  these  (or  the  combinations  of 
equivalent  foci  already  mentioned,  §  25),  will  be  found  to  be  the  powers 
most  useful  for  the  purposes  to  which  it  is  specially  adapted.  As  a  black 
back-ground  is  often  required  in  dissecting  objects  which  are  not  transpa- 
rent, this  may  be  most  readily  provided  by  attaching  a  disk  of  dead-hl^ck 
paper  to  the  back  of  the  mirror.  The  lenses,  mirror,  condenser,  vertical 
stem,  and  milled-head,  all  fit  into  a  drawer  which  shuts  into  the  under- 
side of  the  stage  ;  so  that,  when  packed  together,  and  the  flaps  kept 
down  by  an  elastic  band,  as  shown  at  b,  Fig.  32,  the  instrument  is 
extremely  portable,  furnishing  (so  to  speak)  a  case  for  itself.  It  may  be 
easily  made  with  an  addittional  arm  carrying  a  light  Compound  body. 


^  The  Stage-plate  is  sometimes  made  of  plate-glass  or  ebonite  ;  and  this  is 
decidedly  advantageous  where  Sea- water  or  Acids  are  used. 

^  The  Stage  is  now  more  generally  supported,  either  (as  in  Mr.  Ladd's  model) 
on  four  legs  of  strong  brass  wire,  which  screw  into  its  underside,  and  are  packed 
in  its  drawer  when  dismounted  ;  or  (as  made  by  Mr.  Swift  and  Messrs.  Parkes  of 
Birmingham)  on  four  brass  legs  which  fold  beneath  it ; — either  of  these  construc- 
tions remedying  the  chief  disadvantage  of  the  original  model,  which  consists  in 
the  exclusion  of  side  light  from  the  mirror. 


46 


THE  MICBOSCOPB  AND  ITS  KEVELATIONS. 


furnished  with  objectives  suitable  for  the  examination  of  dissections  or 
other  preparations  made  upon  the  stage,  without  disturbing  them  by 
moval  to  another  instrument. 
45.  Sieiert  and  Kraft's  Dissecting  Microscope. — In  making  minute 
dissections,  however,  the  hands  are  most  advantageously  rested,  not  on  the 
stage  itself,  but  on  supports  at  a  level  intermediate  between  that  of  the 


Siebert  and  Kraft*s  Dissecting  Microscope,  as  folded  in  case. 

stage  and  that  of  the  table.  Such  a  support,  in  some  Continental  Dissect- 
ing Microscopes — as  those  of  Nachet  and  Zeiss — is  attached  to  each  side 
of  the  stage  of  an  ordinary  Simple  Microscope  ;  but  this  arrangement  is 
subject  to  the  disadvantage  of  causing  the  whole  weight  of  the  hands  to  bear 
on  the  stage,  so  as,  by  depressing  it,  to  throw  the  object  out  of  focus,  unless 
the  stage  be  made  of  extraordinary  solidity,  or  be  supported  in  front  as 


CONSTRUCTION  OF  THE  MICROSCOPE. 


47 


well  as  behind.  Hence  the  Author  regards  the  arrangement  adapted  by 
Messrs.  Siebert  and  Kraft  (Fig.  33)  as  preferable  ;  in  which  the  supports 
for  the  hands  are  oblique  wooden  blocks,  altogether  disconnected  from 
the  stage.  These,  being  hinged  to  the  wooden  base  of  tlie  pillar,  can  be 
made  to  turn  up  for  portability  (as  shown  in  Fig.  34),  so  that  the  instru- 
ment packs  into  a  very  small  compass. 

46.  Laboratory  Dissecting  Microscope, — Where,  on  the  other  hand, 
portability  may  bo  altogether  sacrificed,  and  the  instrument  is  to  be 
adapted  to  the  making  of  large  dissections  under  a  low  magnifying  power, 
some  such  form  as  is  represented  in  Fig.  35 — constructed  by  Messrs.  Baker 
on  the  basis  of  that  devised  by  Prof.  Huxley  for  the  use  of  his  Practical 
Class  at  South  Kensington — will  be  found  decidedly  preferable.  The 
framework  of  the  instrument  is  solidly  constructed  in  mahogany,  all  its 
surfaces  being  blackened;  and  is  so  arranged  as  to  give  two  uprights  for 
the  support  of  the  stage,  and  two  oblique  rests  for  the  hands.    Close  to 


the  summit  of  each  of  these  uprights  is  a  groove  into  which  the  stage-plate 
slides;  and  this  may  be  either  a  square  of  moderately  thick  glass,  or  a  plate 
of  ebonite  having  a  central  perforation  into  which  a  disk  of  the  same 
material  maybe  fitted  so  as  to  lie  flush  with  its  surface;  one  of  those  being 
readily  substituted  for  the  other,  as  may  best  suit  the  use  to  be  made  of  it. 
The  magnifier  is  carried  on  an  arm  working  on  a  racked  stem,  which  is 
raised  or  lowered  by  a  milled-head  pinion  attached  to  a  pillar  at  the  fur- 
ther right-hand  corner  of  the  stage.  The  length  of  the  rack  is  sufficient 
to  allow  the  arm  to  be  adjusted  to  any  focal  distance  between  2  inches  and 
l-4th  of  an  inch.  But  as  the  height  of  the  pillar  is  not  sufficient  to  allow 
the  use  of  a  lens  of  3  inches  focus  (which  is  very  useful  for  large  dissections) 
the  arm  carrying  the  lenses  is  made  with  a  double  bend,  which,  when  its 
position  is  reversed  (as  is  readily  done  by  unscrewing  the  milied-head  that 
attaches  it  to  the  top  of  the  racked  stem),  gives  the  additional  inch 
required.  As  in  the  Quekett  Microscope,  a  Compound  body  may  be  easily 
fitted,  if  desired,  to  a  separate  arm  capable  of  being  pivoted  on  the  same 
stem.  The  mirror  frame  is  fixed  to  the  wooden  basis  of  the  instrument; 
and  places  for  the  magnifiers  are  made  in  grooves  beneath  the  hand- 


Laboratory  Dissecting  Microscope. 


48 


THE  MICROSCOPE  AND  ITS  REVELATIONS. 


supports. — The  advantages  of  this  general  design  have  now  been  satis- 
factorily demonstrated  by  the  large  use  that  has  been  made  of  it ;  but 
the  details  of  its  construction  (such  as  the  height  and  slope  to  be  given  to 
the  hand-rests)  may  be  easily  adapted  to  individual  requirements. 

47.  BecFs  Dissecting  Microscope,  iviih  NacUefs  Binocular, — A  sub- 
stantial and  elaborate  form  of  Dissecting  Microscope,  devised  by  the  late 
Mr.  R.  Beck,  is  represented  in  Fig.  36.  From  the  angles  of  a  square  ma- 
hogany base,  there  rise  four  strong  brass  pillars,  which  support,  at  a 
height  of  4  inches^  a  brass  plate  6^  inches  square,  having  a  central  aper- 
Xto^  sa  ture  of  1  inch  across  ; 

upon  this  rests  a  circular 
brass  plate,  of  which 
the  diameter  is  equal  to 
the  side  of  the  preced- 
ing, and  which  is  at- 
tached to  it  by  a  revolv- 
ing fitting  that  sur- 
rounds the  central  aper- 
ture, and  can  be  tight- 
ened by  a  large  milled- 
head  beneath ;  whilst 
above  this  is  a  third 
plate,  which  slides  easi- 
ly over  the  second,  be- 
ing held  down  upon  it 
by  springs  which  allow 
a  movement  of  1^  inch  in 
any  direction.  The  top- 
plate  has  an  aperture  of 

Beck's  Dissecting  Microscope,  with  Nachet's  Binocular         1^  iucll  for  the  reception 

Microscope.  of  yarious  glasscs  and 

troughs  suitable  for  containing  objects  for  dissection;  and  into  it  can 
also  be  fitted  a  spring-holder,  suitable  to  receive  and  secure  a  glass  slide 
of  the  ordinary  size.  By  turning  the  large  circular  plate,  the  object 
under  observation  may  be  easily  made  to  rotate,  without  disturbing  its 
relation  to  the  optical  portions  of  the  instrument;  whilst  a  traversing 
movement  may  be  given  to  it  in  any  direction,  by  acting  upon  the  smaller 
plate.  The  left-hand  back  pillar  contains  a  triangular  bar  with  rack- 
and-pinion  movement  for  focal  adjustment,  which  carries  the  horizontal 
aim  for  the  support  of  the  magnifiers;  this  arm  can  be  turned  away 
towards  the  left  side,  but  it  is  provided  with  a  stop  which  checks  it  in  the 
opposite  direction,  when  the  magnifier  is  exactly  over  the  centre  of  the 
stage-aperture.  Beneath  this  aperture  is  a  concave  mirror,  which  when 
not  in  use,  lies  in  a  recess  in  the  mahogany  base,  so  as  to  leave  the  space 
beneath  the  stage  entirely  free  to  receive  a  box  containing  apparatus; 
whilst  from  the  right-hand  back  corner  there  can  be  raised  a  stem  car- 
rying a  side  condensing-lens,  with  a  ball-and-socket  movement.  In  addi- 
tion to  the  Single  lenses  and  Coddington  ordinarily  used  for  the  purposes 
of  dissection,  a  Binocular  arrangement  was  devised  by  Mr.  K.  Beck,^ 
on  the  principle  applied  by  MM.  Nachet,  about  the  same  date,  in  their 
Stereo-pseudoscopic  Microscope  (§  38).  Adopting  Mr.  Wenham's  method 
of  allowing  half  the  cone  of  rays  to  proceed  to  one  eye  without  interrup- 


^    Transactions  of  the  Microscopical  Society,"  N.  S.,  Vol.  xii.  (1864),  p.  3. 


CONSTRUCTION  OF  THE  MICROSCOPE. 


4:9 


tion,  he  caused  the  other  half  to  be  intercepted  by  a  pair  of  prisms  dis- 
posed as  in  Fig.  27,  2,  and  to  be  by  them  transmitted  to  the  other  eye. 
It  will  be  readily  understood  that  this  arrangement,  though  pseudosco- 
pic  for  the  Compound  Microscope,  is  stereoscopic  for  the  Simple  Micro- 
scope, in  which  there  is  no  reversal  of  the  pictures;  and  the  Author  can 
testify  to  the  fidelity  of  the  eflfect  of  relief  obtainable  by  Mr.  E.  Eeck's 
apparatus,  which,  being  carried  on  an  arm  superposed  upon  that  which 
bears  the  magnifier,  can  be  turned  aside  at  pleasure.    But  he  has  found 
its  utility  to  be  practically  limited  by  the  narrowness  of  its  field  of  view, 
by  its  deficiency  of  light  and  of  magnifying  power,  and  by  the  inconve- 
nience of  the  manner  in  which  the  eyes  have  to  be  applied  to  it. — An  ar- 
rangement greatly  superior  in  air  these  particulars  having  been  since 
worked  out  by  MM.  Nachet,  the  Autlior  has  combined  this  with  Mr.  E. 
Beck's  Stand  and  finds  every  reason  to  be  satisfied  with  the  result;  the 
solidity  of  the  stand  giving  great  firmness,  whilst  the  size  of  the  stage- 
plate  affords  ample  room  for  the  hands  to  rest  upon  it.    The  Objective  in 
Nachet's  arrangement  is  an  achromatic  combination  of  three  pairs,  having 
a  clear  aperture  of  nearly  3-4ths  of  an  inch,  and  a  power  about  equal  to 
that  of  a  single  lens  of  one  inch  focus;  and  immediately  over  this  is  a 
pair  of  prisms,  each  resembling  a.  Fig  27,  having  their  inclined  surfaces 
opposed  to  each  other,  so  as  to  divide  the  pencil  of  rays  passing  upwards 
from  the  objective  into  two  halves.    These  are  reflected  horizontally,  the 
one  to  the  right  and  the  other  to  the  left;  each  to  be  received  by  a  lateral 
prism  corresponding  to  B,  and  to  be  reflected  upwards  to  its  own  eye,  at 
such  a  slight  divergence  from  the  perpendicular  as  to  give  a  natural  con- 
vergence to  the  axes  when  the  eyes  are  applied  to  the  eye-tubes  super- 
posed on  the  lateral  prisms — the  distance  between  these  and  the  central 
prisms  being  made  capable  of  variation,  as  in  the  Compound  Binocular 
of  the  same  makers  (§  38).    The  magnifying  power  of  this  instrument 
may  be  augmented  to  35  or  40  diameters,  by  inserting  a  concave  lens  in 
each  eye-piece,  which  converts  the  combination  into  the  likeness  of  a 
Galilean  telescope  (or  opera-glass);  and  this  arrangement  (originally  sug- 
gested by  Prof.  Briicke  of  Vienna)  has  the  additional  advantage  of 
increasing  the  distance  between  the  object  and  the  object-glass,  so  as  to 
give  more  room  for  the  use  of  dissecting  instruments. — To  all  who  are 
engaged  in  investigations  requiring  very  minute  and  delicate  dissection, 
the  Author  can  most  strongly  recommend  MM.  Nachet's  instrument. 
No  one  who  has  not  had  experience  of  it,  can  estimate  the  immense 
advantage  given  by  the  Stereoscopic  view,  not  merely  in  appreciating  the 
solid  form  of  the  object  under  dissection,  but  also  in  precisely  estimating 
the  relation  of  the  dissecting  instrument  to  it  in  the  vertical  direction. 
This  is  especially  important  when  fine  scissors  are  being  used  horizon- 
tally; since  the  course  of  the  section  can  thus  be  so  regulated  as  to  pass 
through  the  plane  desired,  with  an  exactness  totably  unattainable  by  the 
use  of  any  monocular  magnifier. 

48.  Field'' s  Dissecting  and  Mounting  Microscope, — This  instrument, 
constructed  on  the  plan  of  Mr.  W.  P.  Marshall,  is  a  combination  of  a  Dis- 
secting Microscope  with  a  set  of  apparatus  and  materials  for  the  prepara- 
tion and  mounting  of  microscopic  objects;  the  whole  being  packed  in  a  small 
cubical  case  about  seven  inches  each  way,  convenient  both  for  general  use, 
but  more  particularly  as  a  travelling  case  for  carrying  the  several  requisites 
for  the  examination  and  mounting  of  objects  into  the  country,  or  to  the 
seaside. — The  Microscope  can  be  used  either  Simple  or  Compound,  as 
shown  in  Fig.  37;  and  is  fitted  with  a  mirror,  side-condenser,  and  stage- 
4 


50 


THE  MICROSCOPE  AND  ITS  REVELATIONS. 


forceps,  and  witli  metal  and  glass  stage-plates;  a  dissecting-trougli,  lined 
with  cork,  also  fits  into  the  opening  of  the  stage.  The  Simple  Micro- 
scope, as  used  for  dissecting  and  mounting,  is  shown  in  the  lower  figure; 
it  has  two  powers  used  singly  or  in  combination,  which  are  carried  by  the 
smaller  arm  of  the  stand.  The  Compound  body,  as  shown  in  the  upper 
figure,  screws  into  the  larger  arm  of  the  stand,  and  has  a  divided  objective, 
giving  a  range  of  three  powers;  the  nose  is  made  with  the  standard  screw, 
so  as  to  fit  any  first-class  objectives.  A  telescope  sliding  arm,  fitting  into 
a  socket  on  either  side  of  the  stage,  can  also  be  used  to  carry  the  simple- 
microscope  powers,  as  well  as  a  larger  low-power  lens,  that  serves  also  as  a 


Field's  Dissecting  and  Mounting  Microscope. 


hand-magnifier;  and  the  arm  can  be  readily  fixed  in  any  desired  position 
for  examining  objects  away  from  the  instrument.  A  watch-glass  holder 
used  upon  the  glass  stage-plate,  gives  the  means  of  sliding  steadily  upon 
the  stage  in  any  direction  objects  that  are  under  examination  in  a  watch- 
glass.  A  turn-table  for  mounting  purposes  is  carried  upon  a  long  spindle 
that  works  through  the  corner  of  the  stage  (as  shown  in  the  lower  figure), 
the  arm  of  the  stand  serving  as  a  support  for  the  hand  whilst  using  the 
turn-table;  the  top  is  made  of  the  size  of  an  ordinary  glass  slide,  and  the 
slide  is  held  upon  it  by  an  india-rubber  band.  A  hot  plate  fits  into  the 
opening  of  the  stage,  and  is  heated  by  a  spirit-lamp  placed  in  the  posi- 
tion of  the  mirror,  which  is  then  turned  to  one  side;  and  the  larger  arm 


CONSTRUCTION  OF  THE  MICROSCOPE. 


51 


serves  also  as  a  watch-glass  holder  for  preparing  crystals  hj  evaporation 
over  the  spirit-lamp.  A  selection  of  materials  required  in  preparing 
and  mounting  objects  is  supplied  in  a  rack  of  bottles  sliding  in  the  case; 
and  a  set  of  instruments — dissecting-needles,  knife,  forceps,  dipping 
tubes,  brushes,  etc. — with  a  supply  of  cover-glasses,  cells,  etc.,  are  carried 
in  the  three  drawers;  all  the  different  contents  of  the  case  being  readily 
accessible  when  it  is  set  open,  as  shown  in  the  lower  part  of  the  figure.  ^ 

COMPOUND  MICROSCOPES. 

49.  Of  the  various  forms  of  Compound  Microscope,  the  greater  num- 
ber may  be  grouped  with  tolerable  definiteness  into  three  principal 
Classes  :  the  First  consisting  of  those  high-class  instruments  in  which  the 
greatest  possible  perfection  and  completeness  are  aimed  at,  without 
regard  to  cost ;  the  Second  including  those  which  are  adapted  to  all  the 
ordinary  requirements  of  the  observer,  and  which  can  be  fitted  with  the 
most  important  Accessories  ;^  whilst  to  the  Third  belong  the  Students' 
and  Educational  Microscopes,  in  which  simplicity  and  cheapness  are  made 
the  primary  considerations.  Besides  these,  there  is  a  class  of  Micro- 
scopes devised  for  special  purposes,  but  not  suited  for  ordinary  use. — In 
all,  save  the  last,  the  same  basis  of  support  is  adopted  ;  namely,  a  tripod 
^foot,'  carrying  a  pair  of  uprights,  between  which  the  Microscope  itself  is 
swung  in  such  a  manner,  that  the  weight  of  its  different  parts  may  be  as 
nearly  as  possible  balanced  above  and  below  the  centres  of  suspension  in 
all  the  ordinary  positions  of  the  instrument.  This  double  support  was 
first  introduced  by  Mr.  George  Jackson,  who  substituted  two  pillars  (a 
form  which  Messrs.  Beck  retain  in  their  Large  Microscope,  Plate  vii., 
and  is  now  adopted  by  Messrs.  Eoss,  Plate  v.)  for  the  single  pillar,  con- 
nected with  the  Microscope  itself  by  a  ^  cradle  joint,^  which  was  previously 
in  use,  and  which  is  still  employed  in  many  Continental  models  (Fig.  45). 
But  in  place  of  pillars  screwed  into  the  tripod  base,  the  uprights  are  now 
usually  cast  in  one  piece  with  the  base,  both  for  greater  solidity  and  for 
facility  of  construction  (Pig.  39);  while  in  most  of  the  more  recent 
models  an  open  framework  is  adopted  (more  or  less  resembling  that  first 
devised  by  Mr.  Swift,  Pig.  50),  which  combines  great  steadiness  with 
lightness.  Messrs.  Powell  and  Lealand,  it  will  be  observed,  adopt  a  tri- 
pod support  of  a  different  kind  (Pig.  48  and  Plate  vi.);  still,  however, 
carrying  out  the  same  fundamental  principle  of  swinging  the  Microscope 
itself  between  two  centres.  An  entirely  new  and  very  effective  mode  of 
swinging  the  body  has  lately  been  introduced  by  Mr.  George  Wale  of 
New  York  (Fig.  44). — Two  different  modes  of  giving  support  and  motion 
to  the  '  body  ^  will  be  found  to  prevail.  In  the  first,  which  may  be  called 
the  Ross  model  (as  having  been  originally  adopted  by  Mr.  Andrew  Boss), 
the  ^  body '  is  attached  at  its  base  only  to  a  transverse  ^  arm,^  which,  being 
pivoted  to  the  top  of  the  '  stem,^  is  raised  or  lowered  with  it  by  the  rack- 
and-pinion  action  that  works  in  the  pillar  to  which  the  stage  is  fixed 
(Fig.  52).    The  fundamental  objection  to  this  method  is,  that  unless  the 


1  The  whole  of  this  apparatus  is  supplied  complete  at  the  moderate  cost  of  £4. 
or,  without  the  Compound  body  and  inclined  movement  of  the  stand,  at  £3  10s. 

2  It  is  true  that  the  most  important  of  these  accessories  may  be  applied  to  some 
of  the  smaller  and  lighter  kind  of  Microscopes  ;  but  when  it  is  desired  to  render 
the  instrument  complete  by  the  addition  of  them,  it  is  far  preferable  to  adopt  one  of 
those  larger  and  more  substantial  models,  which  have  been  devised  with  express 
reference  to  their  most  advantageous  and  most  convenient  employment. 


52 


THE  MICROSCOPE  AND  ITS  REVELATIONS. 


transverse  arm  and  the  body  are  constructed  with  great  solidity,  the  ab- 
sence of  support  along  the  length  of  the  latter  leaves  its  ocular  end  sub- 
ject to  vibration,  which  becomes  unpleasantly  apjiarent  when  highpoweri35 
are  used,  giving  a  dancing  motion  to  the  objects.  With  the  view  of 
preventing  this  vibration,  the  top  of  the  ^  body  ^  is  sometimes  connected 
with  the  back  of  the  transverse  arm  by  a  pair  of  oblique  ^  stays  ^  (Fig-  48); 
but  the  usual  plan  is  to  obtain  the  requisite  firmness  by  the  thickness 
and  weight  of  the  several  parts.  In  the  other,  which  may  be  termed  the 
Jackson  model,  and  which  was  first  adopted  by  Mr.  James  Smith  (the 
predecessor  of  Messrs.  Beck),  the  body  is  supported  along  a  great  part  of 
its  length  on  a  solid  ^  limb '  whereby  its  ^  vibration  ^  is  reduced  to  a 
minimum  ;  and  the  rack,  which  is  acted  on  by  a  pinion  working  in  that 
limb,  is  attached  to  the  body  itself  ;  a  construction  that  gives  a  great 
smoothness  and  easiness  of  working  (Plate  Yii.). — Having  made  use  of 
instruments  constructed  by  the  best  makers  on  both  models,  the  Author 
has  no  hesitatation  in  expressing  his  preference  for  the  second,  which  is 
now  employed  by  most  English  makers  (having  been  adopted  by  Messrs. 
Eoss  themselves  in  their  more  recent  instruments),  and  by  nearly  all 
American.  He  regards  it  as  certain  that  greater  freedom  from  vibration 
can  be  obtained  in  lightly-framed  Microscopes  constructed  on  the  Jack- 
son model,  than  in  any  but  the  most  solid  and  cumbrous  of  the  old  Ross 
pattern  ;  and  feels  assured  that  the  principle  of  supporting  the  '  body^ 
along  a  great  part  of  its  length  (which  may  be  applied  in  a  variety  of 
modes)  will  in  time  supersede  that  of  fixing  it  by  its  base  alone,  which  is 
oviously  the  mode  least  adapted  to  prevent  vibration  at  its  ocular  end. 


In  describing  the  Instruments  which  he  has  selected  as  typical  of  the 
several  groups  above  enumerated,  the  Author  wishes  not  to  be  under- 
stood as  giving  any  special  preference  to  these,  above  what  may  be  the 
equally  good  instruments  of  other  Makers.  The  number  of  those  who 
now  construct  really  excellent  Microscopes  has  of  late  years  increased 
greatly  ;  but  their  models  are  for  the  most  part  copied  more  or  less  closely 
from  those  previously  adopted  for  their  high-class  Microscopes  by  the 
three  principal  Firms  which  long  had  exclusive  possession  of  the  field. 
Where  any  individual  Maker  has  introduced  a  real  novelty,  either  in  plan 
of  construction,  or  in  simplification  leading  to  reduction  of  price,  the 
Author  has  thought  this  worthy  of  special  notice  ;  whilst  the  limits  within 
which  he  is  restricted  oblige  him  to  content  himself  with  a  bare  mention 
of  other  Makers  whose  productions  are  favorably  known  to  him.  It  will 
be  found  most  advantageous  to  commence  with  the  Educational  and 
Students'  Microscopes,  as  the  most  simple  in  construction  ;  and  to  pro- 
ceed from  these  through  the  Second  to  the  First- Class  Microscopes, 
reserving  to  the  last  the  group  of  instruments  adapted  for  Special  pur- 
poses. 

THIRD-CLASS  MICROSCOPES. 

50.  Very  important  contributions  to  our  knowledge  of  Nature  have 
unquestionably  been  made  by  the  assistance  of  instruments  not  surpassing 
the  least  jierfect  of  those  now  to  be  described.  And  there  is  this  advan- 
tage in  commencing  Microscope-work  with  a  simple  and  low-j^riced 
instrument — that  the  risk  of  injury  to  a  more  costly  Microscope,  which 
necessarily  arises  from  want  of  experience  in  its  use,  is  avoided;  whilst 
the  inferior  instrument  will  still  be  found  serviceable  for  many  purposes. 


CONSTRUCTION  OF  THE  MICROSCOPE. 


•  53 


after  a  better  one  has  been  acquired.  Microscopes,  of  whatever  Class, 
should  be  provided  with  the  '  Society's  screw  ^  now  used  not  only  by 
British  and  American,  but  also  by  several  Continental  Makers;  so  that 
any  of  their  Objectives  may  be  fitted  to  them.    (See  Note,  p.  58.) 

Educational  Microscopes. 

51.  FieWs  Educational  Microscope, — This  instrument  is  known  as 
the  '  Society  of  Arts  Microscope,^  in  consequence  of  its  having  gained  the 
medal  awarded  by  that  Society,  in  1855  (at  the  suggestion  of  the  Author) 
for  the  best  three-guinea  Compound  Microscope  that  was  then  produced. 
It  has  two  Eye-pieces,  and  two  achromatic  Objectives,  Condenser,  Live- 
box,  etc.,  and  retains  its  place  amongst  useful  instruments  of  low  price. 
It  is  within  the  knowledge  of  the  Author,  that  the  production  of  this 
instrument  has  greatly  promoted  the  spread  of  Microscopy  among  many 
to  whom  the  pursuit  has  proved  most  valuable  as  a  refreshing  and  elevat- 
ing occupation  for  hours  that  might  have  been  otherwise  either  spent  in 
idleness  or  turned  to  much  worse  account. 

52.  Crouches  Educational  Microscope, — This  is  a  very  simple  and  at 
the  same  time  serviceable,  instrument  (Fig.  38);  well  suited  for  the  dis- 
play of  Botanical  objects,  small  Insects  or  parts  of  larger  ones,  Zoophytes 
and  Polyzoa  that  may  be  picked  up  on  almost  any  sea-shore,  or  the  Cir- 
culation in  a  Frog's  foot.  In  order  to  minimize  its  cost,  the  ordinary 
modes  of  focal  adjustment  are  dispensed  with;  the  ^  coarse '  adjustment 
being  made  by  sliding  the  body 

through  the  tube  which  grasps  'Em^S^ 
it,  and  which  is  lined  with  velvet 
to  secure  a  smooth  and  equable 
'  slip;'  and  the  '  fine  '  by  slight- 
ly drawing-out  the  Eye-pieces. 
This  method  answers  very  well 
for  the  low  powers  for  which  this 
insbrument  is  intended;  and  it 
has  the  advantage  of  not  allowing 
the  adjustment  which  a  Teacher 
has  made,  to  be  readily  disturbed 
by  the  PujDils  to  whom  an  object  is 
being  exhibited.  It  is  provided 
with  a  side-condenser  for  illumi- 
nating opaque  objects;  and  with 
a  diaphragm-plate  fitted  into  a 
tube  which  is  screwed  into  the 
aperture  of  the  stage,  and  which 
is  adapted  also  to  receive  a  po- 
larizing prism  and  spot-lens.* 

53.  Parkes^s  Educational 
Microscope, — Such  as  desire  a 
large  and  more  substantial  in- 
strument, which  may  be  advan- 
tageously used  for  higher  pow- 
ers, and  made  to  serve  a  greater 

variety  of  purposes,  will  find  the  crouch's  Kducational  Microscope. 

Microscope  represented  in  Fig. 

^  The  cost  of  this  instrument,  with  a  dividing  object-glass  of  \  inch  and  1  inch 
focus,  in  mahogany  case,  is  only  £2  10s. 


54  • 


THE  MICROSCOPE  AND  ITS  REVELATIONS. 


39  very  suitable  to  such  requirements.  It  is  solidly  built,  without  being 
unduly  weighty,  carries  a  body  of  full  diameter  (which  can  be  lengthen- 
ed by  a  draw-tube  to  ten  inches),  and  stands  well  upon  its^  base.  The 
'  coarse  ^  adjustment  is  made  (as  in  the  preceding  case)  by  sliding  the  body 
within  the  tube  that  grasps  it,  the  lining  of  which  with  cloth  makes  it  work 
very  easily  (Fig.  39);  but  a  rack  and  pinion  movement  may  be  added  at  a 


Parkes's  Educational  Microscope. 


small  additional  cost.  The  ^  fine '  adjustment  is  made  by  a  screw  (turned 
by  the  milled-head  at  the  top  of  the  vertical  pillar),  which  acts  on  the  car- 
nage of  the  body;  and  as  this  carriage  slides  between  dove-tailed  grooves, 
the  adjustment  is  made  with  entire  freedom  from  '  twist.  ^  The  Microscope 
is  furnished  with  two  eye-pieces,  of  which  the  lower  is  preferable  for  objects 
requiring  good  definition;  whilst  the  higher  gives  a  flat  field  of  eight  inches 


CONSTRUCTION  OF  THE  MICROSCOPE. 


55 


diameter,  suitable  for  Sections  of  Wood  and  other  like  objects  viewed  with 
the  low-power  objective.  The  powers  usually  supplied  are  a  separating  com- 
bination of  2  inch  and  1  inch,  which,  by  the  use  of  the  two  eye-pieces  and 
the  di*aw-tube,  gives  a  range  of  magnifying  power  from  15  to  110  diame- 
ters; and  a  l-4tli  inch  of  70°  aperture,  from  which,  by  the  same  means, 
a  range  of  magnifying  power  can  be  obtained  from  140  to  450  diameters. 
The  aperture  of  the  stage  is  furnished  with  a  cylindrical  fitting,  which 
carries  two  diaphragms  (one  with  a  small  aperture,  the  other  with  a 
larger)  for  regulating  the  quantity  of  light  reflected  from  the  mirror  to 
the  object,  a  ground-glass  for  the  equable  diffusion  of  the  light  over  a 
large  field,  and  a  '  spot-lens  ^  for  black-ground  illumination.  The  mir- 
ror is  plane  on  one  side,  and  concave  on  the  other;  and  a  condenser  for 
opaque  objects  is  attached  by  a  jointed  arm,  giving  universal  motion,  to 
the  tube  that  carries  the  body.  The  Objectives  of  this  Microscope,  as  of 
most  of  those  constructed  by  the  same  Makers,  are  made  to  fit  into  the 
nozzle  of  the  body  by  their  '  patent  sliding  adapter,^  which  enables  one 
power  to  be  exchanged  for  another  without  any  screwing  or  unscrewing. 
But  their  Microscopes  can  be  used  with  any  objective  carrying  the 
^  Society's  screw, ^  by  simply  unscrewing  the  special  nozzle  from  the  end 
of  the  body.  And  by  sliding  the  special  nozzle  upon  either  of  its  own 
objectives,  this  may  be  used  with  any  other  instrument  furnished  with 
that  screw.  ^ 

Students^  Microscopes. 

54.  The  principle  is  now  universally  recognized,  that  the  form  of 
Microscope  best  adapted  to  the  wants  of  the  Medical  or  Biological  Stu- 
dent, is  one  in  which  simplicity  and  compactness  of  general  construction 
are  combined  with  excellence  in  optical  performance.  The  demand  for 
instruments  of  this  kind  was  first  met  by  Continental  Opticians;  and  at 
the  time  when  Messrs.  Ross,  Powell  and  Lealand,  and  Smith  and  Beck — 
then  almost  the  only  constructors  of  Microscopes  in  this  country — sold  no 
Objectives  but  such  as  would  stand  the  highest  tests  and  were  costly  in 
proportion,  recourse  was  necessarily  had,  by  such  as  desired  simpler  and 
cheaper  instruments,  to  the  Opticians  of  France  and  Germany;  among 
whom  MM.  JSTachet,  Oberhauser  (succeeded  by  Hartnack),  and  Kellner 
(succeeded  by  Gundlach),  long  shared  the  chief  English  demand.  A 
large  number  of  new  Makers,  however — many  of  them  trained  in  one  or 
other  of  the  three  principal  establishments  just  named — have  now  entered 
the  field;  and  have  put  themselves  in  fair  competition  with  Continental 
Opticians,  and  with  each  other,  alike  in  the  excellence  of  their  work 
(both  mechanical  and  optical),  and  in  moderation  of  price.  A  distinct 
class  of  ^ Students' Microscopes^  of  English  construction,  more  or  less 
framed  upon  Continental  models,  has  thus  come  into  general  use;  afford- 
ing ample  choice,  in  the  varieties  of  their  pattern,  to  such  as  may  have  a 
preference  for  one  or  other  of  them  as  most  suitable  to  the  work  on  which 
they  may  be  engaged.  With  few  exceptions,  the  Microscopes  properly 
belonging  to  this  class  have  the  small  short  '  body '  (capable,  however,  of 
being  lengthened  by  a  '  draw-tube ')  of  the  Continental  instruments; 
and  this  is  grasped  by  a  tube  attached  to  the  '  limb,^  in  such  a  manner  as 

^  The  price  of  this  Microscope  with  the  above-named  Accessories,  in  a  vsrell- 
made  mahogany  Case,  is  £6  10s.  An  Objective  of  l-6th  inch  focus,  giving  a  max- 
imum power  of  560  degrees,  or  one  of  l-7th  inch  giving  a  maximum  power  of  700 
diameters,  may  be  substituted  for  the  l-4th  inch  at  a  very  small  advance  of  cost. 
A  Polariscope  and  Achromatic  Condenser  can  be  easily  added. 


66 


THE  MICROSCOPE  AND  ITS  REVELATIONS. 


to  give  a  support  that  is  free  from  vibration  even  when  high  powers  are 
in  use.  In  the  simplest  models,  such  as  that  of  Messrs.  Baker  (Fig.  40), 
there  is  no  rack-and-pinion  movement  for  the  '  coarse  '  adjustment,  which 
can  be  very  easilj^  made  by  sliding  the  body  through  the  tube  which  holds 
it,  provided  that  this  be  lined  with  cloth  or  velvet;  but  the  rack  move- 
ment can  generally  be  added  at  a  small  cost.  A^fine^  adjustment  for 
exact  focussing,  by  means  of  a  micrometer-screw  worked  by  a  milled-head, 
is  always  provided;  and  this  movement  may  be  given  in  different  Avays. 
In  the  Continental  models,  the  screw  is  usually  contained  within  the  pil- 
lar that  supports  the  arm  or  limb  to  which  the  carriage  of  the  body  is 
attached,  the  milled-head  being  at  its  summit  (Figs.  40,  45);  this  answers 
well  if  due  provision  be  made  to  prevent '  twist  ^  of  the  movable  portion 
(causing  lateral  displacement  of  the  image),  without  interfering  with  its 
freedom  of  vertical  motion.  By  many  British  and  American  makers,  the 
fine  adjustment  is  made  to  act  on  a  tube  within  the  '  nose^  of  the  body, 
into  which  the  objective  is  screwed;  this  being  raised  or  lowered,  either 
by  a  lever  contained  within  the  arm,  which  is  acted-on  by  the  milled-head 
carried  by  it  (as  in  the  original  Ross  model,  Fig.  52),  or  by  a  shorter  lever 
at  the  lower  end  of  the  body,  to  which  the  milled-headed  screw  is  attached 
(as  in  Messrs.  Beck's  Large  Microscope,  Plate  yii.).  This  method  is 
subject  to  two  disadvantages:  (1)  that  the  focussing  tube  which  carries 
the  objective  can  scarcely  be  made  to  work  with  the  requisite  facility, 
without  a  liability  to  '  twist,'  which  becomes  very  perceptible  after  much 
wear,  in  the  displacement  of  the  image  when  a  high  magnifying  power  is 
in  use;  and  (2)  that  by  the  vertical  movement  thus  given  to  the  focussing 
tube,  the  working  length  of  the  body,  and  consequently  the  magnifying 
power  undergoes  change  in  every  adjustment  for  focus.  The  plan  of  fine 
adjustment  which  has  been  adopted  from  an  American  model  by  Messrs. 
Ross  (Plate  v.)  and  is  employed  in  Wale's  New  Working  Microscope 
(Fig.  44),  seems  to  the  Author  in  everyway  preferable.  Here  a  lever 
contained  within  the  limb,  and  acted-on  by  a  micrometer-screw  at  ifs 
back,  gives  motion  to  a  long  slide,  working  in  dove-tailed  grooves, 
behind  the  racked  slide  which  carries  the  body;  and  this  can  be  made  to 
work  very  easily,  without  either  ^  twist'  or  'lost  time.' — The  Stage  of 
Students'  Microscoj)es  is  often  a  simple  plate  of  brass,  with  a  couple  of 
springs  for  holding  down  the  object-slide;  but  in  some  models  (Figs.  40, 
45)  there  is  an  '  upper  stage-plate '  of  glass,  rotating  in  the  optic  axis  of 
the  body.  Into  the  aperture  of  the  stage  a  cylindrical  fitting  is  usually 
screwed,  for  the  purpose  of  receiving  the  Accessories  required  for  giving 
varied  illumination;  the  most  indispensable  of  these  being  Diaphragms  of 
different  apertures.  These  should  be  so  fitted  that  they  can  be  brought 
up  flush  with  the  level  of  the  stage;  the  limitation  of  the  illuminating 
pencil  for  the  purpose  of  obtaining  the  best  definition  being  much  more 
effectively  made  by  a  very  small  aperture  (not  exceeding  a  large  pin-hole) 
close  to  the  under  side  of  the  object-slide,  than  by  a  wider  aperture  at 
some  distance  beneath  it.  For  the  same  reason,  if  a  rotating  '  dia- 
phragm-plate'  (§  98)  be  employed,  containing  a  graduated  series  of  aper- 
tures, it  should  be  attached  to  the  under  side  of  the  Stage  itself,  and  not 
to  the  bottom  of  the  cylindrical  fitting  beneath  it.  For  perfect  regula- 
tion of  the  light,  nothing  is  so  effective  as  the  ^Iris-diaphragm  (§  98); 
and  this,  as  Mr.  Wale  has  shown  (§  60),  may  be  constructed  so  cheaply, 
that  its  general  adoption  seems  very  desirable. — The  mirror  should  be 
double,  one  of  its  surfaces  plane  and  the  other  concave;  and  it  should  be 
so  attached  (I)  that  its  distance  from  the  stage  may  be  varied  sufficiently, 


CONSTRUCTION  OF  THE  MICROSCOPE.  57 

to  allow  the  rays  reflected  from  the  concave  side  to  be  either  brought  to  a 
focus  on  the  object,  or  to  give  a  uniform  illumination  over  a  larger  field, 
and  this  alike  with  the  parallel  rays  of  daylight,  and  the  diverging  rays 
of  a  lamp;  and  (2)  that  it  may  be  thrown  so  far  out  of  the  optic  axis,  as 
to  reflect  rays  of  considerable  obliquity.  The  first  of  these  objects  is 
answered  by  making  the  mirror-frame  slide  upon  a  stem  fixed  into  the 
bottom  of  the  pillar  (Fig.  41);  but  this  does  not  give  sufficient  obliquity. 
The  second  is  readily  provided-for  by  attaching  the  mirror-frame  to  a 
swinging-bar,  pivoted  to  the  under  side  of  the  stage  (Fig.  42);  this  gives 
any  amount  of  obliquity,  but  does  not  enable  the  distance  of  the  mirror 
from  the  stage  to  be  varied.  If  the  mirror-frame  be  made  to  slide  on  a 
stem,  it  should  be  mounted  on  a  jointed  arm,  so  as  to  be  made  capable  of 
reflecting  very  oblique  light;  or,  if  attached  to  a  swinging  bar,  this  bar 
should  be  made  capable  of  elongation  by  a  sliding  piece  working  in  a 
dove-tail  groove  (as  in  Wale's  Microscope,  Fig.  44),  so  as  to  allow  its  dis- 
tance from  the  stage  to  be  varied. — A  very  ingenious  arrangement  of  the 
rotating  ^  upper  stage'  has  been  devised  by  Mr.  John  Phin  (of  New 
York).  It  is  so  fitted  with  a  short  tube,  that  it  may  be  slid  into  the 
cylindrical  fitting,  not  only  from  above,  but  also  from  'below;  and  as  the 
object-slide  rests  upon  the  springs  which  press  it  upwards  against  the 
stage-plate,  not  only  may  light  of  any  degree  of  obliquity  be  throvv^n  upon 
it,  but  the  advantage  of  a  ^safety-stage'  (§  117)  is  obtained,  since  the 
springs  that  support  the  slide  readily  yield  to  any  pressure  exerted  on  it 
by  the  objective.  A  Student's  Microscope  fitted  with  this  form  of  rotat- 
ing stage,  and  with  either  Wenham's  ^  disk  illuminator,'  or  '  Woodward's 
prism'  (§  101),  and  having  the  mirror  hung  in  the  manner  just  recom- 
mended, will  be  found  capable — if  furnished  with  good  Objectives — of 
resolving  all  but  the  most  difficult  Diatom-tests. 

55.  In  regard  to  the  qualities  of  the  Objectives  desirable  for  a  Stu- 
dent's Microscope,  the  Author  feels  assured  that  he  expresses  the  convic- 
tion of  the  most  experienced  workers  in  various  departments  of  Biological 
inquiry,  when  he  re-affirms  the  doctrine  of  which  nearly  half  a  century's 
varied  experience  has  satisfied  him,  but  which  has  been  of  late  vehe- 
mently contested  (not  always  very  cautiously)  by  Microscopists  whose 
range  of  study  has  been  less  extended — that  good  definition,  with  mode- 
rate  angle  of  aperture,  is  the  essential  requisite;  Objectives  of  this  class 
being  not  only  much  more  easy  to  use  by  the  inexperienced,  but  fre- 
quently also  giving  much  more  information  even  to  the  experienced  (in 
virtue  of  their  greater  ^penetration'  or  ^ focal  depth'),  than  can  be  ob- 
tained from  Objectives  of  the  very  wide  angles  required  for  the  resolution 
of  difficult  diatom-tests  (see  §  161).  Every  one  who  is  at  all  conversant 
with  the  recent  history  of  Micro-Zoology,  Micro-Botany,  Micro-Geology, 
or  Animal  or  Vegetable  Histology,  must  know  that  at  least  ninety-nine 
hundredths  of  the  enormous  additions  made  to  each  of  these  departments 
of  inquiry  during  the  last  quarter  of  a  century,  have  been  Avorked-out  by 
Objectives  of  the  kind  here  recommended;  and  those  who  affirm  that  all 
this  work  is  so  imperfect  that  it  will  have  to  be  done  over  again  with  Ob- 
jectives of  excessively  wide  aperture,  have  to  prove  the  fact.  Doubtless 
neio  methods  of  preparation  are  constantly  revealing  novelties  in  whole 
classes  of  objects  which  (it  was  supposed)  had  been  already  studied 
exhaustively;  and  no  one  can  affirm  that  he  has  made  out  everything,  in 
any  object,  which  it  is  capable  of  being  thus  made  to  show.  But  the 
Author  feels  confident  that  no  such  extension  of  our  knowledge  is  likely 
to  take  place  in  this  direction,  as  will  require  the  habitual  use  of  the  very 


58  THE  MICROSCOPE  AND  ITS  REVELATIONS. 

costly  wide-angled  Objectives,  which  certain  Microscopists,  especially  in 
the  United  States,  are  now  extolling  as  alone  trustworthy/  In  confirma- 
tion of  the  foregoing  remarks,  the  following  additional  authorities  may 
be  cited: — Dr.  Beale,  whose  Histological  experience  no  one  can  call  in 
question,  says  (^^How  to  Work  with  the  Microscope,'^  5th  ed.,  p.  10): — 
'^For  ordinary  work  it  will  be  found  inconvenient  if  the  object-glass, 
^'  when  in  focus,  comes  too  close  to  the  object.  This  is  a  defect  in 
glasses  having  a  high  angle  of  aperture.  Such  glasses  admit  much 
light,  and  define  many  structures  of  an  exceedingly  delicate  nature 
which  look  confused  when  examined  with  ordinary  powers.  For  gen- 
eral  microscoinc  work,  however,  glasses  of  medium  angular  aperture 
''are  to  be  recommended.  Glasses  having  an  angle  of  150°  and  upwards 
^^are  valuable  for  investigations  upon  many  very  delicate  and  thin  struc- 
^^tures,  such  as  the  Diatomacece;  hut  such  powers  are  not  well  adapted 
''for  ordinary  work,^^  So  Dr.  Heneage  Gibbes,  who  has  been  trained 
under  Dr.  Klein,  one  of  the  most  distinguished  Histologists  of  the  pres- 
ent day,  recommends  the  Student  Practical  Histology  and  Pathology," 
p.  6)  to  get  some  good  Microscopist  to  test  the  object-glasses  he  thinks  of 
purchasing;  "  and  he  should  see  that  they  are  tested  on  some  Histological 
object,  and  not  on  Diatoms,  as  the  wide  angles  necessary  for  resolving 
"test  DiatomacecB  are  the  reverse  of  useful  to  the  you7ig  histologistJ^ 
And  Dr.  Leidy,  of  Philadelphia,  everywhere  well  known  as  a  most  able 
Biological  worker  of  large  and  varied  experience,  who  has  lately  produced 
an  admirable  Monograph  (illustrated  by  48  beautiful  quarto-plates)  on 
the  "  Fresh-water  Ehizopods  of  North  America,"  makes  a  point,  in  his 
Introduction  (p.  3),  of  informing  Students  that  Microscopic  observa- 
"  tions,  such  as  those  which  form  the  basis  of  the  present  work,  do  not 
require  elaborate  and  high-priced  instruments;"  the  Student's  Micro- 
scopes of  Zentmayer,  Beck,  or  Hartnack,  with  a  power  of  l-4th  or  l-5th 
inch,  and  the  occasional  use  of  a  l-8th  or  1-lOth  inch,  furnishing  all  that 
is  needed.  ^^I  give  the  above  statement,"  he  adds,  ^^not  with  any  dis- 
^^positionto  detract  from  the  value  of  the  various  magnificent  in stru- 
^^ments  so  much  in  vogue,  but  with  the  object  of  dispelling  a  common 
impression  widely  prevalent,  at  least  among  those  with  whom  I  habitu- 
^^ally  come  into  contact,  that  the  kind  of  work  such  as  I  now  put  forth 
^^can  be  done  only  with  the  help  of  elaborate  and  expensive  instru- 
^^ments."2 

^  The  cost  of  the  Objective  of  l-4th  inch  focus  and  170°  aperture,  made  by  Mr. 
Tolles,  of  Boston,  is  70  dollars  (about  £14);  which  would  purchase  a  very  good 
English  Student's  Microscope,  with  a  series  of  excellent  Objectives  up  to  1-lOth 
*  immersion. 

2  Now  that  the  requirements  of  a  Student's  Microscope  are  so  definitely  under- 
stood, the  Author  would  suggest  whether  it  would  not  be  better  that  a  new  stand- 
ard screw  of  much  smaller  size  than  the  *  Society's'  should  be  adopted  for  it,  so 
as  to  enable  Students'  *  Objectives '  to  be  set  in  the  small  light  '  mounts '  used  on 
the  Continent,  instead  of  in  the  massive  mounts  which  the  Socciety's  screw  neces- 
sitates; especially  as,  on  the  construction  already  recommended  (§  17)  no  adjust- 
ment for  thickness  of  covering-glass  is  required,  even  for  high  powers.  A  small 
light  '  nosepiece,'  for  change  of  Objectives,  could  then  be  added  at  a  low  cost, — 
to  the  great  convenience  of  the  worker.  Such  Microscopists  as,  commencing 
with  *  Students'  Microscopes,'  afterwards  provide  themselves  with  more  complete 
instruments,  would  readily  employ  their  Students'  objectives  with  the  latter  by 
means  of  an  *  adapter.'  But  the  Author's  experience  would  lead  him  to  recom- 
mend any  one  engaged  in  research  to  keep  his  Student's  Microscope,  with  its  own 
series  of  objectives,  constantly  on  his  table;  and  to  have  recourse  to  his  larger 
instrument,  with  its  first-class  Objectives  and  varied  methods  of  Illumination, 
only  for  the  more  complete  scrutiny  of  the  preparations  h^  has  made  with  his 
simpler  model. 


CONSTRUCTION  OF  THE  MICROSCOPE. 


59 


56.  Baher^s  Students  Microscope. — Most  of  the  conditions  above 
specified  as  desirable,  are  well  fulfilled  in  the  instrument  represented  in 
Fig.  40;  which  might  easily  be  brought  into  entire  conformity  with  them. 
It  is  extremely  light  and  handy;  and  is  so  well  hung  as  to  be  very  steady 
in  all  positions.  It  is  provided  with  a  rotating  glass  stage;  and  this  car- 
ries a  cj^lindrical  fitting  (not  represented  in  the  figure)  for  the  usual 
Accessories,  ^ 


Baker's  Student's  Microscope.  CoUins's  Student's  Microscope. 


57.  Collinses  Student^ s  Microscope. — This  instrument  (Fig.  41)  is  con- 
structed on  a  plan  altogether  different;  the  body  having  the  diameter  of 
that  of  the  larger  Microscopes  by  the  same  maker  (Fig.  49),  so  as  to  receive 
their  eye- pieces,  and  being  capable  of  elongation  by  a  draw-tube  to  the 
full  ordinary  length.  It  is  provided  with  a  rack-movement  acting  on  a 
carriage  attached  along  the  length  of  the  body  (as  in  the  Jackson  model); 
and  the  top  of  this  carries  the  milled-head  for  the  fine  adjustment,  which 
acts  upon  a  lever  near  the  bottom  of  the  carriage,  so  as  to  raise  or  lower 
a  focussing  tube  within  the  nozzle  of  the  body. 

58.  Pillischer^s  International  Microscope, — The  Student  who  may  be 
willing  to  incur  a  slight  additional  expense,  for  the  sake  of  obtaining  a 
substantial  and  well-constructed  instrument,  cannot  do  better  (in  the 
Author's  judgment)  than  possess  himself  of  the  International  Micro- 
scope of  Mr.  Pillischer  (Fig.  42),  in  which  the  advantages  of  British  and 
Continental  methods  are  ingeniously  combined.    The  pillar,  carrying  a 


^  The  price  of  this  instrument,  with  one  Eye-piece  and  two  Objectives  (1  inch 
and  l-4th  inch),  in  Case,  is  5  guineas;  or,  with  rack  movement  for  coarse  adjust- 
ment, 6  guineas 


60 


THE  MICROSCOPE  AND  ITS  REVELATIONS. 


rack-movement  with  double  milled-head,  is  swung  on  two  uprights  set 
on  a  solid  foot,  in  such  a  manner  as  to  be  well  balanced;  and  at  the  top 
of  the  racked  stem  is  the  milled-head  that  works  the  screw  for  fine 
adjustment,  raising  or  lowering  the  horizontal  arm  which  carries  the 
body,  without  twist  or  loss  of  time.  This  arm  carries  a  tube  firmly 
screwed  into  it,  through  which  the  body  slides;  and  while  this  arange- 
ment,  by  giving  additional  support  to  the  lower  part  of  the  body,  effect- 
ually antagonizes  vibratiou,  it  allows  the  body  to  be  raised  to  a  height 
that  permits  the  use  of  objectives  of  3  or  4  inches'  focus,  for  which  the 
rack-movement  is  not  long  enough  to  provide.  On  the  outside  of  this 
tube  is  a  clip  having  attached  to  it  a  jointed  arm  that  carries  a  condens- 


Pillischer's  International  Microscope.  Ross's  (Zentmayer)  Student's  Microscope. 


ing  lens  for  opaque  objects;  which,  by  raising  or  lowering  the  clip,  or 
turning  it  round  the  tube,  can  be  brought  into  any  required  position. 
The  stage  is  simple,  and  carries  a  rotating  diaphragm-plate  on  its  under 
side.  The  mirror  is  attached  to  a  swinging  bar,  which  might  easily  be 
made  to  elongate  like  that  of  the  Wale  Microscope  (§  60). — The  special 
merit  of  this  model  (of  which  the  Author  can  speak  from  considerable 
experience  of  its  use),  lies  in  the  facility  with  which  both  the  coarse  and 
the  fine  movements  may  be  worked  with  either  of  the  hands,  while  rest- 
ing on  the  table  in  the  position  most  convenient  for  manipulating  the 


CONSTRUCTION  OF  THE  MICROSCOPE. 


61 


object  on  the  stage,  an  advantage  which  every  real  tuorher  with  a  simple 
instrument  of  this  class  will  appreciate.* 

59.  Rosses  {Zentmayer)  Shtdenfs  Microscope. — Another  instrument 
of  superior  make  (Fig.  43),  has  lately  been  introduced  by  Messrs.  Eoss, 
with  the  view  of  affording  to  the  Student  the  advantage  of  the  '  swinging 
tail-piece  for  oblique  ilUimination/  devised  by  Mr.  Zentmayer;  of  which 
a  fuller  description  will  be  given  in  its  application  to  their  First-class 
Microscope  (§  72).  This  tail-piece  swings  round  a  pivot  which  serves  for 
the  attachment  of  the  stage  to  the  limb;  and  at  the  back  of  the  limb  is  a 
milled-head  working  on  the  projecting  end  of  this  pivot,  by  tightening 
which  the  stage  may  be  firmly  fixed  in  its  ordinary  horizontal  position, 
whilst  by  loosening  it  the  stage  may  be  made  to  incline  to  one  side  or  the 
other.  The  '  tail-piece '  carries,  between  the  mirror  and  the  stage,  a  '  sub- 
stage,'  fitting  into  which  may  be  screwed  an  ordinary  1  inch,  1\  inch,  or  2 
inch  Objective,  which  answers  the  purpose  of  an  Achromatic  condenser; 
and  when  a  pencil  of  light  reflected  from  the  mirror  has  been  made  by  it  to 
focus  in  the  object,  the  swinging  of  the  ^tail-piece'  to  one  side  or  the 
other  will  give  any  degree  of  obliquity  to  the  illuminating  pencil  that 
may  be  desired,  without  throwing  its  focus  off  the  object,  as  this  lies  in 
the  plane  of  the  centre  round  which  it  turns.  The  '  tail-piece'  may  even 
be  carried  round  above  the  stage,  so  that  light  of  various  degrees  of 
obliquity  may  be  concentrated  upon  opaque  objects.  The  object-plat- 
form of  the  stage  is  of  glass,  and  rotates  round  the  optic  axis  of  the 
microscope;  so  that  the  object  may  be  illuminated  by  oblique  rays  from 
any  azimuth.  A  mechanical  stage  may  be  added,  if  desired. — The  work- 
manship of  this  simple  model  is  of  the  highest  class;  and  there  is  little 
real  tvorh,  of  which,  in  the  hands  of  an  observer  who  knows  how  to  turn 
the  instrument  to  the  best  account,  it  may  not  be  made  capable,  by  the 
addition  of  a  Polariscope,  Paraboloid,  and  other  accessories,  which  its 
Sub-stage  adapts  it  to  receive.^ 

60.  Wale's  Neio  Working  Microscope, — A  Student's  Microscope  lately 
brought  out  by  Mr.  George  Wale  (IJ.  S.),  deserves  special  notice,  on 
account  of  several  ingenious  improvements  which  he  has  introduced  into 
its  construction. — In  the  first  place,  the  Himb  '  which  carries  the  body 
and  the  stage,  instead  of  being  swung  by  pivots — as  ordinarily — on  the 
two  lateral  supports  (so  that  the  balance  of  the  Microscope  is  greatly 
altered  when  it  is  much  inclined),  has  a  circular  groove  cut  on  either 
side,  into  which  fits  a  circular  ridge  cast  on  the  inner  side  of  each  sup- 
port. The  two  supports,  each  having  its  own  fore-foot,  are  cast  separately 
(in  iron),  so  as  to  meet  to  form  the  hinder  foot,  where  they  are  held 
together  by  a  strong  pin;  while  by  turning  the  milled-head  on  the  right 
support,  the  two  are  drawn  together  by  a  screw,  which  thus  regulates 
the  pressure  made  by  the  two  ridges  that  work  into  the  two  grooves  on 
the  limb.  When  this  pressure  is  moderate,  nothing  can  be  more  satis- 
factory than  either  the  smoothness  of  the  inclining  movement,  or  the 
balancing  of  the  instrument  in  all  positions;  while,  by  a  slight  tighten- 


1  The  cost  of  the  above  Microscope,  with  two  Eye-pieces  (B  and  C),  and  two 
Objectives  (5-8ths  and  l-7th  inch)  giving — vrith  the  Draw-tube — a  range  of  powers 
from  50  to  423  diameters,  packed  in  a  very  compact  Case,  is  only  £7  10s.  Od.,  or, 
with  the  addition  of  an  A  Eye-piece,  a  1^  or  2-inch  Objective,  Polarizing 
Apparatus,  and  Beale's  Drawing  Camera,  10  guineas. 

2  The  price  of  the  Microscope,  as  above  figured,  in  Case,  is  10  guineas.  None 
but  first-class  Objectives  are  supplied  by  Messrs.  Ross;  but  the  Student  wlio  finds 
these  too  costly  may  obtain  elsewhere  such  as  suit  his  requirements. 


62 


THE  MICROSCOPE  AND  ITS  REVELATIONS. 


ing  of  the  screw,  it  can  be  firmly  fixed  either  horizontally,  verticallyj, 
or  at  any  inclination.  The  '  coarse '  adjustment  is  made  by  a  smooth- 
working  rack;  whilst  the  ^fine/  made  by  a  milled-head  at  the  back  of 
the  ^limb/  raises  or  lowers  the  body  by  acting  on  the  slide  that  carries 
the  rack-and-pinion  movement.  The  body  is  furnished  with  a  long 
draw-tube,  which  carries  a  screw  at  its  lower  end  for  the  reception  of 
objectives  of  foci  too  long  to  be  worked  from  the  nose  of  the  outside  body. 
The  stage,  though  thin  enough  to  admit  very  oblique  light,  is  very  firm; 


Wale's  New  Working  Microscope. 

it  is  circular,  and  has  an  all-round  groove  near  its  edge,  alike  on  its  upper 
and  its  under  side.  Into  this  groove  there  fits  a  spring-clip  for  holding 
down  the  slide  upon  the  stage;  and  this  may  not  only  be  turned  round 
into  any  position  above  the  stage,  but  may  be  reversed  so  as  to  hold  the 
slide  against  its  imder  side,  thus  enabling  light  of  any  degree  of  obliquity 
to  be  thrown  on  the  object.  A  removable  *  Iris-diaphragm '  (§  98), 
which  is  made  to  open  or  close  by  a  screw-action,  is  fitted  into  the  stage 
in  such  a  manner  that  its  aperture  is  very  close  to  the  under  side  of  the 


CONSTRUCTION  OF  THE  MICROSCOPE. 


63 


object-slide — an  arrangement  than  which,  in  the  Anther's  opinion, 
nothing  can  be  better.  This  may  be  replaced  by  a  cylindrical  fitting  for 
the  reception  of  a  Polariscope,  Paraboloid,  etc.  The  donble  mirror  is 
carried  npon  an  arm  which  swings  on  a  pivot  from  the  front  of  the  limb 
beneath  the  stage,  and  is  capable  of  extension  by  a  dovetail  sliding  bar. 
— Altogether,  this  instrument  (so  far  as  its  mechanical  arrangements 
are  concerned),  comes  nearer  than  any  others  that  the  Author  has  seen, 
to  his  idea  of  a  model  Student's  Microscope.* 

61.  NaclieVs  Studenfs  Microscope. — This  instrument  deserves  special 
mention  for  certain  peculiarities  of  construction  which  distinguish  it 
from  the  ordinary  Continental  model  of  Microscopes  of  this  class. 
While  most  of  these  can  be  used  only  in  the  vertical  position,  the  Micro- 
scope of  MM.  Nachet  is  attached  to  the  supporting  pillar  by  a  cradle- 
joint,  which  allows  it  to  be  inclined  at  any  angle.  The  body  is  furnished 
with  a  draw-tube,  by  which  it  is  shortened  for  packing;  and.  is  embraced 
by  a  tube  which  carries  the  rack,  so  that  it  is  well  supported,  and  maybe 
readily  drawn  out  and  replaced  by  the  Binocular  already  described. 
(§  38,  Fig.  28).  The  ^slow  motion^  is  given  by  a  milled-head  placed  at 
the  top  of  the  sliding-stem,  so  as  to  be  near  that  which  gives  the  rack- 
and-pinion  adjustment.  The  chief  peculiarity  of  this  instrument,  how- 
ever, lies  in  its  Stage,  which  the  Author  has  no  hesitation  in  pronouncing 
to  be  the  most  perfect  of  its  Tcind  that  has  been  yet  devised.'^  Its  base  is 
formed  of  a  thick  plate,  3J  inches  square,  having  a  large  circular 
aperture;  and  on  this  is  superposed  a  circular  plate  of  3  inches  in 
diameter,  to  which  a  rotary  movement,  concentric  with  the  optic  axis  of 
the  Microscope,  can  be  given  with  great  facility.  In  this  circular  plate 
a  disk  of  thin  plate-glass  is  cemented  with  black  cement,  the  united 
thickness  of  the  two  around  the  central  aperture  being  not  more  than 
l-8th  of  an  inch,  so  that  light  of  the  greatest  obliquity  can  be  trans- 
mitted to  the  object  from  beneath.  The  rotating  plate  is  furnished  with 
a  projection  at  the  back,  to  which  is  attached  a  strong  V-shaped  pair 
of  springs,  having  their  extremities  armed  beneath  with  small  ivory 
knobs,  which  press  down  on  the  Object-carrier.  This  last  consists  of  a 
brass  frame  furnished  with  tongues  and  springs  projecting  forward  for 
the  reception  of  the  slide,  and  also  wi^h  a  pair  of  knobs,  to  v^liich  the 
fingers  may  be  applied  in  giving  motion  to  it;  whilst  the  frame  incloses 
a  piece  of  plate-glass  a  little  thicker  than  itself.  Thus  the  under  surface 
of  the  glass-plate  of  the  Object-carrier  slides  over  the  upper  surface  of 
the  circular  glass  stage-plate;  being  held  down  upon  it  and  retained  in 
any  position  by  the  pressure  of  the  ivory  knobs.  The  advantages  of  this 
arrangement  lie  (1)  in  the  perfect  facility  with  which  the  Object-carrier 
may  be  moved,  and  the  steadiness  with  which  it  keeps  its  place  when  not 
unduly  weighted;  (2)  in  the  facility  with  which  it  can  be  readjusted,  in 
case  the  movement  should  become  too  easy,  by  bending  down  the  V 
springs;  and  (3)  by  the^absence  of  liability  to  derangement  by  rust — a 
point  of  great  importance  when  work  is  being  done  with  sea-water  or 
chemicals.    The  front  portion  of  the  rotating  plate  bears  a  small  pro- 


^  This  Microscope,  with  two  Eye-pieces,  and  with  fairly  good  Objectives  of 
2-3ds  and  l-5th  inch,  is  sold  in  New  York  for  35  dollars,  or  little  more  than  £7. 
It  could  probably  be  made  in  this  country  (if  there  were  a  considerable  demand 
for  it)  for  5  guineas. 

2  This  Stage,  which,  on  the  Author's  recommendation,  has  been  copied,  first 
by  Mr.  Crouch,  and  now  by  other  English  opticians,  seems  to  have  been  originally 
invented  by  Mr.  Zentmayer  of  Philadelphia. 


64 


THE  MICROSCOPE  AND  ITS  REVELATIONS. 


jecting  piece  on  either  side,  into  which  may  be  screwed  a  pin  that 
carries  a  sliding-sprin^;  this  arrangement  is  suited  for  securing  a 
Zoophyte-trough  or  other  piece  of  apparatus  not  suitable  to  being 
received  by  the  object  carrier,  which  can  be  easily  slipped  away  from 
beneath  the  ivory  knobs,  thus  leaving  the  stage  free.  To  the  under  side 
of  the  stage  is  firmly  pivoted  a  broad  bar,  into  which  is  screwed  a  short 
sprung  tube,  that  becomes  exactly  concentric  with  the  optic  axis  of  the 
instrument,  when  the  bar  (which  is  shown  turned  away  in  the  figure)  is 
pushed  beneath  the  stage  until  checked  by  a  firm  stop;  and  as  this  bar  is 
composed  of  two  pieces,  held  together  by  a  pair  of  screws  working 
through  slots,  the  centering  of  the  tube  may  be  precisely  readjusted  if  it 
should  at  any  time  become  faulty.  Into  this  tube  may  be  inserted 
another  that  carries  either  (1)  a  Diaphragm,  sliding  with  caps  of  differ- 


Nachet's  Student's  Microscope.  Browning's  Rotating  Microscope. 


ent  apertures;  (2)  a  Polarizing  prism;  (3)  a  Ground-glass  for  diffusing 
the  light,  which  may  be  either  plane,  or  a  plano-convex  lens  ground  on 
its  flat  side  Avhich  is  directed  upwards;  (4)  au  Achromatic  Condenser; 
and  (5)  a  Glass  Cone,  having  its  apex  pointing  downwards,  and  a  large 
black  spot  in  the  centre  of  the  convex  base  directed  towards  the  object, 
which  gives  au  excellent  '  black-ground  ^  illumination.  Lastly,  the 
Mirror  is  attached  to  a  stem  which  is  so  jointed  as  to  enable  it  to  reflect 
rays  of  very  great  obliquity. — To  those  who  wish  a  compact  instrument 
of  great  completeness  and  capability,  which  may  be  worked  advantage- 
ously even  with  high  powers,  the  Author  can  strongly  recommend  this 


C?OJ^STRUCTION  OF  THE  MICROSCOPE. 


65 


Microscope.  The  Objectives  supplied  with  it  are  of  great  excellence  and 
very  moderate  cost,  and  are  quite  adequate  for  all  the  ordinary  purposes 
of  scientific  investigation. 

62.  Broionincf  s  Rotating  Microscope. — The  peculiarity  of  this  instru- 
ment is  that,  as  in  many  of  the  Continental  models,  the  object-platform 
(b),  with  the  limb  carrying  the  body  above  it,  revolves  together;  whilst 
the  lower  ])Lite  of  the  stage  (o),  with  any  apparatus  fitted  into  it,  as  like- 
wise the  mirror,  remains  fixed.  Thus  the  object  is  enabled  to  receive 
illumination  in  every  azimuth  without  any  derangement  either  in  its  cen- 
tering, or  in  its  focal  adjustment.  Tho  body  is  supported,  as  in  the  Jack- 
son model,  upon  a  limb.  A,  which  is  firmly  fixed  to  the  rotating  plate  B  of 
the  stage.  In  the  simplest  form  of  the  instrument,  shown  in  the  figure, 
the  rotation  is  effected  by  pressing  a  finger  on  the  projecting  pins  attached 
to  b;  but  if  required,  B  can  be  made  to  move  by  a  pinion  and  toothed 
wheel,  with  graduated  scale  attached;  and  a  sub-stage  for  carrying  illumi- 
nating apparatus  can  be  fixed  to  an  arm  below  c.  This  Microscope  is  fur- 
ther characterized  by  the  solidity  of  its  several  parts,  and  the  care" taken  in 
its  construction  to  secure  it  against  derangement  from  an  accidental  strain. 
It  is  particularly  adapted  to  the  use  of  those  who  work  with  high  powers 
upon  objects  requiring  the  varied  illumination  for  which  this  rotating 
arrangement  gives  special  facilities. 

63.  Croucli's  Student's  Binocular. — This  instrument  (Plate  in.)  was 
devised  at  a  time  when  the  construction  of  the  Binocular  was  still  almost 
exclusively  confined  to  the  makers  of  First-class  instruments;  and  it  had 
the  great  merit  of  bringing  within  reach  of  the  Student  a  convenient  and 
well-constructed  Binocular,  at  a  moderate  cost.  With  the  improvements 
it  has  since  received,  it  still  remains  one  of  the  best  instruments  of  its 
class;  and  the  Author,  after  considerable  use  of  it,  can  strongly  recom- 
mend it  to  such  as  desire  to  possess  a  Binocular  at  once  cheap,  good,  and 
portable.  Its  general  arrangement  is  shown  in  Plate  in.,  but  a  mechani- 
cal stage  can  be  substituted,  if  desired.  The  rotating  stage  and  object- 
holder  resemble  those  of  MM.  Nachet's  Microscope  (Fig.  45). — An  Achro- 
matic Condenser,  Paraboloid,  Polarizing  apparatus,  etc.,  can  be  added  to 
this  instrument;  or  it  may  be  fitted  with  Mr.  Crouch's  '  Universal  Sub- 
stage  Illuminator,'  which,  like  that  of  Mr.  Swift  (Fig.  85),  combines 
the  different  Accessories  ordinarily  required  for  the  examination  of  trans- 
parent objects.' 

64.  Baker's  Student's  Erecting  Binocular. — With  a  special  view  to 
the  wants  of  Students  in  various  departments  of  Biology,  Messrs.  Baker 
have  adapted  a  Stephenson  Binocular  (§  35)  to  the  stand  of  their 
Student's  Microscope,  as  shown  in  Fig.  47;  with  which  the  stand  of 
their  Laboratory  Dissecting  Microscope  (Fig.  35)  may  be  so  combined  as 
to  afford  the  requisite  support  to  the  hands,  when  they  are  engaged  in 
dissecting  (or  otherwise  manipulating)  objects  on  the  stage  of  the  Binocu- 
lar. An  ordinary  Monocular  body  may  be  readily  substituted  for  the 
Binocular;  and  the  same  Eye-pieces  and  Objectives  serve  for  both.  The 
low  cost  at  which  this  instrument  is  made,  will  doubtless  cause  many  to 
possess  themselves  of  it,  whose  pursuits  will  be  specially  facilitated  by  its 
use.^ 


^  The  price  of  this  instrument,  with  one  pair  of  Eye-pieces,  two  Objectives  (a 
best  1-inch  and  a  l-4th  of  110°),  and  a  Condenser  for  opaque  objects,  in  case,  is 
£12  15s.  Od. 

2  The  price  of  this  Binocular,  with  one  pair  of  Eye-pieces,  a  dividing  Objective 
of  1  inch  and  2  inches,  and  a  l-4th  inch  of  70%  in  Case,  is  10  guineas. 
5 


crouch's  students'  binocular. 


CONSTRUCTION  OF  THE  MICROSCOPE. 


67 


Excellent  Students'  Microscopes  are  now  produced  by  many  other  Makers; 
among  whom  Messrs.  Beck  should  be  particularly  mentioned,  as  having  led  the 
way  in  supplying  low-priced  but  really  serviceable  instruments,  such  as  could  at 
that  time  only  be  obtained  on  the  Continent.  Their  *  Economic '  Microscope 
framed  on  the  Continental  model,  and  furnished  with  good  Objectives  of  1  inch 
and  l-4th  inch,  is  sold  for  5  Guineas;  and  other  Objectives  specially  constructed, 
for  it,  ranging  to  the  l-16th  inch,  with  a  complete  set  of  Accessories,  are  supplied' 
at  a  very  moderate  cost.  The  same  Makers  supply  an  *  Economic' Wenhamj 
Binocular,  having  two  pairs  of  Eye-pieces,  three  Objectives,  a  glass  rotating' 
Stage,  and  a  jointed  lengthening  arm  to  the  Mirror  (which  allows  it  to  be  used 
above  the  Stage  for  the  illumination  of  opaque  objects)  for  10  Guineas. — Mr* 
Collins  also  supplies  a  10  guinea  Wenham  Binocular,  with  Objectives  of  1  inch 
and  l-4th  inch  (80''),  the  latter  being  specially  adapted  for  use  with  the  Binocular, 
by  a  short  mount  which  brings  it  close  to  the  Wenham  prism. — Mr.  Swift  makes 


Baker's  Student's  Erecting  Binocular. 

a  *  College '  Microscope,  in  which  the  Stage  is  fitted  with  a  revolving  diaphragm- 
plate  of  ingenious  construction,  that  brings  its  apertures  up  to  the  level  of  the 
object-slide.  Of  Mr.  Crouch's  and  Messrs.  Parkes's  Students'  Microscopes  also, 
the  Author  can  speak  with  approval,  as  regards  both  the  mechanical  and  the 
optical  part  of  their  work. 

65.  Second-Class  Microscopes. — ^Under  this  head  may  be  ranked  those 
instruments  which  combine  first-rate  workmanship  with  simplicity  in  the, 
plan  of  construction;  and  which  may  be  consequently  designated  as  ^  Supe- 
rior Students'  Microscopes.'  Among  these  the  first  place  should  be  given 
to  Messrs.  Powell  and  Lealand's  Smaller  Microscope  (Fig.  48),  which  was 
long  the  favorite  instrument  of  British  Histologists,  and  which,  though 
not  adapted  for  objects  requiring  very  oblique  light,  is  still  in  demand 


68 


THE  MICROSCOPE  AND  ITS  REVELATIONS. 


among  those  who  value  first-rate  workmanship,  with  all  convenient  appli- 
ances for  ordinary  Biological  research.  A  Sub-stage  (not  shown  in  the 
figure)  carrying  every  kind  of  illuminating  apparatus,  can  be  attached 
beneath  the  stage;  and  the  large  angular  aperture  now  given  by  Messrs. 
Powell  and  Lealand  to  their  Immersion  Achromatic  Condenser,  enables 
,  this  instrument  to  resolve  the  most  difficult  test-objects.  The  stand  is 
\  well  suited  to  carry  a  Bino- 

cular body;  which  may  be 
fitted  not  *^only  with  the  or- 
dinary stereoscopic  ^  Wen- 
ham^  prism,  but  also  with 
the  non-stereoscopic  arrange- 
ment of  these  Makers  (§  81), 
which  enables  even  the 
highest  powers  to  be  used 
binocularly,  though  not  ste- 
reoscopicaily. 

66.  The  value  of  Stereo- 
scopic  Binocular  vision  in 
Scientific  investigation  be- 
ing now  admitted  by  all 
who  have  really  worked  with 
it  tipon  siiitable  objects,  the 
Author  would  earnestly  re- 
commend every  one  about  to 
provide  himself  with  even  a 
Second-class  Microscope,  to 
incur  the  small  expense  of 
the  Binocular  addition. 
This  addition,  however,  will 
lose  an  important  element  of 
its  value,  if  the  Stage  of  the 
instrument  be  not  adapted 
to  rotate  in  the  oi3tic  axis  of 
the  Body;  so  that  objects 
which  are  being  viewed  by 
incident  light  may  be  pre- 
sented to  the  illuminating 
rays  in  every  direction. 
Among  the  first  to  recog- 
nize this  principle,  and  to 
apply  it  in  practice,  were  Messrs.  Beck;  whose  Popular  Microscope 
(Plate  IV.),  devised  by  the  late  Mr.  E.  Beck,  will  be  found  very  suitable 
to  the  wants  of  such  as  work  with  low  and  moderate  powers  upon  objects 
for  the  study  of  which  Binocular  vision  is  peculiarly  advantageous;  and 
especially  serviceable  to  Travellers,  as  the  ingenious  way  in  which  it  is 
framed  and  supported  enables  it  to  bear  a  good  deal  of  rough  usage 
without  injury.  The  original  Eoss  model  here  adopted  in  the  support 
and  movement  of  the  body,  is  sufficiently  steady  when  only  moderate 
powers  are  employed;  and  the  stem  that  forms  the  centre  of  the  whole,  is 
swung  immediately  behind  the  stage  on  a  broad  stay  G,  which,  again,  is 
attached  by  a  pair  of  centres  at  its  lower  angles  to  the  triangular  base  r. 
The  lower  end  h  of  the  stem  carries  a  stout  projecting  pin,  which  fits 
into  various  holes  along  the  median  line  of  the  base;  whereby  the  instru- 


Powell  and  Lealand's  Smaller  Microscope. 


CONSTRUCTION  OF  THE  MICROSCOPE. 


69 


EDATE  ly. 


beck's  popular  microscope. 


70 


THE  MICROSCOPE  AND  ITS  REVELATIONS. 


ment  may  be  firmly  steadied  in  positions  more  or  less  inclined,  or  may  be 
fixed  upright.  It  may  be  also  fixed  in  the  horizontal  position  required 
for  drawing  with  the  Camera  Lucida;  for  the  pin  at  the  bottom  of  the 
stem  then  enters  the  hole  at  the  top  of  the  stud  k,  and  the  stay  g  falls 
flat  down,  resting  on  the  top  of  the  stout  pin  L.  The  advantages  of  this 
construction  are  that  it  is  strong,  firm,  and  yet  light;  that  the  instru- 
ment rests  securely  at  the  particular  inclination  desired,  which  is  often 
not  the  case  on  the  ordinary  construction  when  the  joint  has  worked 
loose;  and  that  in  every  position  there  is  the  needful  preponderance  of 
balance.  The  Stage  D  is  circular,  and  upon  it  fits  a  circular  plate  T, 
which  rotates  in  the  optic  axis  of  the  Microscope.  On  the  plate  T  there 
slides  the  Object-holder  u,  which  is  so  attached  to  it  by  a  wire  spring 
that  bears  against  its  under  surface,  as  to  be  easily  moved  by  either  or 
both  hands;  and  as  access  can  be  readily  gained  to  this  spring  by  detach- 
ing the  plate  T  from  the  stage,  it  may  either  be  removed  altogether  so  as 
to  leave  the  stage  free,  or  may  be  adjusted  to  any  degree  of  stiffness 
desired  by  the  observer.  The  object-holder  has  a  ledge  V  for  the  support 
of  the  slide;  and  it  is  also  provided  with  a  small  spring  w,  attached  to  it 

by  a  milled-head,  by  turning 
which  the  spring  may  be 
brought  to  bear  with  any  re- 
quired pressure  against  the 
edge  of  the  slide  laid  upon  the 
object-holder,  so  as  to  prevent 
it  from  shifting  its  place  when 
rotation  is  given  to  the  stage, 
or  when,  the  instrument  being 
placed  in  the  horizontal  posi- 
tion, the  stage  becomes  verti- 
cal. The  central  tube  of  the 
Stage  is  furnished  with  a  rota- 
ting Diaphragm-plate,  and  is 
adapted  to  receive  various 
^ther  fittings;  and  a  Side-Con- 
denser on  a  separate  stand  is 
also  supplied.  * 

67.  Collinses  Harley  Bin- 
ocular.— This  instrument,  as 
represented  in  Eig.  49,  is  sub- 
stantially framed  and  well 
hung  on  the  Eoss  model;  but 
is  now  made  also  on  the  Jack- 
son model  at  the  same  price. 
The  caps  of  the  Eye-i)ieces  are 
provided  with  shades,  which 
cut  off  the  outside  lights  from 
each  eye;  these  can  be  adapted 
to  any  instrument,  and  the 
Author  can  speak  strongly  of 
coiiins's  Harley  Binocular.  their  value  from  his  own  ex- 

i  ;  ^  

^  The  price  of  this  instrument,  with  two  pairs  of  Eye-pieces,  three  Objectives 
(a  2-inch  of  10%  a  1-inch  of  22%  and  a  l-4th  of  75°),  and  Side-Condenser  on  stand, 
in  Case,  is  £16  10s. 


CONSTRUCTION  OF  THE  MICROSCOPE. 


71 


perience.  The  Wenham  prism  at  the  common  base  of  the  bodies  is  fitted 
into  an  oblong  box,  which  slides  through  the  arm  that  carries  them;  this 
contains,  in  addition,  a  Nicol  analyzing  prism,  and  is  also  pierced  with  a , 
vacant  aperture;  so  that,  by  merely  sliding  this  box  transversely  until  its 
aperture  comes  into  the  axis,  the  instrument  may  be  used  as  an  ordinary 
Monocular;  or,  if  the  analyzing  prism  be  made  to  take  the  place  of  the 
Wenham,  whilst  the  polarizing  prism  beneath  the  stage  is  brought  into 
position  by  rotating  the  Diaphragm-plate  in  which  it  is  fixed,  it  is  at  once 
converted  into  a  Polarizing  Microscope — with  the  disadvantage,  however, 
of  not  being  then  Binocular.  It  has  also  a  ^nose-piece'  carrying  two 
Objectives,  by  a  sliding  movement  of  which  one  power  may  be  substi- 
tuted  for  the  other/ 

68.  Swiff s  Challenge  Micro- 
scope,— The  instrument  con- 
structed under  this  designation 
by  Messrs.  Swift,  is  one  of  which 
it  may  be  fairly  said  that  it  is 
surpassed  by  no  other  of  its  price 
in  the  excellence  of  its  work- 
manship, and  its  suitability  to 
the  general  wants  of  the  Micro- 
scopist.  The  support  on  which 
it  is  hung  is  extremely  firm  and 
substantial  without  being  heavy; 
and  when  the  limb  is  brought  to 
the  horizontal  position,  resting 
on  the  cross  plate  between  the 
two  uprights,  the  instument  is 
still  well  balanced.  The  rack 
and  pinion  movement  is  made 
with  oblique  teeth;  a  construc- 
tion which  favors  smoothness 
and  sensitiveness  in  the  adjust- 
ment, so  that  a  l-4th  inch  objec- 
tive may  be  focussed  by  it  alone. 
The  fine  adjustment  is  made  by 
the  milled-head  at  the  lower  end 
of  the  body. — It  is  a  peculiarity 
in  this  instrument,  which  espe- 
cially fits  it  for  those  who  work 

much  with  Polarized  light,  that  Swift's  challenge  Microscope. 

the  analyzing  prism  is  fitted 

into  the  body  above  the  Wenham  prism,  in  such  a  manner  that,  when 
its  fitting  is  drawn  out  (without  being  removed),  it  is  completely 
out  of  the  way  of  the  light-rays;  whilst,  when  the  use  of  the  Polariscope 
is  required,  the  prism  can  be  at  once  pushed  into  the  body,  working  in 
conjunction  with  the  Wenham  prism.  This  mode  of  mounting  the 
analyzer  is  found  to  interfere  much  less  with  the  definition  of  the  objec- 
tive, than  the  insertion  of  it  between  the  objective  and  the  Wenham 


^  The  price  of  this  instrument,  with  Mechanical  rotatinj^  Stage,  two  pairs  of 
Eye-pieces,  two  Objectives  (either  a  2-inch  of  12°,  or  a  1-inch  of  18°,  with  a  l-4th 
of  95°),  Side-Condenser  on  Stand,  and  Polarizing  apparatus  in  Cabinet,  is  £19. 
Accessories  of  various  kinds  can  be  readily  fitted  to  it. — A  *  first-class '  Binocular 
is  also  constructed  by  the  same  Maker  on  the  Jackson  model. 


72 


THE  MICROSCOPE  AND  ITS  KEVELATIONS. 


prism.  The  stage  rotates  in  the  optic  axis;  and  may  either  bear  (as  in 
the  figure)  a  sliding  object-carrier,  or  may  be  furnished  with  mechanical 
actions*  The  mirror  is  attached  to  the  stem  by  a  crank-arm,  allowing  it 
to  be  so  placed  as  to  reflect  light  of  considerable  obliquity.  Beneath  the 
Stage  is  a  broad  horizontal  dovetail  groove,  into  which  is  very  exactly 
fitted  a  firm  (sprung)  slide  that  cames  a  Sub-stage  for  illuminating  appa- 
ratus, fitted  with  a  vertical  rack  movement,  and  with  horizontal  center- 
ing screws;  this  arrangement  (devised  by  Mr,  Swift)  enables  the  sub- 


Browning's  Smaller  Stephenson  Binocular. 


stage  to  be  placed  in  position  or  removed,  without  disturbing  either  the 
stage  or  the  mirror.  The  extremely  ingenious  Universal  Sub-stage — 
combining  Achromatic  Condenser,  Black-ground  Illuminator,  and  Pola- 
rizer with  varied  adaptations — devised  by  Mr.  Swift  for  this  Microscope, 
but  capable  of  being  applied  to  any  other,  will  be  described  hereafter 
(§  112).  The  Author,  having  had  his  instrument  (thus  fitted)  in  constant 


CONSTRUCTION  OF  THE  MICROSCOPE. 


73 


use  for  several  years  past,  feels  justified  in  unreservedly  expressing  his 
high  appreciation  of  it/ 

69.  Browning^s  Smaller  8teplienso7i  Binocular. — This  instrument, 
represented  in  Fig.  61,  is  of  more  substantial  build  than  the  Students' 
Binocular  of  Messrs.  Baker  (§  64);  and  is  further  distinguished  by.its 
special  adaptation  for  use  with  Polarized  light.  In  place  of  the  reflecting 
prism  at  the  junction  of  the  inclined  bodies,  a  plane  piece  of  dark  glass, 
silvered  on  one  face,  is  hung  on  a  horizontal  axis  at  the  polarizing  angle; 
its  silvered  face  being  turned  in  front  v^hen  it  is  used  for  ordinary  pur- 
poses, so  as  to  reflect  into  the  two  inclined  bodies,  the  light-rays  which 
proceed  to  it  from  the  pair  of  dividing  prisms;  whilst,  when  it  is  to  act  as 
an  analyzer,  it  is  turned  on  its  axis  by  means  of  a  milled-head  so  as  to 
bring  the  dark-glass  surface  to  the  front.  Further,  by  fixing  into  the 
arm  the  tube  which  carries  the  objective,  with  its  fine  adjustment,  and 
by  making  that  which  contains  tlie  dividing  prisms  and  mirror,  and 
which  also  carries  the  double  body,  slide  over  it,  the  latter  can  either  be 
turned  half  round,  so  as  to  point  the  eye-pieces  in  the  reverse  direction 
(for  the  exhibition  of  the  object  to  an  observer  sitting  at  the  opposite  side 
of  a  small  table)  without  any  disturbance  of  the  adjustments;  or  it  can 
be  lifted  off  altogether,  and  replaced  by  an  ordinary  Monocular  body,^ 

FIEST-CLASS  MICROSCOPES. 

70.  "We  now  pass  to  an  entirely  different  class  of  Instruments— those 
of  which  the  aim  is,  not  simplicity,  but  perfection;  not  the  production  of 
the  best  effect  compatible  with  limited  means,  but  the  attainment  of 
everything  that  the  Microscope  can  accomplish,  without  regard  to  cost 
or  complexity.  To  such,  of  course,  the  Stereoscopic  Binocular  is  an  in- 
dispensable addition;  and  it  is  not  less  essential  that  the  Stage  should 
have  a  rotatory  movement  in  the  Optic  axis  of  tlie  instriment; — not  only 
for  the  due  examination  of  opaque  objects,  as  already  mentioned  (§  66), 
but  also  because  this  movement  is  requisite  for  the  effective  examination 
of  very  delicate  transparent  objects  by  Oblique  light,  allowing  the  effect 
of  light  and  shadow  to  be  seen  in  every  direction;  and,  in  addition,  be- 
cause in  the  examination  of  objects  under  Polarized  light,  a  class  of  ap- 
pearances is  produced  by  the  rotation  of  the  object  between  the  prisms, 
which  is  not  developed  by  the  rotation  of  either  of  the  prisms  themselves. 

71.  Boss's  First-class  Microscope,— As  what  is  known  as  the  Boss 
model  is  still  made,  being  preferred  by  some  purchasers,  we  shall  com- 
mence with  a  notice  of  the  original  form  of  the  instrument  which  has 
gained  so  high  a  celebrity. — The  general  plan  of  this  Microscope,  as 
shown  in  Fig.  52,  is  carried  out  with  the  greatest  attention  to  solidity  of 
construction,  in  those  parts  especially  which  are  most  liable  to  tremor, 

^  The  price  of  this  instrument  in  the  simple  form  here  figured,  with  one  pair  of 
Eye-pieces  and  best  1-inch  and  l-4th  inch  (80°)  Objectives,  and  Condensing  lens 
on  separate  stand,  in  Case,  is  £14.  A  mechanical  stage  costs  £2  10s,  additional, 
and  the  sub-stage  (without  fittings)  £2  2s. — A  very  ingenious  *  swinging  sub- 
stage  '  has  been  lately  devised  by  Mr.  Swift  (''  Journ.  of  Koy.  Microsc.  8oc.,"  vol. 
iii.,  1880,  p.  867)  for  obtaining  illumination  of  any  degree  of  obliquity,  even  by 
two  pencils  at  once.  The  Condenser  is  made  to  slide  on  an  arc-piece  (as  in  Mr. 
Grubb's  arrangement,  §  72),  which  is  prolonged  above  the  Stage  for  opaque 
illumination;  and  with  this  may  be  combined  a  second  arc-piece  at  right  angles 
to  the  first,  carrying  a  second  Condenser,  which  is  found  serviceable  in  the  reso- 
lution of  difficult  Diatom-tests. 

^  The  price  of  this  instrument,  with  one  pair  of  Eye-pieces  and  Objectives  of  1 
inch  (16°)  and  l-4th  inch  (75°),  is  £20.  Any  Accessories  can  readily  be  added  to  it. 


74 


THE  MICROSCOPE  AND  ITS  REVELATIONS. 


as  also  to  the  due  balancing  of  the  weight  of  its  different  parts  upon  the 
horizontal  axis.  Any  inclination  may  be  given  to  it;  and  it  may  be  fixed 
in  any  position  by  a  clamping  screw,  turned  by  a  short  lever  on  the  right- 
hand  upright.  The  ^fine^  adjustment  is  effected  by  the  milled-head  on 
the  transverse  arm  just  behind  the  base  of  the  '  body;'  this  acts  upon  the 
'  nose '  or  tube  projecting  below  the  arm,  wherein  the  objectives  are 
screwed.    The  other  milled-head,  seen  at  the  summit  of  the  stem,  serves 


the  circular  rack,  moreover,  enables  it  to  be  used  as  a  Goniometer  (§  92). 
Below  the  stage,  and  in  front  of  the  stem  that  carries  the  mirror,  is  a 
dovetail  sliding-bar,  which  is  moved  uj)  and  down  by  the  milled-head 
shown  at  its  side;  this  sliding-bar  carries  what  is  termed  by  Mr.  Boss  the 
^Secondary  stage '  (shown  separately  at  b),  which  consists  of  a  tube  for 
the  reception  of  the  Achromatic  Condenser,  Polarizing  prisms,  and  other 
fittings.  To  this  secondary  stage  a  traversing  movement  of  limited  extent 
is  given  by  means  of  two  screws,  one  on  the  front  and  the  other  on  the 
left-hand  side  of  the  frame  which  carries  it,  in  order  that  its  axis  may  be 
brought  into  perfect  coincidence  with  the  axis  of  the  body;  and  a  rotatory 
movement  also  is  given  to  it  by  the  turning  of  a  milled-head,  which  is 
occasionally  useful,  and  the  exact  amount  of  which  is  measured  by  a 
graduated  circle. — The  special  advantages  of  this  instrument  consist  in 


Ross's  First-class  Microscope. 


Tig.  52. 


to  secure  the  transverse  arm 
to  this,  and  may  be  tightened 
or  slackened  at  pleasure,  so 
as  to  regulate  the  traversing 
movement  of  the  arm;  this 
movement  is  only  allowed  to 
take  place  in  one  direction, 
namely,  towards  the  right 
ride,  being  checked  in  the 
opposite  by  a  '  stop,'  which 
secures  the  coincidence  of 
the  axis  of  the  principal 
^body'  with  the  centre  of 
the  stage,  and  with  the  axis 
of  the  illuminating  appa- 
ratus beneath  it.  The  ob- 
ject-platform, to  which  rect- 
angular traversing  motions 
are  given  by  the  two  milled- 
heads  at  the  right  of  the 
stage,  is  also  made  to  rotate 
in  the  optic  axis  by  a  milled- 
head  placed  underneath  the 
stage  on  the  left-hand  side; 
this  turns  a  pinion  which 
works  against  a  circular 
rack,  whereby  the  whole 
apparatus  above  is  carried 
round  about  two-thirds  of 
a  revolution,  without  in  the 
least  disturbing  the  place  of 
the  object,  or  removing  it 
from  the  field  of  the  Micro- 
scope.   The  graduation  of 


CONSTRUCTION  OF  THE  MICROSCOPE. 


75 


its  general  steadiness,  in  the  admirable  finish  of  its  workmanship.,  and  in 
the  variety  of  movements  which  may  be  given  both  to  the  object  and  to 
the  fittings  of  the  secondary  or  sub-stage.  Its  disadvantages  consist  in 
the  want  of  portability  that  necessarily  arises  from  the  substantial  mode 
of  its  construction;  and  in  the  liability  to  tremor  in  the  image,  when  the 
liighest  powers  are  used,  through  the  wanfe  of  support  to  the  body  along 
its  length  (§  49). — This  last  consideration  has  induced  Messrs.  Boss  to 
adopt  the  *  Jackson-model^  in  their  more  recent  Microscopes;  the  newest 
and  most  complete  form  of  which  will  be  next  described. 

72.  Boss's  Improved  Jackson- Zeiitmuyer  Microscope. — In  this  admir- 
able instrument  (Plate  v.)  the  Jackson-model  is  followed  as  to  general 
construction,  whilst  it  is  improved-on  in  various  important  particulars. 
The  ^limb^  that  supports  the  principal  body  with  the  usual  rack-and- 
pinion  slide  for  coarse  adjustment,  carries  also  a  second  (or  focussing) 
slide  at  the  back  of  the  first,  to  which  a  slow  up  and-down  movement  is 
given  by  a  lever  passing  through  a  channel  in  the  limb,  which  is  acted- on 
by  a  micronometer  screw  with  a  large  milled-head  placed  in  a  very  acces- 
sible position.  This  arrangement  renders  the  fine  adjustment  quite 
free  from  either  ^  twist'  or  ^  loss  of  time,'  whilst  permitting  it  to  work 
with  sufficient  freedom;  and  has  the  advantage  of  not  affecting  the  mag- 
nifying power  by  altering  the  length  of  the  body.  Further,  if  a  divided 
scale  (with  a  vernier)  be  engraved  on  the  edge  of  the  limb,  the  thickness 
of  any  uncovered  object  lying  on  the  stage  can  be  measured  with  great 
exactness.  The  rotating  stage-plate  (graduated  at  its  edge  to  serve  as  a 
Goniometer),  is  supported  upon  a  firm  ring  composed  of  metal  of  pecu- 
liar inflexibility;  and  to  this  it  can  be  secured  in  any  azimuth  by  a 
clamping-screw  beneath.  Its  single  traversing  platform  is  moved  in 
rectangular  directions  by  two  milled-heads  placed  on  the  same  axis,  that 
work  a  combination  of  screw  and  pinion  (devised  by  the  ingenuity  of 
Mr.  Wenham),  which  is  placed  above  instead  of  beneath  it;  and  in  this 
device  more  oblique  light  (it  is  affirmed)  can  be  brought  to  bear  upon  the 
lower  surface  of  the  object,  than  in  any  other  mechanical  stage  yet  con- 
structed. The  stage-ring  is  not  immovably  fixed  to  the  limb,  but  is  at- 
tached to  a  conical  stem,  which  passes  through  the  tubular  pivot  of  the 
swinging  '  tail-piece '  to  be  presently  described,  and  is  clamped  at  the 
back  of  the  instrument  by  a  strong  screw  and  nut.  Thus  the  stage  may 
be  made  to  incline  toward  either  side  at  any  angle,  so  that  a  view  may 
be  gained  of  the  sides  and  edges  of  a  solid  object,  as  well  as  of  its  front; 
or  it  may  be  removed  altogether,  and  replaced  by  any  other  form  of  ob- 
ject-support more  suitable  to  the  special  requirements  of  the  individual 
Microscopist. — The  most  important  novelty,  however,  consists  in  the 
adoption  of  the  (patented)  Zentmayer  method  of  giving  to  the  entire 
illuminating  apparatus  any  desired  degree  of  obliquity.  The  Mdea'  is 
by  no  means  new;  and  it  was  carried- out  many  years  ago  by  the  late  Mr. 
Grubb  of  Dublin,  who  fixed  beneath  the  stage  a  sector  or  arc-piece  of 
nearly  a  semi-circle  having  its  centre  in  the  object,  upon  which  the  at- 
tachments of  the  mirror  and  condenser  were  made  to  slide.  But  tlie 
arrangement  devised  by  Mr.  Zentmayer  is  not  only  far  simpler,  but  also 
more  effective.  It  consists  in  swinging  the  '  tail-piece'  which  carries  the 
mirrow  and  the  secondary  or  sub-stage,  upon  a  pivot  placed  at  the  back 
of  the  stage,  the  horizontal  axis  of  which  is  in  a  line  with  the  point  of 
intersection  of  the  optic  axis  of  the  body  with  the  plane  of  the  object  on 
the  stage;  so  that  the  axis  of  the  condenser  shall  always  pass  through 
that  point,  whatever  may  be  its  inclination  to  the  perpendicular.  By 


76 


THE  MIOKOSCOPE  AND  ITS  REVELAl'IOilS. 


OOfTPTRUCTION  OF  THE  MICROSCOPE. 


7T 


means  of  this  arrangement,  every  kind  of  illuminating  apparatus  adapted 
to  the  sub-stage  can  be  made  to  act  at  any  obliquity  whatever;  and  as 
the  tail-piece  may  be  swung  round  on  the  side  opposite  to  that  of  the 
milled-heads  of  the  traversing  stage,  until  it  is  brought  considerably 
above  the  stage,  oblique  illumination  may  be  thrown  by  the  condenser, 
not  only  on  the  under  but  also  on  the  upper  surface  of  any  object.  It 
is  one  great  advantage  of  this  method,  that  condensers  of  large  angle  of 
aperture  are  not  required  for  the  purpose  of  oblique  illumination;  the  con- 
verging pencils  given  by  ordinary  Objectives  of  1  inch  or  1 J  inch  focus,  used 
as  condensers,  being  fully  adequate.  Further,  the  swinging  tail-piece  may 
be  used  to  measure  the  angular  aperture  of  Objectives  in  the  manner  to 
be  hereafter  described,  its  inclination  to  the  optic  axis  being  marked  by 
a  divided  arc  on  its  upper  segment,  which  also  enables  the  illuminating 
angle  at  which  any  particular  object  is  best  seen  to  be  observed  and  re- 
corded.— Altogether,  it  may  be  unhesitatingly  affirmed,  that  the  Zent- 
mayer  system  enables  the  best  results  of  oblique  illumination  to  be  ob- 
tained with  greater  facility  than  any  other  of  equal  effectiveness;  while 
the  simplicity  of  the  construction  of  the  whole  instrument  enables  Messrs. 
Eoss  to  reduce  its  cost  considerably  below  that  of  the  old  Eoss  or  Eoss- 
Jackson  models. 

73.  Poioell  and  Leland^s  Large  Microscope, — These  eminent  Makers 
have  not  made  any  essential  modification  in  the  construction  of  their 
large  Microscope,  represented  in  Plate  yii. ;  preferring  to  furnish  the 
very  oblique  illumination  now  in  general  demand  by  enlarging  the  angu- 
lar aperture  of  their  Achromatic  Condenser  (§  99).  The  chief  peculiar- 
ity of  their  model  consists  in  the  attachment  both  of  the  Stage  and  Sub- 
stage  to  a  large  solid  brass  ring,  which  is  firmly  secured  to  the  stem  of 
the  instrument.  The  upper  side  of  this  ring  bears  a  sort  of  carriage  that 
supports  the  stage;  and  to  this  carriage  a  rotatory  movement  around  the 
optic  axis  of  the  principal  body  is  given  by  a  milled-liead,  the  amount  of 
this  movement  (which  may  be  carried  through  an  entire  revolution)  being 
exactly  measured  by  a  graduated  circle.  The  stage,  which  is  furnished 
with  the  usual  traversing  movements,  worked  by  two  milled-heads  on  the 
same  axis,  is  made  thin  enough  to  admit  of  the  mirror  being  so  placed, 
by  means  of  its  extending  arm,  as  to  reflect  light  on  the  object  from  out- 
side the  large  brass  ring  that  supports  the  stage  and  sub-stage.  Light 
of  the  greatest  obliquity,  however,  may  be  more  conveniently  obtained 
by  an  Amici's  prism  (§  102)  placed  above  the  supporting  ring.  The 
sub-stage  is  furnished  with  rotatory  and  rectangular,  as  well  as  with  ver- 
tical movements.  The  instrument  is  so  well  balanced  on  its  horizontal 
axis,  that  it  remains  perfectly  steady  without  clamping,  in  whatever  po- 
sition it  may  be  placed. 

74.  Beclc's  First-class  Microscope, — It  was  by  this  Firm  that  the 
Jackson  model  was  first  adopted,  for  which  the  Author  has  already  ex- 
pressed his  preference  (§  49).  Besides  the  steadiness  imparted  to  the 
double  body  by  the  support  given  to  it  by  the  limb  along  the  greater  part 
part  of  its  length,  it  is  an  additional  advantage  of  this  construction,  that 
by  continuing  the  limb  beneath  the  stage,  the  secondary  body  or  Sub- 
stage  (which  carries  the  illuminating  apparatus)  is  made  to  work  in  a 
dovetailed  groove  that  is  ploughed-out  in  continuity  with  that  in  which 
the  rack  of  the  principal  body  slides,  an  arrangement  obviously  favorable 
to  exactness  of  centering.  The  Stage  has  a  nearly  complete  rotation  in 
the  optic  axis  of  the  instrument,  motion  being  given  to  it  by  a  milled- 
head  beneath  the  stage,  the  pinion  attached  to  which  can  be  readily 


78 


THE  MICKOSCOPE  AND  ITS  KEVELAVIONS. 


CONSTRUCTION  OF  THE  MICROSCOPE. 


ELATE  YIL 


POWELL  AND  LEALAND'S  LARGE  MICROSCOPB. 


80 


THE  MICROSCOPE  AND  ITS  REVELATIONS. 


thrown  out  of  gear  when  a  more  rapid  rotation  of  the  stage  by  hand  is 
desired;  and  it  bears  a  graduated  circle  at  its  margin  for  the  measure- 
ment of  angles.  It  is  fitted  immediately  beneath  the  object-platform 
with  an  iris-diaphragm^  worked  by  a  lever  action. 

75.  Bgc¥s  Improved  First- class  Microscope, — In  order  to  meet  the 
demand  for  very  oblique  illumination,  and  to  supply  this  in  a  mode  yet 
more  perfect  than  the  Zentmayer  system,  Messrs.  Beck  have  adapted  to 
the  preceding  instrument  a  swinging  sub-stage,  carried  by  an  arm  that 
works  radially  upon  a  large  vertical  disk  attached  to  the  limb,  on  the  plan 
originally  suggested  by  Mr.  Grubb;  his  semi-circle  being  extended,  how- 
ever, into  a  nearly  complete  circle,  so  as  to  allow  the  arm  carrying  the 
sub-stage  and  mirror  to  be  brought  round  to  the  upper  side  of  the 
stage,  for  the  illumination  of  opaque  objects.  The  essential  feature  of 
their  construction,  however,  which  differentiates  it  from  every  other  yet 
devised,  consists  in  a  provision  for  adjusting  the  illuminating  apparatus 
to  the  thickness  of  the  glass  slide  on  which  the  object  is  mounted.  This 
is  effected  by  making  the  disk  with  its  radial  arm,  slide  vertically  in  a  dove- 
tail fitting;  the  illuminating  apparatus  attached  to  it,  at  whatever  degree 
of  obliquity  it  may  be  placed,  being  raised  or  lowered  (by  a  lever-handle) 
in  the  optical  axis  of  the  instrument,  so  as  to  enable  the  illuminating 
cone  to  be  exactly  focussed  in  the  object  itself — which  on  the  Zentmayer 
model,  can  only  be  done  with  precision  when  the  upper  surface  of  the 
slide  is  exactly  in  the  plane  of  the  horizontal  axis  of  the  swinging  ^  tail- 
piece.'— The  Stage  also,  in  this  elaborate  instrument,  is  so  attached  to 
the  limb  by  a  firm  pivot,  as  to  be  capable  not  only  of  being  inclined  to- 
ward either  side  at  any  angle,  but  also  of  being  turned  completely  over, 
so  as  to  allow  the  object  to  be  viewed  from  its  under  side — a  provision  to 
which  the  Author's  experience  makes  him  attach  a  special  value. 

First-class  Binocular  Microscope-Stands,  copied  (more  or  less  closely)  from 
either  the  Ross  or  the  Jackson  models,  are  also  made  by  Messrs.  Baker,  Collins, 
Crouch,  Pillischer,  and  Swift,  as  well  as  by  other  makers  of  whose  work  the 
Author  has  no  personal  knowledge. — That  of  Mr.  Crouch  is  distinguished  by  a 
provision  for  meeting  the  difficulty  which  is  continually  experienced,  of  keeping 
the  image  in  place  during  the  rotation  of  the  stage,  especially  with  high  powers; 
the  adjustment  which  suits  one  Objective,  not  being  good  for  another  somewhat 
differently  centered.  This  defect  presents  itself  still  more  frequently  when  a 
*  nose-piece '  is  in  use;  its  centering  being  rarely  so  exact  as  to  be  free  from  an 
error  that  makes  itself  very  perceptible  when  a  high  power  is  exchanged  for  a 
low  one.  By  means  of  two  diagonal  screws  beneath  the  stage,  worked  by  two 
milled-heads  at  its  hinder  margin,  Mr.  Crouch  affords  a  ready  means  by  which 
the  observer  can  adapt  the  centering  of  his  stage  to  any  objective  he  may  have 
in  use. — Mr.  Browning  also  constructs  a  First-class  Stand  for  his  Stephenson 
Binocular. 

MICROSCOPES  POR  SPECIAL  PURPOSES, 

Of  the  large  number  of  instruments  which  have  been  ingeniously  de- 
vised, each  for  some  particular  use,  it  would  be  quite  foreign  to  the  pur- 
pose of  this  Treatise  to  attempt  to  give  an  account.  A  few  forms,  how- 
ever, may  be  noticed,  as  distinguished  either  by  their  special  adaptiveness 
to  very  common  wants,  or  by  the  ingenious  manner  in  which  the  require- 
ments of  particular  classes  of  investigators  have  been  met. 

76.  Dr,  Beale's  Pocket  Microscope. — This  instrument  consists  of  an 
ordinary  Microscope-body,  the  Eye-piece  of  which  is  fitted  with  a  draw- 
tube  that  slides  smoothly  and  easily;  whilst  its  lower  end  is  fitted  into  an 
outer  tube,  of  which  the  end  projects  beyond  the  objective.  Against  this 
projecting  end  the  object-slide  is  held  by  a  spring,  as  shown  in  Fig.  53, 


CONSTBCCTION  OF  THE  MICKOSCOPE. 


81 


being  fixed  (if  necessary)  by  a  screw-clip.  The  coarse  adjustment  is 
made  by  sliding  the  body  through  the  outer  tube  which  carries  the  ob- 
ject; and  the  fine  adjustment  by  sliding  the  eye-tube  in  or  out.  The 
object,  if  transparent,  is  illuminated  either  by  holding  up  the  Microscope 
to  a  window  or  lamp,  from  which  the  rays  may  pass  directly  through  it, 
or  by  directing  it  towards  a  mirror  laid  on  the  table  at  such  an  angle  as 
to  reflect  light  from  either  of  these  sources:  if  opaque,  it  is  allowed  to 
receive  direct  light  through  an  aperture  in  the  outer  tube.  The  extreme 
simplicity  and  portability  of  this  instrument  (which  when  closed  is  only 
six  inclics  long)  constitutes  its  special  recommendation.  "With  due  care 
even  high  powers  may  be  use,  the  eye-piece  adjustment  giving  the  power 
of  very  exact  focussing.  Hence  this  Pocket  Microscope  may  be  conve- 
niently applied  to  the  ]nirposes  of  Clinical  observation  (the  examination 
of  Urinary  Deposits,  Blood,  Sputa,  etc.),  either  in  hospital  or  in  private 
practice;  whilst  it  may  also  be  advantageously  used  by  the  Field  Natural- 
ist in  examining  specimens  of  Water  for  Animalcules,  Protophytes,  etc. 


Dr.  Beale's  Demonstrating  Microscope. 


77.  Dr.  Beale^s  Demonstrating  Microscope. — The  same  instrument 
may  be  used  for  the  purposes  of  Class-demonstration,  by  attaching 
its  outer  tube  on  a  wooden  support  to  a  horizontal  board,  which  also 
carries  a  small  lamp  attached  to  it  in  the  required  position  (Fig.  53). 
The  object  having  been  fixed  in  its  place,  and  the  coarse  adjustment 
made  by  sliding  the  body  in  the  outer  tube,  these  parts  may  then  be  im- 
movably secured,  nothing  being  left  movable  except  the  eye-tube,  by 
sliding  which  m  or  out  the  fine  adjustment  may  be  effected.  Thus  the 
whole  apparatus  may  be  passed  from  hand  to  hand  with  the  greatest  facil- 
ity, and  without  any  probability  of  disarrangement;  and  every  observer 
may  readily  ^  focus '  for  himself,  without  any  risk  of  injuring  the  object.^ 

78.  Bahefs  Travelling  Microscope, — An  instrument  has  been  devised 
by  Mr.  Moginie,  which  is  but  little  inferior  in  portability  to  the  Pocket 
Microscope  of  Prof.  Beale,  and  has  some  advantages  over  it.  The  body 
(Fig.  54)  slides  in  a  tube  which  is  attached  to  a  stem  that  carries  at  its 
end  a  small  Stage  and  Mirror.  The  stem  itself  contains  a  fine  adjustment 
that  is  worked  by  a  milled-head  at  its  summit;  and  near  to  this  is  attached 
by  pivot-joint  a  pair  of  legs,  which,  when  opened-out,  form  with  the  stem 
a  firm  tripod  support.  The  coarse  adjustment  having  been  made  by 
sliding  the  body  through  the  tube  which  grasps  it,  the  fine  adjustment  is 


^  The  price  of  Dr.  Beale's  Clinical  Microscope,  as  made  by  Mr.  Collins,  without 
Objectives,  is  £1  lis.  6d.    That  of  the  same  instrument  fitted  up  as  a  Demonstrat- 
ing Microscope,  is  £3  3s. — Mr.  Collins  also  makes  another  Class  and  Demonstra- 
tion Microscope,  or  a  pattern  of  Dr.  Lawson's  for  £3  10s.,  without  Objectives. 
6 


82 


THE  MICROSCOPE  AND  ITS  REVELATIONS. 


made  by  the  milled-head;  and  thus  even  high  powers  may  be  very  con- 
viently  worked.  The  legs  being  tubular,  one  of  them  is  made  to  hold 
glass  dipping-tubes,  whilst  the  other  contains  needles  set  in  handles,  with 
three  short  legs  of  steel  wire,  by  screwing  which  into  the  stem  and  stage, 
the  Microscope  may  be  used  (though  not  without  risk  of  overturn)  in  the 


marvel  of  ingenuity;  while  its  workmanship  is  so  excellent  that  its  joints 
do  not  easily  become  loosened  by  wear,  and  can  all  be  readily  tightened 
when  required.  It  is  so  steady  as  to  bear  being  worked  (as  a  Monocular) 
with  even  high  powers;  but  its  great  advantage  consists  in  its  suitability 
to  the  Traveller,  who  either  wishes  (as  often  happens  to  the  Author)  to 
display  to  scientific  friends  in  other  countries  a  set  of  objects  that  can  be 
most  advantageously  seen  by  the  Binocular  under  low  powers,  or  to  avail 
himself  of  opportunities  of  examining  on  the  spot  any  interesting  speci- 
mens he  may  meet  with.  The  instrument  also  carries  Mr.  Swift's 
Combination  Sub-stage  (Fig.  85),  which  can  be  packed,  together  with 
three  Objectives,  Side-Condenser,  and  several  other  Accessories,  into  a 
Case  only  11  inches  long,  G|-  inches  wide,  and  3|-  inches  deep,  the  whole 
weighing  only  7^  lbs. 

80.  NaclieVs  Chemical  Microscope, — The  inverted  Microscope  origi- 
nally constructed  by  MM.  Nachet  on  the  plan  devised  by  Dr.  J.  Lawrence 
Smith,  of  Louisiana,  U.S.,  for  the  purpose  of  viewing  objects  from  their 
under  side  when  heat  or  re-agents  are  being  applied  to  them,^  has  lately 
been  improved  by  its  constructor  with  a  special  view  to  meeting  the  re- 
quirements of  observers  engaged  in  the  '  cultivation '  of  the  minute  organ- 

^  Instruments  nearly  resembling  the  above  are  made  by  Messrs.  Murray  and 
Heath,  Mr.  Browning,  and  Mr.  Swift. 

^  This  idea  was  suggested  at  nearly  the  same  time  by  Dr.  Leeson;  and  was 
carried  out  in  an  instrument  constructed  for  him  by  Messrs.  Smith  and  Beck, 


vertical  position.  This  in- 
strument may  be  specially 
recommended  to  those  who, 
already  possessing  a  superior 
Microscope,  desire  neither  to 
encumber  themselves  with  it 
whilst  travelling,  nor  to  ex- 
pose it  to  risk  of  injury,  but 
wish  to  utilize  its  Objectives 
by  means  of  a  simple  and 
portable  arrangement.^ 


Baker's  Travelling  Microscope. 


79,  Simft's  Portable  Bin- 
ocular.— Carrying  still  further 
an  idea  originally  worked-out 
by  Messrs.  Powell  and  Lea- 
land,  Mr.  Swift  has  devised 
a  very  complete  Portable  Bin- 
ocular, which  can  be  folded 
into  a  very  small  compass, 
without  any  screwing  or  un- 
screwing, and  can  be  thus  set 
up,  as  in  Pig.  55  A,  or  packed 
away,  as  at  Fig.  55  b,  with 
great  facility,  when  once  the 
manner  of  doing  so  has  been 
learned.    Its  construction  is  a 


CONSTRUCTIOK  OF  THE  MICROSCOPE. 


isms  which  act  as  fer- 
ments. The  general  ar- 
rangement of  this  instru- 
ment is  shown  in  Fig.  56. 
On  the  table  which  forms 
its  base,  there  rests  a  box 
containing  a  glass  mirror 
silvered  on  its  upper  sur- 
face, which  is  phiced  at 
such  an  angle  as  to  reflect 
the  light-rays  received 
through  the  inverted  Ob- 
jective mounted  on  the 
top  of  the  box,  into  the 
body  fixed  into  its  oblique 
face.  Over  the  objective 
is  placed  tlie  Stage,  above 
which  again  is  the  Mirror 
for  reflecting  light  down- 
wards through  the  object 
placed  upon  it.  The 
focal  adjustment  is  made 
in  the  first  place  by 
means  of  a  sliding  tube 
which  carries  the  objec- 
tive, and  then  by  the 
micromet er-screw  v, 
Avhich  raises  or  lowers  the 
stage.  The  platform  on 
which  the  optical  appar- 
atus rests,  can  be  moved 
in  rectangular  directions 
by  the  two  milled-heads, 
0,  t;  and  is  furnished 
with  two  graduated  scales, 
by  means  of  which  it  may 
be  brought  with  exact- 
ness into  any  position  pre- 
viously recorded,  so  that 
any  point  of  the  object 
may  be  immediately  re- 
found — an  arrangement 
of  special  value  in  cultiva- 
t  i  o  n  -  experiments.  On 
the  stage  is  a  circular 
glass  cell,  G,  for  holding 
the  fluid  to  be  examined; 
in  the  bottom  of  this  in 
a  n  aperture,  which  i  s 
closed  by  a  piece  of  thin 
cover-glass  well  cemented 
round  its  edges,  thus  al- 
lowing the  use  of  high 
magnifying  powers  hav- 


Swift's  Portable  Binocular,  as  set  up  for  use. 


Swift's  Portable  Binocular,  as  packed  in  case. 


84  THE  MICROSCOPE  AND  ITS  REVELATIONS. 

ing  a  short  yery  focus;  while  its  top  is  ground  flat,  so  that  a  cover 
of  thin  plate-glass  may  be  closely  fitted  to  it  by  the  intervention  of 
a  little  grease  or  glycerine;  the  whole  being  secured  in  its  place  by  three 
small  uprights.    The  cell  is  furnished  also  with  two  small  glass  taps,  r, 
R,  with  which  india-rubber  tubes  are  connected.    By  this  cell,  which 
may  be  made  to  serve  as  a  moist,  a  warm,  and  a  gas-chamber,  experi- 
ments on  the  rarefaction 
and  compression  of  air, 
and  on  the  absorption  of 
gases,  can  be  made  with 
great  facility.    For  '  cul- 
tivation '  experiments, 
smaller  cells  are  provid- 
ed, which  are  attached  to 
brass-plates  so  arranged 
as  to  have  always  a  fixed 
position  on  the  stage. ^ 

81.  Non- Stereoscopic 
Binoculars, — The  great 
comfort  which  is  expe- 
rienced by  the  Microsco- 
pist  from  the  conjoint  use 
of  both  eyes,  has  led  to 
the  invention  of  more 
than  one  arrangement  by 
which  this  comfort  can 
be  secured,  when  those 
high  powers  are  required 
which  cannot  be  employ- 
ed with  the  ordinary 
Stereoscopic  Binocular. 
This  is  accomplished  by 
Messrs.  Powell  and  Lea- 
land  by  taking  advantage 
of  the  fact  already  ad- 
verted to  (§  1),  thatwhen 

Nachet's  Chemical  Microscope.  a    pencil    of    Pays  falls 

obliquely  upon  the  sur- 
face of  a  refracting  medium,  a  part  of  it  is  reflected  without  entering 
that  medium  at  all.  In  the  place  usually  occupied  by  the  Wenham  prism, 
they  interposed  an  inclined  plate  of  glass  with  parallel  sides,  through 
which  one  portion  of  the  rays  proceeding  upwards  from  the  whole  aper- 
ture of  the  Objective  passes  into  principal  body  with  very  little  change 
in  its  course,  whilst  another  portion  is  reflected  from  its  surface  into  a 
rectangular  prism  so  placed  as  to  direct  it  obliquely  upwards  into  the 
secondary  body  (Fig.  57).  Although  there  is  a  decided  difference  in 
brightness  between  the  two  images,  that  formed  by  the  reflected  rays 
being  the  fainter,  yet  there  is  marvellously  little  loss  of  definition  in  either, 
even  when  the  25th-inch  objective  is  used.  The  disc  and  prism  are  fixed 
in  a  short  tube,  which  can  be  readily  substituted  in  any  ordinary  Binocu- 
lar Microscope  for  the  one  containing  the  Wenham  prism.  —  Other  arrange- 


'  A  Mineralogical  Microseope  specially  contrived  by  M.  Nachet  for  minute 
Petrological  researches,  will  be  described  at  the  end  of  Chap.  xxi. 


CONSTRUCTION  OF  THE  MICROSCOPE. 


85 


ments  were  long  since  devised  by  Mr.  Wenham,^  with  a  yiew  to  obtain 
a  greater  equality  in  the  amount  of  light-rays  forming  the  two  pictures; 
and  he  has  lately  carried  one  of  these  into  practical  effect^  with  the  advan- 
tage that  the  compound  prism  of  which  it  consists,  has  so  nearly  the  same 
shape  and  size  as  his  ordinary  stereoscopic  prism,  as  to  be  capable  of  being 
mounted  in  precisely  the  same  manner,  so  that  the  one  may  be  readily  ex- 
changed for  the  other.  The  axial  ray  preceding  upwards  from  the 
objective,  enters  the  prism  a  b  d  e  f  (Fig.  58)  at  right  angles  to  its  lower 
face,  and  passes  on  to  where  it  meets  the  inclined  face  A  b,  at  which 
this  prism  is  nearly  in  contact  with  the  oblique  face  of  the  right-angled 
prism  ABC.  By  internal  reflection  from  the  former,  and  external  reflec- 
tion   from    the  latter, 

about  half  the  beam  h  is  ^j^^  ^ 
reflected  within  the  first 
prism  in  the  direction  c  h, 
while  the  other  half  pro- 
ceeds straight  onwards 
through  the  second  prism, 
in  the  direction  c  a\  so  as 
to  pass  into  the  "principal 
body.  The  reflected  half, 
meeting  at  d  the  oblique 
(silvered)  surface  d  e,  of 
the  first  prism,  is  again 
reflected  in  the  direction 
d  V ;  and  passing  out  of 
that  prism  perpendicular- 
ly to  its  surface  a  F,  prO-PowellandLea- 

ceeds  towards  the  J^^^scopic  b^^^ 

<Zn/body.    The  two  prisms  cular  Arrange- 

must  not  be  in  absolute 

contact  along  the  plane  A  b,  since,  if  they  were,  Newton's  rings  would  be 
formed;  and  much  nicety  is  required  in  their  adjustment,  so  that  the  two 
reflections  may  be  combined  without  any  blurring  of  the  image  in  the 
secondary  body.  Being  (by  Mr.  Wenham^s  kindness)  the  possessor  of 
a  prism  thus  adjusted  by  himself,  the  Author  can  bear  testimony  to  the 
excellence  of  its  performance;  and  he  feels  sure  that  for  the  prolonged 
observation,  under  high  powers,  of  objects  not  requiring  the  extreme  of 
perfection  in  definition — such,  for  example,  as  the  study  of  the  Cyclosis 
in  Plants, — great  advantage  is  gained  from  the  conjoint  use  of  Ijotli  eyes 
by  one  of  the  above  arrangements. 


i  "  Transactions  of  the  Microsc.  Soc."  N,S.,  Vol.  xiv.  (1866),  p.  105. 


86 


♦the  microscope  and  its  revelations. 


CHAPTEE  III. 
ACCESSORY  APPARATUS. 

In  describing  the  yarious  pieces  of  Accessory  Apparatus  with  which 
the  Microscope  may  be  furnished,  it  will  be  convenient  in  the  first  place 
to  treat  of  those  which  form  (when  in  use)  part  of  the  instrument  itself, 
being  appendages  either  to  its  Body  or  to  its  Stage,  or  serving  for  the 
Illumination  of  the  objects  which  are  under  examination;  and  secondly, 
to  notice  such  as  have  for  their  function  to  facilitate  that  examination, 
by  enabling  the  Microscopist  to  bring  the  objects  conveniently  under  his 
inspection. 

Section  1.  Appendages  to  the  Microscope. 

83.  Amplifier, — It  is  obvious  that  if,  by  the  use  of  a  concave  lens 
interposed  between  the  Objective  and  the  Eye-piece,  the  divergence  of 
the  rays,  in  the  course  from  the  former  to  the  latter,  be  increased,  the 
magnifying  power  of  the  instrument  will  be  augmented  in  proportion; 
and  such  an  addition  (which  was  long  since  introduced  into  Telescopes, 
and  also  into  the  Solar  Microscope)  has  been  brought  into  general  use  in 
the  United  States,  having  been  first  made  effective  by  Mr.  Tolles.  As  con- 
structed by  him,  the  Amplifier  is  an  achromatic  concavo-convex  lens  of 
small  diameter,  screwed  into  the  lower  end  of  the  draw-tube,  so  as  to  be 
at  no  great  distance  behind  the  objective,  the  power  of  which  it  doubles, 
without  (it  is  affirmed)  producing  sensible  deterioration  of  the  image. 
Dr.  Devron,  of  New  Orleans,  states  that  two  photographs  having  been 
taken  of  Amphipleura  pelhicida,  the  one  by  a  Tolles'  l-12th  with  amplifier, 
the  other  with  a  Tolles'  1-25  without  amplifier,  they  proved  to  be  scarcely 
distinguishable;  and  that  the  19th  band  of  Robert's  ruled  plate  could  be 
resolved  with  its  aid,  by  objectives  under  which  without  it  no  resolution 
could  be  obtained.^  It  is  obvious  that  if  the  magnifying  power  of  our 
Microscopes  can  be  thus  doubled,  without  the  strain  of  eyes,  and  the  loss 
of  light  and  of  definition,  produced  by  deep  Eye-piecing,  and  without 
the  necessity  of  employing  Objectives  of  inconveniently  short  focus  and 
great  cost,  a  great  advantage  will  have  been  gained;  while  those  who 
wish  to  possess  a  graduated  range  of  powers,  need  only  supply  themselves 
with  half  the  number  of  Objectives  needed  to  give  it,  since  each  can  be 
made  to  do  double  work  (a  1  inch,  for  example,  serving  also  as  a  half-inch) 
without  change  either  of  the  eye-piece  or  the  focal  adjustment. — Dr. 
Wythe,  of  San  Francisco,  states  that  he  has  obtained  very  good  results  by 
placing  a  double-concave  or  a  concavo-convex  lens  of  about  6  inches  focus, 
and  of  as  large  a  diameter  as  the  tube  will  allow,  about  3  inches  below  the 


^    American  Monthly  Journal  of  Microscopy,"  Vol.  iii.  (1878^,  p.  38. 


ACCESSORY  APPARATUS. 


87 


eye-piece;  counteracting  its  aberrations  by  substituting  a  convexo-concave 
lens  for  the  plano-convex  which  forms  the  field-glass  of  the  ordinary 
Huyghenian  eye-piece.  ^ 

83.  Draw-Tube, — It  is  advantageous  for  many  purposes  that  the  Eye- 
piece should  be  fitted,  not  at  once  into  the  ^bqdy  ^  of  the  Microscope,  but 
into  an  intermediate  tube;  the  drawing-out  of  which,  by  augmenting  the 
distance  between  the  objective  and  the  image  which  it  forms  in  the  focus 
of  the  eye-glass,  still  further  augments  the  size  of  the  image  in  relation 
to  that  of  the  object  (§  25).  For  although,  as  a  general  rule,  the  magni- 
fying power  cannot  be  thus  increased  with  advantage  to  any  considerable 
extent,  yet,  if  the  corrections  of  low  objectives  have  been  well  adjusted, 
their  performance  is  not  seriously  impaired  by  a  moderate  lengthening  of 
the  body;  and  recourse  may  be  conveniently  had  to  this  on  many  occa- 
sions in  which  some  amplification  is  desired,  intermediate  between  the 
powers  furnished  by  any  two  Objectives.  Thus,  if  one  objective  give  a 
power  of  80  diameters,  and  another  a  power  of  120,  by  using  the  first  and 
drawing  out  the  Eye-piece,  its  power  may  be  increased  to  100.  Again, 
it  is  often  very  useful  to  make  the  object  fill  up  the  whole,  or  nearly  the 
whole,  of  the  field  of  view;  so  as  to  prevent  the  vividness  and  distinct- 
ness of  its  image  from  being  interfered  with  by  extraneous  light.  In  the 
use  of  the  Micrometric  eye-pieces  to  be  presently  described  (§§  90,  91), 
very  great  advantage  is  to  be  derived  from  the  assistance  of  the  Draw- 
tube;  as  enabling  us  to  make  a  precise  adjustment  between  the  divisions 
of  the  Stage-micrometer  and  those  of  the  Eye-piece  micrometer;  and  as 
admitting  the  establishment  of  a  more  convenient  numerical  relation  be- 
tween the  two,  than  could  be  otherwise  secured  without  far  more  elabo- 
rate contrivances.  Moreover,  if,  for  the  sake  of  saving  room  in  packing, 
it  be  desired  to  reduce  the  length  of  the  body,  the  draw-tube  (in  a  Mono- 
cular Microscope)  affords  a  ready  means  of  doing  so. — Objectives  of  higli 
power,  however,  require  special  adjustment  when  any  considerable  length 
of  Draw- tube  is  used.  fig.  59. 

84.  — Lister^s  Erector. — This  instrument,  first  ajiplied  to 
the  Compound  Microscope  by  Mr.  Lister,  consists  of  a  tube 
about  three  inches  long,  having  a  meniscus  at  one  end  and  a 
plano-convex  lens  at  the  other  (the  convex  sides  being  upwards 
in  each  case),  with  a  diaphragm  nearly  half  way  between  them; 
and  this  is  screwed  into  the  lower  end  of  the  draw-tube,  as 
^liown  in  Fig.  59.  Its  effect  is  (like  the  corresponding  erector 
A)f  the  Telescope),  to  antagonize  the  inversion  of  the  image 
formed  by  the  object-glass  by  producing  a  second  inversion, 
%o  as  to  make  the  Image  presented  to  the  eye  correspond  in 
position  with  the  Object — an  arrangement  of  great  service  in 
cases  in  which  the  object  has  to  be  subjected  to  any  kind  of 
manipulation.  The  passage  of  the  rays  through  two  addi- 
tional lenses  of  course  occasions  a  certain  loss  of  light  and 
impairment  of  the  distinctness  of  the  image;  but  this  need 
not  be  an  obstacle  to  its  use  for  the  class  of  purposes  for 
which  it  is  especially  adapted  in  other  respects,  since  these 
seldom  require  a  very  high  degree  of  defining  power.  By  the 
position  given  to  the  Erector,  it  is  made  subservient  to  an- 
other purpose  of  great  utility;  namely,  the  procuring  a  very 
extensive  range  of  Magnifying  power,  without  any  change  in  ^^^^^^'^"^ 
the  Objective.    For  when  the  draw- tube,  with  the  erector  fit-  Erector. 


'  Op.  cit,  Vol.  V.  (1880),  p.  81. 


88 


THE  MICROSCOPE  AND  ITS  KEVELATIONS. 


ted  to  it,  is  completely  pushed-in,  the  acting  length  of  the  body  (so  to 
speak)  is  so  greatly  reduced  by  the  formation  of  the  first  image  much  nearer 
the  objective,  that,  if  a  lens  of  2-3ds  of  an  inch  focus  be  employed,  an  ob- 
ject of  the  diameter  of  1^  inch  can  be  taken  in,  and  enlarged  to  no  more 
than  4  diameters;  whilst,  on  the  other  hand,  when  the  tube  is  drawn-out  4-^ 
inches,  the  object  is  enlarged  100  diameters.  Of  course  every  interme- 
diate range  can  be  obtained  by  drawing-out  the  tube  more  or  less;  and  the 
facility  with  which  this  can  be  accomplished,  especially  when  the  Draw- 
tube  is  furnished  with  a  rack-and-pinion  movement  (as  in  Messrs.  Beck's 
Compound  Dissecting  Microscope),  renders  such  an  instrument  very  use- 
ful in  various  kinds  of  research. 

85.  Micro-Megascope. — This  designation  has  been  applied  by  Dr.  J. 
Matthews,^  to  an  arrangement  of  the  ordinary  Microscope,  whereby  such 
alow  amplification  may  be  obtained,  as  gives  a  general  view  of  large  ob- 
jects, without  the  need  of  any  special  apparatus.  The  method  consists 
in  employing  the  ordinary  microscope  to  magnify — not  the  object  itself — 
but  an  image  of  it  formed  by  a  lens  placed  between  the  object  and  the 
front  of  the  objective.  In  the  principle  of  this  method  there  is  nothing 
new,  for  every  Microscopist  who  has  focussed  an  Achromatic  Condenser, 
upon  a  transparent  object,  has  seen  the  image  formed  by  it  of  his  win- 
dow-frame, blind-tassel,  or  (it  may  be)  of  sharply  defined  clouds.^  And 
Dr.  Eoyston-Pigott  has  been  accustomed  to  employ  sucli  images  of  hairs, 
fine  wires,  etc.,  as  ^  tests  ^  for  the  defining  quality  of  Objectives  of  high 
magnifying  j)ower.  The  novelty  consists  in  the  mode  of  applying  it  to 
the  purpose  just  named.  This  answers  best  when  an  Objective  of 
2-inches  or  l|-inch  focus  is  used  in  tlie  microscope,  and  a  1-inch  Objec- 
tive is  placed  in  the  Sab-stage  with  its  front-lens  upward.  The  object  to 
be  imaged  by  the  latter  is  to  be  placed  either  at  some  distance  behind 
it,  the  mirror  being  turned  aside,  or,  if  the  Mirror  be  employed,  at  some 
distance  from  it  on  either  side;  the  distance,  in  either  case,  being  adapted 
to  give  to  the  Microscoj)ic  image  the  amplification  required.  The  former 
arrangement  is  most  convenient  if  the  Microscope  is  being  used  in  a  hori- 
zontal position  ;  the  latter  is  most  suitable  when  the  Microscope  is  inclined, 
the  distance  of  an  object  placed  in  the  optic  axis  being  then  limited.  If 
exact  definition  is  required,  the  Mirror  should  be  replaced  by  a  right- 
angled  Prism  (§  2).  Tiie  object,  whether  transparent  or  opaque,  must 
be  suitably  illuminated;  and  it  will  be  found  convenient  to  use  a  special 
support  so  made  that  its  position  and  height  may  be  conveniently  varied. 

86.  Nachefs  Erecting  Prism, — An  extremely  ingenious  arrangement 
has  been  made  by  MM.  Nachet,  on  the  basis  of  an  idea  first  carried  into 
practice  by  Prof.  Amici,  by  which  the  inverted  image  given  by  the  Com- 
pound Microscope  is  erected  by  a  single  rectangular  prism  placed  over  the 
eye-piece.  The  mode  in  which  this  prism  is  fitted  up  is  shown  in  Fig. 
GO  (2);  the  rationale  of  its  action  is  explained  by  the  diagram  (1).  The 
prism  is  interposed  between  the  two  lenses  of  the  Eye-piece,  and  has 
somewhat  the  form  of  a  double  wedge,  with  two  pentagonal  sides,  A  B  c  D 
E,  and  A  B  H  G  p,  which  meet  each  other  along  the  common  edge  A  b, 
and  two  facets,  D  e  E  g,  and  c  D  a  H,  which  meet  along  the  common 
edge  D  G,  the  edges  a  b  and  r>  g  being  23erpendicular  to  each  other.  The 
rays  emerging  from  the  field-glass  enter  this  prism  by  Its  lower  surface, 
and  are  reflected  at  i,  upon  the  face  a  b  h  g  e,  from  which  they  are  again 


^    Journal  of  Quekett  Microscopical  Club,"  July,  1879. 

^  The  Author  has  thus  exhibited  to  his  friends  a  Microscopic  view  of  the  Moon. 


ACCESSORY  APPARATUS. 


89 


reflected  upon  the  lower  surface  at  the  point  k,  and  thence  to  the  point 
L  upon  the  vertical  face  c  d  G  H,  and  lastly  at  the  point  m,  upon  the 
other  yertical  face  D  e  f  g;  from  which  the  image  normally  and  com- 
pletely erected,  i^  again  sent  back,  to  issue  by  the  superior  surface  upon 
which  the  eye-glass  is  placed.  All  the  reflections  are  total  except  tlie  first 
at  i;  and  the  loss  of  light  is  far  less  than  would  be  anticipated. — The  ob- 
liquity which  this  Prism  gives  to  the  visual  rays,  when  the  Microscope  is 
placed  yertically  for  dissecting  or  for  the  examination  of  objects  in  fluid, 
is  such  as  to  bring  them  to  the  eye  at  an  angle  very  nearly  corresponding 
with  that  at  which  the  Microscope  may  be  most  conveniently  used  in  the 
inclined  position  (§  41,  iii.);  so  that,  instead  of  being  an  objection,  it  is 
a  real  advantage. 

87.  Sorby-Broiuning  Ificro-jSpectroscope,'— When  the  Solar  ray  is 
decomposed  into  a  colored  spectrum  by  a  prism  of  sufficient  dispersive 
power  to  which  the  light  is  admitted  by  a  narrow  slit,  a  multitude  of 
dark  lines  make  their  appearance.  The  existence  of  these  was  originally 
noticed  by  Wollaston;  but  as  Fraunhofcr  first  subjected  them  to  a  thor- 
ough investigation,  and  mapped  tlicm  out,  they  are  known  as  Fraicn- 


Nachet's  Erecting-Prism. 

hofer  lines.  The  greater  the  dispersion  given  by  the  multiplication  of 
prisms  in  the  Spectroscope,  the  more  of  these  lines  are  seen;  and  they 
bear  considerable  magnification.  They  result  from  the  interruption  or 
absorption  of  certain  rays  in  the  Solar  atmosphere,  according  to  the  law, 
first  stated  by  Angstrom,  that  ''rays  which  a  substance  absorbs  are  pre- 
cisely those  which  it  emits  when  made  self-luminous/'  KirchhofE  showed 
that  while  the  incandescent  vapors  of  Sodium,  Potassium,  Lithium,  etc., 
give  a  spectrum  Avith  characteristic  Iriglit  lines,  the  same  vapors  intercept 
portions  of  white  light,  so  as  to  give  darh  lines  in  place  of  the  bright 
ones,  absorbing  their  own  special  color,  but  allowing  rays  of  other  colors 
to  pass  through. — Again,  when  ordinary  light  is  made  to  pass  through 
colored  bodies  (solid,  liquid,  or  gaseous),  or  is  reflected  from  their  sur- 
faces, so  as  to  affect  the  eye  with  the  sensation  of  color,  its  spectrum  is 
commonly  found  to  exhibit  absorption  bands,  which  differ  from  the  Fraun- 
liofer  lines,  not  only  in  their  greater  breadth,  but  in  being  more  or  less 
nebulous  or  cloudy,  so  that  they  cannot  be  resolved  into  distinct  lines  by 
magnification,  while  too  much  dispersion  thins  them  out  to  indistinct- 

*  For  general  information  on  the  Spectroscope  and  its  uses,  the  student  is  re- 
ferred to  Professor  Roscoe's  "  Lectures  on  Spectrum  Analysis,"  or  the  translation 
of  Dr.  Schellen's  ''Spectrum  Analysis." 


90 


THE  MICROSCOPE  AND  ITS  REVELATIONS. 


ness.  Now,  it  is  by  the  character  of  these  bands,  and  by  their  position 
in  the  spectrum,  that  the  colors  of  different  substances  can  be  most  ac- 
curately and  scientifically  compared;  many  colors  whose  impressions  on 
the  eye  are  so  similar  that  they  cannot  be  distinguished,  being  readily 
discriminated  by  their  spectra.    The  purpose  of  the  Micro-Spectroscope 

is  to  apply  the  spectroscopic  test  to 
very  minute  quantities  of  colored  sub- 
stances; and  it  fundamentally  con- 
sists of  an  ordinary  Eye-piece  (which 
can  be  fitted  into  any  Microscope)  with 
certain  special  modifications.  As 
originally  devised  by  Mr.  Sorby,  and 
worked-out  by  Mr.  Browning,  the 
Micro-Spectroscope  is  constructed  as 
follows  (Fig.  61):— AboYC  its  Eye- 
glass, which  is  achromatic,  and  made 
Micro-specti^scope.  capable  of  focal  adjustment  by  the 

milled-head  b  lor  rays  oi  dmerent  re- 
frangibilities,  there  is  placed  a  tube  A,  containing  a  series  of  five  prisms, 
two  of  flint-glass  (Fig.  62,  F  f)  interposed  between  three  of  crown  (c  c  c), 
in  such  a  manner  that  the  emergent  rays  r  r,  which  have  been  separated 

by  the  dispersive  action  of  the  flint-glass 
^-^^^  prisms,  are  parallel  to  the  rays  which  enter 

the  combination.  Below  the  eye-glass,  in 
the  place  of  the  ordinary  stop,  is  a  dia- 

  phragm  with  a  narrow  slit,  which  limits  the 

Arrangement  ofprismsin  Spectroscope  admission  of  light;  this  cau  be  adjusted  in 
Eye-piece.  Vertical  positiou  by  the  milled-head  h,  whilst 

the  breadth  of  the  slit  is  regulated  by  c.  The  foregoing,  with  an  Objective 
of  suitable  power,  would  be  all  that  is  needed  for  the  examination  of  the 
spectra  of  objects  placed  on  the  stage  of  the  Microscope,  whether  opaque 
or  transparent,  solid  or  liquid,  provided  that  they  transmit  a  sufficient 
amount  of  light.  But  as  it  is  of  great  importance  to  make  exact  compa- 
risons of  such  artificial  spectra,  alike  with  the  ordinary  or  natural  Spec- 
trum, and  with  each  other,  provision  is  made  for  the  formation  of  a 
second  spectrum,  by  the  insertion  of  a  right-angled  prism  that  covers 
one-half  of  this  slit,  and  reflects  upwards  the  light  transmitted  through 
n  aperture  seen  on  the  right  side  of  the  eye-piece.  For  the  production 
vyf  the  ordinary  spectrum,  it  is  only  requisite  to  reflect  light  into  this 
aperture  from  the  small  mirror  i,  carried  at  the  side;  whilst  for  the  pro- 
duction of  the  spectrum  of  any  substance  through  which  the  light  re- 
flected from  this  mirror  can  be  transmitted,  it  is  only  necessary  to  place 
the  slide  carrying  the  section  or  crystalline  film,  or  the  tube  containing 
the  solution,  in  the  frame  D  d  adapted  to  receive  it.  In  either  case,  this 
second  spectrum  is  seen  by  the  eye  of  the  observer  alongside  of  that  pro- 
duced by  the  object  viewed  through  the  body  of  the  Microscope,  so  that 
the  two  can  be  exactly  compared. 

88.  The  exact  position  of  the  Absorption-bands  is  as  important  as  that 
of  the  Fraunhofer-lines;  and  some  of  the  most  conspicuous  of  the  latter 
afford  fixed  points  of  reference,  provided  the  same  Spectroscope  be 
employed.  The  amount  of  dispersion  determines  whether  the  Fraunhofer- 
lines  and  Absorption-bands  are  seen  nearer  or  farther  apart;  their  actual 
positions  in  the  field  of  view  varying  according  to  the  dispersion,  while 
their  relative  positions  are  in  constant  proportion. — The  best  contrivance 


ACCESSORY  APPARATUS. 


91 


for  measuring  the  spectra  of  absorption  bands  is  Browning's  Bright-line 
Micrometer  shown  m  Fig.  63.  At  R  is  a  small  mirror  by  which  light  from 
the  lamp  employed  can  be  reflected  through  E  D  to  the  lens  c,  which,  by 
means  of  a  perforated  stop,  forms  a 
bright  pointed  imago  on  the  surface 
of  the  upper  prism,  whence  it  is  reflected 
to  the  eye  of  the  observer.  The  rotation 
of  a  wheel  worked  by  the  milled-head  M 
carries  this  bright  point  over  the  spec- 
trum, and  the  exact  amount  of  motion 
may  be  read  off  to  l-10,000th  inch  on 
the  graduated  circle  of  the  wheel.  To 
use  this  apparatus,  the  Fraunhof  er  lines 
must  be  viewed  by  sending  bright  day- 
light through  the  spectroscope,  and  the 
positions  of  the  principal  lines  care- 
fully measured,  the  reading  on  the 
micrometer-wheel  being  noted  down. 
A  Spectrum-map  may  then  be  drawn 
on  cardboard,  on  a  scale  of  equal  parts; 
and  the  lines  marked  on  it,  as  shown  in 
the  upper  half  of  Fig.  64.  The  lower 
half  of  the  same  figure  shows  an  Ab- 
sorption-spectrum, with  its  bands  at 
certain  distances  from  the  Fraunhofer 
lines.  The  cardboard  Spectrum-map, 
when  once  drawn,  should  be  kept  for 
reference.' 

89.  A  beginner  with  the  Micro- 
Spectroscope  should  first  hold  it  up  to 
the  sky  on  a  clear  day,  without  the 
intervention  of  the  microscope,  and  note 
the  effects  of  opening  and  closing  the  slit  by  rotating  the  screw  c  (Fig.  61); 
the  lines  can  only  be  well  seen  when  the  slit  is  reduced  to  a  narrow  open- 
ing. The  screw  H  diminishes  the  length  of  the  slit,  and  causes  the 
spectrum  to  be  seen  as  abroad  or  a  narrow  ribbon.  The  screw  e  (or  in 
some  patterns  two  small  sliding-knobs)  regulates  the  quantity  of  light 
admitted  through  the  square  aperture  seen  between  the  points  of  the 
springs  D  D. — Water  tinged  with  port  wine,  madder,  and  blood,  are  good 
fluids  with  which  to  commence  this  study  of  absorption-bands.  They 
may  be  placed  in  small  test  tubes,  in  flat  glass  cells,  or  in  wedge-shaped 
cells.^  As  each  color  varies  in  refrangibility,  the  focus  must  be  adjusted 
by  the  screw  b.  Fig.  61,  according  to  the  part  of  the  spectrum  that  is 
examined. — When  it  is  desired  to  see  the  spectrum  of  an  exceedingly 


Bright-line  Spectro-Micrometer. 


^  Mr.  Swift  has  devised  an  improved  Micro-Spectroscope,  in  which  the  Micro- 
metric  apparatus  is  combined  with  the  ordinary  Spectroscopic  Eye-piece,  and  two 
spectra  can  be  brought  into  the  field  at  once. — Otiier  improvements  devised  by  Mr. 
Sorby,  and  a  new  form  devised  by  Mr.  F.  H.  Ward,  have  been  carried  into  execution 
,  by  Mr.  Hilger.  (See  Journ.  of  Roy.  Microsc.  Soc,"  Vol.  i.,  1878,  p.  326,  and  Vol. 
ii.,  1879,  p.  81.)  Another  construction  possessing  some  advantages  over  the  origi- 
nal form,  has  been  devised  by  Zeiss  of  Jena  (See  ''Journ.  of  Roy.  Microsc.  Soc," 
Vol.  iii.,  1880,  p.  703). 

-A  series  of  specimens,  in  small  tubes,  for  the  study  of  Asorption- spectra,  is 
kept  on  sale  by  Mr.  Browning  ;  and  the  directions  given  in  his  *'  How  to  Work 
with  the  Micro-Spectroscope  "  should  be  carefully  attended  to. 


92 


THE  MICROSCOPE  AND  ITS  EEVELATIONS. 


minute  object,  or  of  a  small  portion  only  of  a  larger  one,  tlie  prisms  are  to 
be  removed  by  withdrawing  the  tube  containing  them  ;  the  slides  should 
then  be  opened  wide,  and  the  object,  or  part  of  it,  brought  into  the  centre 
of  the  field  ;  the  vertical  and  horizontal  slits  can  then  be  partly  shut,  so  as 
to  inclose  it,  and  if  the  prisms  are  then  replaced,  and  a  suitable  objective 
employed,  the  required  spectrum  will  be  seen  unaffected  by  adjacent 
objects.  For  ordinary  observations,  Objectives  of  from  2  inches  to  2-  ^ 
3ds  inch  focus  will  be  found  most  suitable  ;  but  for  very  minute  quan- 
tities of  material  a  higher  power  must  be  employed.  Even  a  single  Eed 
Blood-corpuscle  may  be  made  to  show  the  characteristic  Absorption -bands 
represented  (after  Prof.  Stokes)  in  Fig.  65/ 

90.  MicrometriG  Apparatus, — Although  some  have  applied  their 
micrometric  api)aratus  to  the  Stage  of  the  Microscope,  yet  it  is  to  the 
Eye -piece  that  it  may  be  most  advantageously  adapted.^  The  Cohweb 
Micro?netery  invented  by  Eamsden  for  Telescopes,  is  probably,  when  well 
constructed,  the  most  perfect  instrument  that  the  Microscopist  can  em- 


upper  half,  Map  of  Solar  Spectrum,  showing  Fraunhofer  lines.    Lower  half,  Absorption  Spefe 
trum,  showing  position  of  Bands  in  relation  to  lines. 

ploy.  lb  is  made  by  stretching  across  the  field  of  an  Eye-piece  two  verj 
delicate  parallel  wires  or  spider's  threads,  one  of  which  can  be  separated 
from  the  other  by  the  action  of  a  micrometer  screw,  the  head  of  which 
is  divided  at  its  edge  into  a  convenient  number  of  parts,  which  success- 
ively pass-by  an  index,  as  the  milled-head  is  turned.  A  portion  of  the 
field  of  view  on  one  side  is  cut  off  at  right  angles  to  the  filaments,  by  i\ 
scale  formed  of  a  thin  plate  of  brass  having  notches  at  its  edge, whoso 
distance  corresponds  to  that  of  the  threads  of  the  screw,  every  fifth  notch 
being  made  deeper  than  the  rest  for  the  sake  of  ready  enumeration. 
The  object  being  brought  into  such  a  jiosition  that  one  of  its  edges  seems 

^  For  further  information  on  The  Spectrum  Method  of  Detecting  Blood,"  see 
an  important  paper  by  Mr.  Sorby,  in  Monthly  Micros.  Journal,"  Vol.  vi.  (1871), 
p.  9. 

2  The  Stage-Micrometer  constructed  by  Fraunhofer  is  employed  by  many 
Continental  Microscopists  ;  but  it  is  subject  to  this  disadvantage— that  any  error 
in  its  performance  is  augmented  by  the  whole  magnifying  power  employed ; 
whilst  a  like  error  in  the  Eye-piece  Micrometer  is  increased  by  the  magnifying 
power  of  the  eye-piece  alone. — Dr.  Royston-Pigott  has  pointed  out  ("  Monthly 
Micros.  Journ.,"  Vol.  ix.,  1873,  p.  2.),  that  by  placing  the  Cobweb  Micrometer  at 
some  distance  beneath  the  stage,  and  by  forming  an  aerial  image  of  it  (by  an 
interposed  lens)  in  the  plane  of  the  object  ;  the  delicacy  and  accuracy  of  its 
measurements  may  be  greatly  increased  ;  the  numerical  value  of  each  division 
being  reduced,  in  proportion  to  the  reduction  in  the  size  of  the  aerial  image, 
which  will  of  course  be  determined  by  the  focal  length  of  the  lens  that  forms  it, 
and  by  the  distance  of  the  Micrometer  beneath  it. 


ACCESSORY  APPARATUS. 


93 


to  touch  the  stationary  filament,  the  other  thread  is  moved  by  the  micro- 
meter-screw until  it  appears  to  lie  in  contact  with  the  other  edge  of  the 
object ;  the  number  of  the  entire  divisions  on  the  scale  shows  how  many 
complete  turns  of  the  screw  must  have  been  made  in  thus  separating  the 
filaments,  while  the  number  to  which  the  index  points  on  the  milled- 
head  shows  what  fraction  of  a  turn  may  have  been  made  in  addition.  It 
is  usual,  by  employing  a  screw  of  100  threads  to  the  inch,  to  give  to 
each  division  of  the  scale  the  value  of  1-lOOth  of  an  inch,  and  to  divide 
the  milled -head  into  100  parts  ;  but  the  absolute  value  of  the  divisions  is 
of  little  consequence,  since  their  micrometric  value  depends  upon  the 
Objective  with  which  the  instrument  may  be  employed.  This  must  be 
determined  by  means  of  a  ruled  slip  of  glass  laid  upon  the  stage  ;  and  as 
the  distance  of  the  divisions  even  in  the  best-ruled  slips  is  by  no  means 
uniform,  it  is  advisable  to  take  an  avarage  of  several  measurements,  both 
upon  different  slips,  and  upon 
different  parts  of  the  same  slip. 
Here  the  Drawtube  Avill  be  of  es- 
sential use,  in  enabling  the  Micro- 
scopist  to  bring  the  value  of  the 
divisions  of  his  Micrometer  to 
even  numbers, — The  Microscopist 
who  applies  himself  to  researches 
requiring  micrometric  measure- 
ment, should  determine  the  value 
of  his  Micrometer  with  each  of 
the  Objectives  he  is  likely  to  use 
for  the  purpose  ;  and  should  keep 
a  table  of  these  determinations, 
recording  in  each  case  the  extent 
to  which  the  tube  has  been  drawn 
out,  as  marked  by  the  graduated 
scale  of  inches  which  it  should 
possess.  And  he  should  also  make 
an  accurate  estimate  of  the  thick- 
ness of  the  Cobweb-threads  them- 
selves :  since,  if  this  be  not  prop-  ^  ^   ^  ^    ,  ^ 

in         T    P  •        -L      J        1    Spectroscopic  appearance  or  fresn  bcarlet 

erly  allowed  for,  a  serious  error  bw-  2  of  Deoxydized  Biood  (cmorine) ;  3,  of 

will  be  introduced  into  the  mea-  H^Bmatm;  oj^tained  by  acting  on  cruorine  with  an 
.  T      ,       ,  1  •     •     ,       acid  :  4,  of  Hsematm  reoxydized. 

surements  made  by  this  instru- 
ment, especially  when  the  spaces  measured  are  extremely  minute.  (See 
Michell,  in    Transact.  Micros.  Soc/'  N.  S.,  Vol.  xiv.,  p.  71.) 

91.  The  costliness  of  the  Cobweb  Micrometer  being  an  important 
obstacle  to  its  general  use,  a  simpler  method  (devised  by  Mr.  G.  Jackson) 
is  more  commonly  adopted;  which  consists  in  the  insertion  of  a  transpar- 
ent scale  into  an  ordinary  Huyghenian  Eye-yiece  in  the  focus  of  the  eye- 
glass, so  that  the  image  of  the  object  is  seen  to  be  projected  upon  it. 
This  scale  is  ruled  like  that  of  an  ordinary  measure  {i.e.,  Avith  every  tenth 
line  lo7ig,  and  every  fifth  line  half  its  length)  on  a  slip  of  glass,  which  is 
so  fitted  into  a  brass  frame  (Pig.  66,  b),  as  to  have  a  slight  motion  towards 
either  end;  one  of  its  extremities  is  pressed-upon  by  a  fine  milled-head 
screw  which  works  through  the  frame,  and  the  other  by  a  spring  (con- 
cealed in  the  figure)  with  antagonizes  the  screw.  The  scale  thus  mounted 
is  introduced  through  a  pair  of  slits  in  the  Eye-piece  tube,  immediately 
above  the  diaphragm  (Eig.  66,  a),  so  as  to  occupy  the  centre  of  the  field; 


THE  MICROSCOPE   AND  ITS  REVELATIONS. 


and  it  is  brought  accurately  into  focus  by  unscrewing  the  eye-glass  until 
the  lines  of  the  scale  are  clearly  seen.  The  value  of  the  divisions  of  this 
scale  must  be  determined  by  means  of  a  ruled  Stage-micrometer,  as  in 
the  former  instance,  for  each  Objective  employed  in  micrometry,  the  use 
of  the  Draw-tube  enabling  the  proportions  to  be  adjusted  to  even  and  con- 
venient numbers);  and  this  having  been  accomplished,  the  scale  is  brought 
to  bear  upon  the  object  to  be  measured,  by  moving  the  latter  as  nearly  as 
possible  into  the  centre  of  the  field,  and  then  rotating  the  Eye-piece  in 
such  a  manner  that  the  scale  may  lie  across  that  diameter  which  it  is  de- 
sired to  measure.  The  pushing  screw  at  the  extremity  of  the  scale  being 
then  turned  until  one  edge  of  the  object  appears  to  be  in  exact  contact 
with  one  of  the  long  lines,  the  number  of  divisions  which  its  diameter 

occupies  is  at  once  read-off  by 
directing  the  attention  to  the 
other  edge — the  operation  be- 
ing nothing  more  than  laying 
a  rule  across  the  body  to  be 
measured.^  This  method  of 
measurement  may  be  made 
quite  exact  enougli  for  all  ordi- 
nary purposes,  provided,  in  the 
first  place,  that  the  Eye-piece 
scale  be  divided  with  a  fair  de- 
gree of  accuracy;  and  secondly, 
that  the  value  of  its  divisions 
be  ascertained  (as  in  the  case  of 
the  Cobweb-Micrometer)  by  sev- 
eral comparisons  with  a  ruled 
scale  laid  upon  the  Stage. 
Thus  if,  by  a  mean  of  numer- 

Jackson's  Eye-piece  Micrometer.  ^^^g    observations,   WC  establish 

the  value  of  each  division  of  the  eye-piece  scale  to  be  1-12, 500th  of  an  inch, 
then,  if  the  image  of  an  object  be  found  to  measure  tS^  of  those  divisions, 
its  real  diameter  will  be  3^+  ysItto  Wtt  ii^ch.^  With  an  Objective  of 
l-12th-inch  focus,  the  value  of  the  divisions  of  the  Eye-j)iece  scale  may 
be  reduced  to  1-25, 000th  of  an  inch;  and  as  the  eye  can  estimate  a  fourth 
part  of  one  of  the  divisions  with  tolerable  accuracy,  it  follow  that  a  mag- 
nitude of  as  little  as  1-100, 000th  of  an  inch  can  be  measured  with  a  near 
approach  to  exactness. — Even  this  exactness  may  be  increased  by  the 
Application  of  the  diagonal  scale  (Fig.  67)  devised  by  M.  Hartnack.  The 


^  Dr.  Royston-Pigott  (Zoc.  cit.)  prefers  to  introduce  into  the  aperture  of  the 
diaphragm  a  plano-convex  lens  of  very  long  focus,  with  the  lines  engraved  upon 
its  flat  surface.  The  advantage  of  the  screw-movement  is  sacrificed,  but  a  greater 
distinctness  of  the  lines  is  obtained. 

®  The  calculation  of  the  dimensions  is  much  simplified  by  the  adoption  of  a 
Decimal  scale;  the  value  of  each  divison  being  made,  by  the  use  of  the  Draw-tube 
adjustment,  to  correspond  to  some  aliquot  part  of  a  ten-thousandth  or  a  hundred- 
thousandth  of  an  inch,  and  the  dimensions  of  the  object  being  then  found  byl 
simple  multiplication: — Thus  (to  take  the  above  example)  the  value  of  each  divi- 
sion in  the  decimal  scale  is  .00008,  and  the  diameter  of  the  object  is  .00028.  The 
Metric  system  being  now  universally  employed  on  the  Continent,  many  British 
and  American  Microscopists  prefer  to  record  their  observations  in  parts  of  a  Milli- 
metre; and  with  a  view  to  their  convenience  Messrs.  Beck  supply  Stage-Microme- 
ters ruled  on  one  side  of  a  median  line  to  lOOths  and  lOOOths  of  an  Inch,  and  on 
the  other  side  to  lOOths  of  a  Millimetre. 


ACCESSORY  APPARATUS. 


95 


J 

/ 

•"•1         ,        ■  I  ■  6 

ill 

ili::-ltilt£t 

liilLuliAMini :  ; 

vertical  lines  are  crossed  by  two  parallel  lines,  at  a  distance  from  each 
other  of  five  divisions  of  the  vertical  scale;  and  the  parallelogram  thus 
formed  is  crossed  by  a  diagonal.  It  is  obvious  from  this  construction, 
that  the  lengths  of  the  lower  segments  of  the  50  vertical  lines,  cut  off  by 
the  diagonal,  will  progressively  increase  from  .1  to  5.0;  so  that  when  it  is 
desired  to  obtain  an  exact  measurement  of  an  object  between  these  limits 
it  is  only  requisite  to  find  the  segment  Avhose  length  precisely  coincides 
with  the  diameter  to  be  taken,  which  it  Avill  then  give  in  tenths  of  the 
value  of  the  vertical  divisions,  whatever  these  may  be.  Thus,  at  a,  the 
length  of  the  segment  will  be  1.8;  at  i  it  will  be  3.4. — Whatever  method 
be  adopted,  if  the  measure- 
ment be  made  in  the  Eye-  Pi<3.,67. 
piece  and  not  on  the  stage, 
it  will  be  necessary  to  make 
allowance  for  the  adjust- 
ment of  the  Object-glass 
to  the  thickness  of  the  glass 
that  covers  the  object, 
since  its  magnifying  power 

is  considerably  affected  by  Hartnack's  Eye-piece  Micrometer. 

the  separation  of  the  front 

pair  of  lenses  from  those  behind  it  (§  17).  It  Avill  be  found  convenient 
to  compensate  for  this  alteration  by  altering  the  Draw-tube  in  such  a 
manner  as  to  neutralize  the  effect  produced  by  the  adjustment  of  the 
Objective;  thus  giving  one  uniform  value  to  the  divisions  of  the  Eye- 
piece scale,  whatever  may  be  the  thickness  of  the  covering-glass;  the 
amount  of  the  alteration  required  for  each  degree  must  of  course  be  deter- 
mined by  a  series  of  measurements  with  the  Stage-micrometer. — Micro- 
metric  measurements  may  also  be  made  with  the  Camera  Lucida,  in  the 
manner  to  be  presently  described,  or  with  Dr.  Beale's  neutral  tint 
reflector  (§  94). 

92.  Goniometer, — When  the  Microscope  is  employed  in  researches  on 
minute  Crystals,  their  angles  may  be  measured  by  adapting  a  Goniometer 
to  the  Eye-piece;  but  as  all  First-Class  Microscopes  are  now  provided 
with  rotating  Stages  graduated  at  their  edges,  with  tlie  addition  of  a 
Vernier-scale  if  desired,  the  measurement  may  be  more  conveniently 
made  by  giving  rotation  to  the  object.  An  Eye-piece  is  required  whose 
field  is  traversed  diametrically  by  fixed  line  (either  a  filament  stretched 
across  it,  or  a  line  ruled  on  glass),  and  is  turned  so  as  to  bring  this  line 
into  coincidence  with  one  of  the  lines  forming  the  angle  to  be  measured, 
when  the  Stage  is  at  zero;  the  stage  is  then  rotated  until  the  fixed  line 
coincides  with  the  other  line  of  the  angle,  and  the  amount  of  movement 
is  read  off  on  the  scale. — If  a  higher  degree  of  precision  be  required  than 
either  of  these  methods  is  fitted  to  afford,  the  Douile  Refracting  Gonio- 
meter, invented  by  Dr.  Leeson,  may  be  substituted.^ 

93,  Diapliracpn  Eye-piece, — It  is  often  useful  to  cut  off  the  light  sur- 
rounding the  object  or  part  of  the  object  to  be  examined;  for  the  sake 
alike  of  avoiding  glare  that  is  injurious  to  the  eye,  and  of  rendering  the 
features  of  the  object  more  distinct.  This  may  be  accomplished  on  the 
plan  of  Mr.  Slack,  by  the  introduction,  just  above  the  ordinary  '  stop,'  of 


^  For  a  description  of  this  instrument,  see  Dr.  Leeson 's  description  of  it  in  Part 
xxxiii.  of  the  "Proceedings  of  the  Chemical  Society,"  and  Mr.  Eichard  Beck's 
*•  Treatise  on  the  Microscope,"  p.  65. 


96 


THE  MICROSCOPE   AND  ITS  REVELATIONS. 


four  small  shutters,  worked  by  as  many  milled-lieads  projecting  slightly 
beyond  the  flange  of  the  eye-piece.  By  combining  the  movements  of  these 
shutters  in  various  ways,  it  is  easy  to  form  a  series  of  symmetrical  aper- 
tures, bounded  by  straight  lines,  and  of  any  dimensions  required.  As 
remarked  by  its  inventor,  this  Diaphragm  Eye-piece  may  also  be  used  to 
isolate  one  out  of  many  objects  that  may  be  on  the  same  slide,  and  thus 
to  show  that  object  alone  to  persons  who  might  not  otherwise  distinguish 
it. — For  this  last  purpose  the  Indicator  of  Mr.  Quekett  may  also  be  used; 
which  is  a  small  steel  hand  placed  just  over  the  diaphragm,  so  as  to  point 
to  nearly  the  centre  of  the  field,  whilst  it  may  be  turned  back  when  not 
required,  leaving  the  field  of  view  quite  free.  The  particular  object  or 
portion  of  the  object  to  which  it  is  desired  to  direct  attention,  being 
brought  to  the  extremity  of  the  hand,  is  thus  at  once  ^  indicated  ^  to  any 
other  observer. 


Microscope  arranged  with  Camera  Lucida,  for  Drawing  or  Micrometry, 


94.  Camera  Lucida  and  otlier  Drcming  Apparatus. — Various  contri- 
vances may  be  adapted  to  the  Eye-j)iece,  in  order  to  enable  the  observer 
to  see  the  image  jorojected  upon  a  surface  whereon  he  may  trace  its  out- 
lines. The  one  most  generally  employed  is  the  Camera  Lucida  prism 
contrived  by  Dr.  Wollaston  for  the  general  purposes  of  delineation;  this 
being  fitted  on  the  front  of  the  eye-piece,  in  place  of  the  '  cap  ^  by  which 
it  is  usually  surmounted.  The  Microscope  being  placed  in  a  horizontal 
position,  as  shown  in  Fig.  68,  the  rays  which  pass  through  the  eye-piece 
into  the  prism  sustain  such  a  total  reflection  from  its  oblique  surface,  that 
they  come  to  its  upper  horizontal  surface  at  right  angles  to  their  previous 
direction;  and  the  eye  being  so  placed  over  the  edge  of  this  surface  as  to 
receive  these  rays  from  the  prism  through  part  of  the  pupil,  whilst  it 
looks  with  the  other  half  beyond  the  prism  down  to  a  white  paper  surface 
on  the  table,  it  sees  the  image  so  strongly  and  clearly  projected  upon  that 
surface,  that  the  only  difficulty  in  tracing  it  arises  from  a  certain  incapa- 
city which  seems  to  exist  in  some  individuals  for  seeing  the  image  and 
the  tracing-point  at  the  same  time.  This  difficulty  (which  is  common 
to  all  instruments  devised  for  this  purpose)  is  lessened  by  the  interposition 
of  a  slightly  convex  lens  in  the  position  shown  in  the  figure,  between  the 


ACCESSOKY  APPARATUS. 


97 


eye  and  the  paper,  in  order  that  the  rays  from  the  paper  and  tracing- 
point  may  diverge  at  the  same  angle  as  those  which  are  received  from  the 
prism;  and  it  may  be  generally  got  over  altogether,  by  experimentally 
modifying  the  relative  degrees  of  light  received  from  the  object  and  from 
the  paper.  If  the  image  bo  too  bright,  the  paper,  the  trp-cing- point,  and 
the  outline  it  has  made,  are  scarcely  seen;  and  either  less  light  may  be 
allowed  to  come  from  the  object,  or  more  light  (as  by  a  taper  held  near, 
may  be  thrown  on  the  paper  and  tracing-point.  Sometimes,  on  the 
other  hand,  measures  of  the  contrary  kind  must  be  taken. — Another  in- 
strument for  the  same  jourpose,  invented  by  the  celebrated  anatomist 
Soemmering,  and  ])ref erred  by  some  Microscopists,  is  a  flat  speciilujn  of 
polished  steel  or  speculum-metal,  of  smaller  diameter  than  tli^  ordinary 
j)upil  of  the  eye,  fixed  at  an  angle  of  45°  in  front  of  the  eye-piece.  The 
rays  from  the  eye-piece  are  reflected  vertically  upwards  to  the  central  part 
of  the  pupil  placed  above  the  mirror,  whilst,  as  the  eye  also  receives  rays 
from  the  paper  and  tracer  in  the  same  direction,  through  the  peripheral 


Eis.  Pig.' 70^ 


Chevalier's  Camera  Lucida.  Nachet's  Camera  Lucida, 


portion  of  the  pupil,  the  image  formed  by  the  Microscope  is  visually  pro- 
jected downwards. — In  another  form  of  Camera  Lucida,  devised  by  Amici, 
and  adapted  to  the  horizontal  microscope  by  Chevalier,  the  eye  looks 
through  the  Microscoj)e  at  the  object  (as  in  the  ordinary  view  of  it), 
instead  of  looking  at  its  projection  upon  the  paper,  the  image  of  the 
tracing-point  being  projected  upon  the  field — an  arrangement  which  isiu 
many  respects  more  advantageous.  This  is  effected  by  combining  a  per- 
forated steel  mirror  with  a  reflecting  prism;  and  its  action  will  be  under- 
stood by  the  accompyaning  diagram  (Pig.  69).  The  ray  a  b  proceeding 
from  the  object,  after  emerging  from  the  eye-piece  of  the  Microscope,  passes 
through  the  central  perforation  in  the  oblique  mirror  m,  which  is  placed 
in  front  of  it,  and  so  directly  onwards  to  the  eye.  On  the  other  hand, 
the  ray  a'  proceeding  upwards  from  the  tracing-point,  enters  the  prism  P, 
is  reflected  from  its  inclined  surface  to  the  inclined  surface  of  the  mirror 
M,  and  is  by  it  reflected  to  the  eye  at  i%  in  such  parallelism  to  the  rap  b 
proceeding  from  the  object,  that  the  two  blend  into  one  image. — The 


98 


THE  MICROSCOPE  AND  ITS  REVELATIONS. 


same  effect  is  produced  by  a  contrivance  which  has  been  devised  by  MM. 
Nachet  for  use  with  vertical  Microscopes,  and  is  much  employed  on  the 
Continent.  It  consists  of  a  prism  of  a  nearly  rhomboidal  form  (Fig.  70), 
which  is  placed  with  one  of  its  inclined  sides  A  c,  over  the  eye-piece  of 
the  Microscope;  to  this  side- is  cemented  an  oblique  segment  e,  of  a  small 
glass  cylinder,  which  presents  to  the  ray  a  i,  proceeding  directly  upwards 
from  the  object,  a  surface  at  right  angles  to  it;  so  that  this  ray  passes  into 
the  small  cylinder  E,  and  out  from  the  side  A  B,  of  the  larger  prism,  with- 
out sustaining  any  refraction,  and  with  very  little  loss  by  reflection  from 
the  inclined  surfaces  at  which  they  join.  But  the  ray  a'  V ,  which  comes 
from  the  tracing  point  on  a  paper  at  the  end  of  the  base  of  the  Micro- 
scope, entering  the  rhomboidal  prism,  is  reflected  from  its  inclined  side 
B  D,  to  its  inclined  side  A  c,  and  thence  it  is  again  reflected  to  h,  in  coin- 
cidence with  the  ray  which  has  directly  proceeded  from  the  object.  As 
the  ray  a'  V  is  necessarily  oblique,  the  picture  visually  projected  on  the 
paper  will  be  distorted,  unless  the  right  side  of  the  drawing-board  bo 
raised,  so  that  its  plane  shall  be  at  right  angles  to  a*  V, — Of  the  nume- 
rous contrivances  for  drawing  from  the  Microscope,  the  simplest  and  by 
no  means  the  least  effective,  is  the  Neutral  Tint  Reflector^  recommended 
by  Dr.  Beale,  which  consists  of  a  piece  of  neutral-tint  glass,  set  in  a  cap 
fitted  on  the  Eye-piece,  with  which  it  makes  an  angle  of  45^^.  The 
Microscope  being  arranged  as  in  Fig.  68,  the  eye,  looking  downwards, 
receives  at  the  same  time  the  image-forming  rays  from  the  eye-piece, 
wdiich  come  to  it  by  reflection  from  tlie  surface  of  the  glass,  and  those 
from  the  paper,  tracing-point,  or  rule,  which  pass  to  it  through  the  glass. 
A  simple  and  inexpensive  substitute  for  this,  which  its  inventor  (Mr.  T. 
B.  Jennings,  U.S.)  has  found  very  efficient,  maybe  made  by  taking  a 
flat  cork  about  1^  inch  in  diameter,  cutting  a  hole  in  it  sufficiently  large 
to  enable  it  to  fit  tightly  on  the  Eye-piece  (without  its  cap),  and  then 
making  a  transverse  slit  beneath  the  hole,  into  which  is  to  be  inserted  a 
thin-glass  cover  at  an  angle  of  45°. 

95.  With  one  or  other  of  the  foregoing  contrivances,  every  one  may 
learn  to  draw  an  outline  of  the  Microscopic  image;  and  it  is  extremely 
desirable  for  the  sake  of  accuracy,  that  every  representation  of  an  object 
should  be  based  on  such  a  delineation.  Some  persons  will  use  one 
instrument  most  readily,  some  another;  the  fact  being  that  there  is  a 
sort  of  a  knack''  in  the  use  of  each,  which  is  commonly  acquired  by 
practice  alone,  so  that  a  person  accustomed  to  the  use  of  any  one  of  them 
does  not  at  first  work  well  with  another.  Although  some  persons  at  once 
acquire  the  power  of  seeing  the  image  and  the  tracing-point  with  equal 
distinctness,  the  case  is  more  frequently  otherwise;  and  hence  no  once 
should  allow  himself  to  be  baffled  by  the  failure  of  his  first  attempt.  It 
will  sometimes  happen,  especially  when  the  AVollaston  prism  is  employed, 
that  the  want  of  power  to  see  the  pencil  is  due  to  the  faulty  position  of 
the  eye,  too  large  a  part  of  it  being  over  the  prism  itself.  When  once  a 
good  position  has  been  obtained,  the  eye  should  be  held  there  as  steadily 
as  possible,  until  the  tracing  shall  have  been  completed.  It  is  essential 
to  keep  in  view  that  the  proportion  between  the  size  of  the  tracing  and 
that  of  the  object  is  affected  by  the  distance  of  the  eye  from  the  paper; 
and  hence  that  if  the  Microscope  be  placed  upon  a  support  of  different 
height,  or  the  Eye-piece  be  elevated  or  depressed  by  a  slight  inclination 
given  to  the  body,  the  scale  will  be  altered. — This  it  is,  of  course, 
peculiarly  important  to  bear  in  mind,  when  a  series  of  tracings  is  being 
made  of  any  set  of  objects  which  it  is  intended  to  delineate  on  a  uniform 


ACCESSORY  APPARATUS. 


99 


scale;  or  wlien  the  Camera  Lucida  (or  any  similar  arrangement)  is  em- 
ployed for  the  purpose  of  Micrometry.  All  that  is  requisite  to  turn  it  to 
this  account,  is  an  accurately  divided  Stage-micrometer,  which,  being 
])laced  in  the  position  of  the  object,  enables  the  observer  to  see  its  lines 
projected  upon  the  surface  upon  which  he  has  drawn  his  outline;  for  if 
the  divisions  be  marked  upon  the  paper,  the  average  of  several  taken, 
and  the  paper  then  divided  by  parallel  lines  at  the  distance  thus  ascer- 
tained (the  spaces  being  subdivided  by  intermediate  lines,  if  desirable),  a 
very  accurate  scale  is  furnished,  by  which  the  dimensions  of  any  object 
drawn  in  outline  under  the  same  power  may  be  minutely  determined. 
Thus,  if  the  divisions  of  a  Stage-micrometer,  the  real  value  of  each  of 
Avhich  is  a  100th  of  an  inch,  should  be  projected  on  the  paper  with  such 
a  magnifying  power  as  to  be  at  the  distance  of  an  inch  from  one  another, 
it  is  obvious  that  an  ordinary  inch-scale  applied  to  the  measurement  of 
an  outline  would  give  its  dimensions  in  lOOths  of  an  inch,  whilst  each 
tenth  of  that  scale  would  be  the  equivalent  of  a  1,000th  of  an  inch. 
When  a  sufficient  magnifying  power  is  used,  and  the  dimensions  of  the 
image  are  measured  by  the  ^ diagonal^  scale  (which  subdivides  the  inch 
into  1,000  parts),  great  accuracy  may  be  obtained.  It  was  by  the  use  of 
this  method,  that  Mr.  Gulliver  made  his  admirable  series  of  measure- 
ments of  the  diameters  of  the  Blood -corpuscles  of  different  animals. — In 
using  Nachet's  vertical  Camera  for  Micrometry,  care  must  be  taken  so  to 
adjust  the  slope  of  the  drawing-board,  that  the  Micrometer  scale  shall  be 
projected  on  the  paper  without  distortion. 

96.  Nose-piece. — It  is  continually  desirable  to  be  able  to  substitute 
one  objective  for  another  with  as  little  ex- 
penditure of  time  and  trouble  as  possible; 
so  as  to  be  able  to  examine  under  a  higher 
magnifying  power  the  details  of  an  ob- 
ject of  which  a  general  view  has  been 
obtained  by  means  of  a  lower;  or  to  use 
the  lower  for  the  purpose  of  finding  a 
minute  object  (such  as  a  particular  Dia- 
tom in  the  midst  of  a  slide-full)  which  we 
wish  to  submit  to  high  amplification. 
This  is  effected  by  the  Nose-piece  of  Mr. 
C.  Brooke,  which,  being  screwed  into 
the  object-end  of  the  body  of  the  Micro- 
scope, carries  two  objectives,  either  of 
which  may  be  brought  into  position  by 
turning  the  arm  on  a  pivot.    In  its  origi-        ^^^^^'^  improved  Nose-piece, 

nal  form,  the  arm  was  straight;  so  that  the  Objective  not  in  use  was  often 
brought  own  upon  the  Stage,  unless  the  relative  lengths  of  the  two  objec- 
tives were  specially  adjusted.  This  inconvenience  is  avoided,  however,  in 
the  construction  adopted  by  Messrs.  Powell  and  Lealand,  and  further  sim- 
plified by  Mr.  Swift  (Fig.  71);  the  bend  given  to  the  arm  having  the  effect 
of  keeping  the  Objective  not  in  use  completely  off  the  stage.  ^  The  work- 
ing Microscopist  will  scarcely  find  any  Accessory  more  practically  useful 
to  him  than  this  simple  piece  of  apparatus. 

97.  Finders. — All  Microscopists  occasionally,  and  some  continually, 
feel  the  need  of  a  ready  means  of  -finding,  upon  a  glass  slide,  the  particu- 
lar object,  or  portion  of  an  object,  Avhich  they  desire  to  bring  into  view; 
and  various  contrivances  have  been  suggested  for  the  purpose.  Where 
different  magnifying  powers  can  be  readily  substituted  one  for  another^ 


100 


THE  MICROSCOPE  AND  ITS  REVELATIONS. 


as  by  the  use  of  the  Erector  (§  84)  or  of  the  Nose-piece,  no  special  means 
are  required;  since,  when  the  object  has  been  found  by  a  low  power,  and 
brought  into  the  centre  of  the  field,  it  is  rightly  placed  for  examination 
by  any  other  Objective.  Even  this  slight  trouble,  however,  may  be 
saved  by  the  adoption  of  more  special  methods;  among  the  simplest  of 
which  is  marhing  the  position  of  the  object  on  the  surface  of  the  thin 
glass  which  covers  it.  The  readiest  mode  of  doing  this,  when  the  object 
is  large  enough  to  be  distinguished  by  the  naked  eye  or  under  the 
Simple  Microscope,  is  to  make  a  small  ring  round  it  with  a  fine  cameFs- 
hair  pencil  dipped  in  Asphalte,  or  Brunswick  black  (Indian  ink  being 
objectionable,  as  liable  to  be  washed  off  when  water-immersion  Objectives 
are  in  use);  but  when  the  object  is  not  thus  visible,  the  slide  must  be 
laid  in  position  on  the  stage,  the  object  ^  found  ^  in  the  Microscope,  the 
Condenser  adjusted  to  give  a  bright  and  defined  circle  of  light,  and  then, 
the  Microscope-body  being  withdrawn,  the  black  ring  is  to  be  marked 
around  the  illuminated  spot.  Tliis  method,  however,  has  the  disadvan- 
tage of  concealing  any  other  objects  that  may  lie  in  dose  proximity  to 
the  one  around  which  the  circle  is  drav/n;  and  recourse  must  be  had  in 
such  cases  to  some  other  plan.  Tlie  Mechanical  Stage  may  be  easily 
turned  to  account  as  n> finder,  by  engraving  upon  it  two  scales,  horizontal 
and  vertical,  by  which  the  object-platform  may  be  exactly  set  to  any 
desired  position;  this  platform  being  itself  provided  with  a  removable 
^  stop, ^  against  which  the  glass  slide  (resting  on  its  lower  edge)  may  so 
abut,  as  always  to  occupy  the  same  place  on  the  platform.  Now  sup- 
posing an  observer  to  be  examining  a  newly-mounted  slide,  containing 
any  object  which  he  is  likely  to  wish  to  find  on  some  future  occasion,  he 
first  lays  the  slide  on  the  object-platform,  with  its  lower  edge  resting  on 
the  ledge,,  and  its  end  abutting  against  the  lateral  stop,  and  brings  the 
object=platform  itself  to  the  zero  of  the  scales;  then,  whenever,  on 
moving  the  slide  by  the  traversing  action,  he  meets  with  any  particular 
form  worthy  of  note,  he  reads  off  its  position  upon  the  two  scales,  and 
records  it  in  any  convenient  mode.  The  scale  may  be  divided  to  50ths 
of  an  inch,  and  each  of  these  spaces  may  be  again  halved  by  the  eye;  and 

26 

the  record  may  perhaps  be  best  made  thus, — Triceratium  favus  ^-^^  the 

upper  number  marking  the  ^ latitude^  of  the  object  on  the  vertical  scale, 
and  the  lower  its  longitude  ^  on  the  horizontal.  Whenever  the  Micro- 
scopist  may  wish  again  to  bring  this  object  under  examination,  he  has 
merely  to  lay  the  slide  in  the  same  ])ositon  on  the  platform,  and  to 
adjust  the  platform  by  its  scales  according  to  the  recorded  numbers.^ — 
The  ^ finder^  most  commonly  used  is  that  invented  by  Mr.  Maltwood,^ 
which  consists  of  a  glass  slide  3  inches  by  1^  inch,  on  which  is  photographed 
a  scale  that  occupies  a  square  inch  and  is  divided  by  horizontal  and 
vertical  lines  at  l-50th  of  an  inch  apart  into  2,500  squares,  each  of  which 
contains  two  numbers,  one  marking  its  ^ latitude^  or  place  in  the  vertical 
series,  and  the  other  its  '  longitude  ^  or  place  in  the  horizontal  series. 


^  This  plan,  first  suggested  by  Mr.  Okeden,  might  be  adopted  with  so  little 
trouble  or  expense  in  every  Microscope  possessed  of  a  Mechanical  stage,  that  it 
would  be  very  desirable  for  every  such  Microscope  to  be  furnished  with  these 
graduated  scales.  If  the  different  Makers  would  agree  to  use  the  l-50th  inch 
scale,  Observers  at  a  distance  from  one  another,  who  might  wish  to  examine  each 
other's  objects,  would  have  no  difficulty  in  finding  them  by  the  record  of  their 
positions  accompanying  each  slide. 

2  'Transactions  of  the  Microscopical  Society,"  N.  S.,  Vol.  vi.  (1858),  p.  59. 


ACCESSORY  APPARATUS. 


101 


The  slide,  when  in  use,  should  rest  upon  the  ledge  of  the  stage  of  the 
Microscope,  and  be  made  to  abut  against  a  stop  about  1^  inch  from  the 
centre  of  the  stage. — In  order  to  use  this  ^finder/  the  Object-slide  must 
be  laid  upon  the  Stage  in  such  a  manner  as  to  rest  upon  its  ledge  and  to 
abut  against  the  stop;  and  when  some  particular  object,  whose  place  it 
is  desired  to  record,  has  been  brought  into  the  field  of  view,  the  object- 
slide  being  removed  and  the  ^finder'  laid  down  in  its  place,  the  numbers 
of  the  square  then  in  the  field  are  to  be  read  off  and  recorded.  To  find 
that  object  again  at  any  time,  the  finder^  is  to  be  laid  in  its  place  on  the 
stage,  and  the  stage  moved  so  as  to  bring  the  recorded  number  into 
view;  and  the  object-slide  being  then  substituted  for  the  finder,  the 
desired  object  will  present  itself  in  the  field.  As  care  is  taken  in  the 
production  of  each  MMaltwood,'  that  the  scale  shall  be  at  an  exact  dis- 
tance from  the  bottom  and  left-hand  end  of  the  glass  slide,  the  Micro- 
scopist  may  thus  enable  any  other  observer  provided  with  a  similar 
^finder'  to  bring  into  view  any  desired  object,  by  informing  him  of  the 
numbers  that  mark  its  latitude  and  longitude.  These  numbers  may 
either  be  marked  upon  the  object-slide  itself,  or  recorded  in  a  separate 
list.^ 

98.  Diajjhragms. — Every  Microscope  should  be  provided  with  some 
means  of  regulating  the  amount  of  light  sent  upwards  from  the  Mirror 
through  transparent  objects  under  examination.  This  is  usually  accom- 
plished by  means  of  a  Diaphragm-plate,  perforated  by  apertures  of  dif- 
ferent sizes  (the  smallest  of  which  should  be  no  larger  than  a  pin-hole), 
and  pivoted  to  a  removable  fitting  attached  to  the  under  side  of  the 
Stage,  in  such  a  manner  that  by  rotating  the  plate,  either  of  the  aper- 
tures can  be  brought  into  the  optic  axis  of  the  instrument.  The  larg- 
est of  its  apertures  should  be  made  to  carry  a  ground-glass  (so  fitted  as  to 
be  removable  at  pleasure),  the  use  of  which  is  to  diffuse  a  soft  and  equable 
light  over  the  field  when  large  transparent  objects  are  under  examination 
with,  a  low  power;  Avhile  between  the  smallest  and  the  largest  aperture 
there  should  be  an  unperforated  space,  to  serve  as  a  dark  background  for 
Opaque  objects.  The  edge  of  the  Diaphragm-plate  should  be  notched 
at  certain  intervals,  and  a  spring-catch  fitted  so  as  to  drop  into  the 
notches,  in  order  that  each  aperture  may  be  brought  into  its  proper  cen- 
tral position.  When  the  Diaphragm-plate  is  used  to  imjorovc  the  defini- 
tion of  high  powers,  it  loses  much  of  its  value  if  its  aperture  be  not  very 
close  to  the  under  side  of  the  object-slide;  and  any  arrangement  which  sets 
it  at  some  distance  beneath  the  stage  is  consequently  objectionable.  Its 
best  position  is  in  tlie  thichyiess  of  the  stage,  which,  for  receiving  it,  is 

^  The  only  drawback  to  the  utility  of  the  Maltwood  finder  lies  in  the  fact  that 
a  single  square  more  than  covers  the  field  taken  in  by  l-4th  Objective  with 
the  A  eye-piece;  so  that  with  powers  many  times  as  great,  the  proportion  of  the 
square  viewed  at  once  is  so  small,  as  to  make  it  impossible  to  fix  the  place  of  the 
object  with  any  precision.  To  obviate  this  difficulty,  Mr.  W.  Webb* proposes  a 
fixidev  ruled  with  lines  only  1 -200th  of  an  inch  apart,  so  as  to  divide  a  square  of 
only  3-4th  of  an  inch  into  22,500  squares.  As  it  would  be  impossible  to  mark  dis- 
tinguishing numerals  within  squares  of  such  minuteness,  he  rules  stronger  lines 
at  intervals,  so  as  to  divide  the  whole  area  into  '  blocks'  of  100  squares  in  each; 
and  any  individual  square  can  be  easily  described  (1.)  by  the  block  in  which  it  lies, 
and  (2)  by  its  position  in  that  block.  ("Journ.  of  Roy.  Microsc.  Soc,"  Vol.  iii., 
1880,  p.  750). — To  those  who  prefer  the  simplicity  given  by  the  numbering  of  each 
square  in  the  Maltwood  finder,  the  Author  would  suggest  that  Cxie  object  may  be 
always  *  found'  by  it  with  the  l-4th  Objective;  and  that,  if  thus  brought  into  the 
centre  of  its  field,  the  object  will  lie  within  the  field  of  any  Objective  of  higher 
power,  provided  the  centering  of  the  two  be  conformable. 


102 


THE  MICROSCOPE  AND  ITS  REVELATIOiNS. 


made  of  two  plates  screwed  together.— A  'different  arrangement  may  be 
adopted  with  advantage,  when  the  Stage  is  provided  with  a  cyhndrical 
fitting  for  the  reception  of  Illuminating  and  Polarizing  apparatus.  A 
short  tube  sliding  into  this  may  carry  a  shoulder  at  its  upper  end,  upon 
which  may  be  fitted  two  or  more  caps  with  apertures  of  different  sizes,  so 
that  these  perforated  caps  may  be  either  pushed  up  flush  with  the  sur- 
face of  the  stage,  or  may  be  lowered  to  any  distance  beneath  it,  according 
as  the  best  effect  is  produced.  A  ground-glass  for  diffusing  light  may 
also  be  adapted  to  lie  on  the  shoulder  in  the  place  of  the  perforated 
caps;  and  there  should  also  be  an  mipevforsited  cap  to  serve  as  a  back- 
ground to  opaque  objects. — Such  great  advantage  is  often  derivable  from 
a  gradational  modification  of  the  light,  that  the  Microscopist  who  de- 
sires to  avail  himself  of  this  will  do  well  to  provide  himself  with  one  of 
the  forms  of  graduating  diaphragm  which  have  been  recently^ntroduced. 
That  long  ago  invented  by  DoUond  for  Telescopic  purposes  is  equally 
applicable  to  the  Microscope;  the  circumstance  that  its  aperture  is  square 
instead  of  round,  not  constituting  any  practical  objection  to  its  use.  In 
another  form,  introduced  by  Mr.  Collins  (Fig.  72),  four  shutters  are 


made  to  move  inwards  simultaneously,  by  acting  on  a  lever-handle,  so  as 
to  narrow  the  aperture,  the  shape  of  which  always  remains  more  nearly  cir- 
cular than  square.  And  in  the  '  Iris  Diaphragm  ^  devised  by  Mr.  J.  II. 
Brown, ^  the  multiplication  of  the  number  of  shutters  makes  the  aperture 
practically  circular.  The  new  construction  of  this,  devised  by  Mr.  G-eo. 
Wale,  U.  S.,  is  so  simple,  inexpensive,  and  effectual,  that  its  general  adop- 
tion in  place  of  the  Diaphragm-plate  may  be  anticipated. 

99.  Achromatic  Condensers, — In  almost  every  case  in  which  an  Objec- 
tive of  l-4th  inch  or  any  shorter  focus  is  employed,  its  performance  is 
greatly  improved  by  the  interposition  of  an  Achromatic  combination 
between  the  mirror  and  the  object,  in  such  a  manner  that  the  rays  re- 
flected from  the  former  shall  be  brought  to  a  focus  in  the  spot  to  which 
the  object  is  directed.  A  distinct  picture  of  the  source  of  light  is  thus 
thrown  on  the  object,  from  which  the  rays  emanate  again  as  if  it  were 
self-luminous.  The  Achromatic  combination,  which  (at  least  in  all 
First-class  Microscopes)  is  one  specially  adapted  to  the  purpose,  is  fur- 
nished with  a  Diaphragm-plate  immediately  beneath  its  lowest  lens  (Fig. 
73);  and  this  is  pierced  Avith  holes  of  such  forms  and  sizes  as  to  cut  off 
in  various  degrees,  no  merely  the  peripheral  but  also  the  central  part  of 


Collins's  Graduating  Diaphragm. 


Beck's  Achromatic  Condenser. 


^  ''Transactions  of  the  Microscopical  Society,"  Vol.  xv  ,  p.  74. 


ACCESSORY  APPARATUS. 


103 


the  illuminating  pencil,  or  to  allow  oblique  light  to  pass  only  in  some 
one  azimuth,  or  in  two  azimuths  at  right  angles  to  each  other.  The 
Achromatic  Condenser  of  Messrs.  Beck  is  a  combination  of  three  pairs, 
of  which  the  first  and  second  are  removable,  so  that  the  back  pair  may 
be  used  alone  for  the  illumination  of  objects  viewed  with  low  or  medium 
powers. — The  Achromatic  Condenser  of  Messrs.  Powell  and  Lealand  has 
an  angular  aperture  of  170°,  and  thus  transmits  rays  of  extreme  obliquity 
through  objects  mounted  on  thin  ghiss;  all  other  rays  being  excluded  (if 
desired)  by  a  special  arrangement  of  stops.  The  Diaphragm-plate  being 
perforated  by  apertiires  of  different  sizes,  the  largest  of  these  (which 
transmits  the  entire  pencil)  can  be  partially  closed  by  centric  or  eccentric 
stops  attached  to  a  separate  arm,  any  one  of  which  can  be  brought  into 
the  optic  axis;  and  thus,  whilst  the  graduated  apertures  of  the  dia- 
phragm-plate limit  periplieral  portion  of  the  pencil,  the  stops  cut  off 
its  central,  allowing  the  transmission  either  of  its  entire  peripheral  por- 
tion, or  of  the  rays  proceeding  only  from  some  special  part  or  parts  of  it. 


The  same  eminent  makers  have  lately  introduced  a  JSTon-achromatic  Oil- 
immersion  Condenser;  which,  at  a  much  lower  cost,  serves  for  the  reso- 
lution of  the  most  difficult  tests,  their  illumination  by  colored  rays  not 
being  found  practically  objectionable. — In  the  Achromatic  Condenser 
now  made  by  Messrs.  Ross,  extreme  obliquity  of  the  illuminating  rays 
is  not  provided  for,  this  being  obtained  by  means  of  their  swinging  '  tail- 
piece' (§  72).  Its  combination  has  a  focus  of  about  4-lOths  inch;  and 
beneath  its  back-lens,  which  has  an  aperture  of  half  an  inch,  is  an  Iris- 
diaphragm  for  reducing  it  in  any  desired  degree,  with  a  rotating  dii> 
phragm-plato  having  a  set  of  stops  adapted  to  limit  the  aperture  and  to 
give  a  '  black-ground  '  illumination  under  objectives  of  different  angular 
apertures. — Messrs.  Beck  have  recently  introduced  a  new  Achromatic 
Condenser  with  a  front  revolving  eccentrically  (Fig.  72),  by  which 
means  its  focus  may  be  varied,  and  a  '  black-ground*'  illumination  may 
be  obtained  suitable  for  objectives  having  angles  as  high  as  120°. 

100 .  Weister  Condenser, — Though  the  original  idea  of  the  arrangement 
which  has  come  into  general  use  under  this  designation,  and  which  is 
at  the  same  time  comparatively  inexpensive  and  applicable  to  a  great 
variety  of  purposes,  was  given  by  Mr.  J.  Webster  (''Science  Gossip," 
April  1st,  1865),  it  has  received  important  modifications  at  the  hands  of 
the  Opticians  by  whom  the  instrument  is  manufactured;  and  has,  per- 
haps, not  even  yet  undergone  its  full  development.    In  its  present  form 


104 


THE  MICROSCOPE  AND  ITS  REVELATIONS. 


the  arrangement  of  the  lenses  strongly  resembles  that  used  in  the  Kell- 
ner  eye-piece  (§  28);  the  field-.s^lass  of  the  latter  serving  as  a  condenser 
to  receive  the  cone  of  rays  reflected  upwards  from  the  mirror,  and  to 
make  it  converge  upon  a  small  Achromatic  combination,  whicli  consists 
of  a  double-convex  lens  of  crown,  with  a  plano-convex  lens  of  flint,  the 
plane  side  of  the  latter  being  next  the  object.  These  lenses  are  of  large 
size  and  deop  curvature;  so  that  when  their  central  part  is  stopped-out, 
the  rays  transmitted  from  their  peripheral  portion  meet  at  a  wide  angle  of 
convergence,  and  have  the  elfect  of  those  transmitted  through  the  peri- 
pheral portion  of  the  ordinary  Achromatic  Condenser.  When,  on  the 
other  hand,  this  combination  is  used  with  a  diaphragm  that  allows  only 
the  central  rays  to  pass,  these  rays  meet  at  a  small  angle;  and  the  illu- 
mination thus*^  given  is  very  suitable  for  objects  viewed  with  low  powers. 
Again,  by  stopping-out  the  central  portion  of  the  combination;  and  re- 
moving the  Condenser  to  a  short  distance  beneath  the  object,  the  effect 
of  a  ^  blaok-ground '  illumination  (§104)  can  be  very  satisfactorily  ob- 
tained witli  Objectives  of  low  or  moderate  angular  aperture.  Further, 
by  stopping-out  not  only  the  central  but  also  a  great  part  of  the  periphe- 
ral rays,  so  as  only  to  allow  the  light  to  enter  from  a  small  portion  or 
portions  of  the  margin,  illumination  of  considerable  obliquity  can  be  ob- 
tained. All  this  can  be  provided  for  by  a  Diaphragm-plate  made  to 
rotate  at  as  short  a  distance  as  possible  beneath  the  condensing-lens;  but 
as  the  number  of  apertures  in  this  plate  is  necessarily  limited,  a  greater 
variety  is  obtained  by  the  use  of  a  Graduating  Diaphragm  (§  98)  for  the 
regulation  of  the  centric  aperture,  and  by  making  the  apertures  in  the 
rotating  plate  subservient  to  the  other  purposes  already  named,  as  is  done 
in  the  arrangement  of  Mr.  Collins  (Fig.  75). — Still  greater  variety  can 
be  obtained  by  substituting  for  the  Diaphragm-plate  a  short  tube  sliding 
within  the  one  that  carries  the  lenses;  its  summit  being  furnished  with 
a  socket  into  which  may  be  inserted  a  diaphragm  of  blackened  card  or 
of  thin  metal,  with  an  aperture  or  apertures  of  any  shape  or  size  that  may 
be  desired.  In  this  manner  the  diaphragm  may  be  carried  up  quite  close 
to  the  condensing  lens,  which  is  a  great  advantage;  and  when  oblique 
illumination  is  desired,  the  light  may  be  transmitted  from  any  azimuth, 
by  giving  rotation  to  the  tube  carrying  a  diaphragm  with  a  marginal 
aperture. — The  Webster  Condenser  thus  improved  (which  may  also  be 
used  in  combination  with  the  Polariscope)  will  be  found  one  of  the  most 
universally-useful  accessories  with  which  a  Student's  Microscope  can  be 
provided.^ 

101.  OUiqiie  Illuminators, — The  extremely  oblique  illumination  re- 
quired for  the  resolution  of  the  more  difficult  lined  ^  tests,'  may  be  pro- 
vided, as  has  been  shown,  either  by  the  employment  of  a  Condenser  of 
very  wide  angular  aperature  (§  99);  or  by  giving  to  the  whole  Illuminat- 
ing apparatus  (as  originally  suggested  by  Mr.  Grulfb,  of  Dublin)  a  posi- 


^  A  form  of  condenser  specially  adapted  for  very  oblique  and  also  for  '  black- 
ground  '  illumination  was  devised  a  few  years  ago  by  Prof.  Abbe  of  Jena 
("Monthly  Microsc.  Journ.,"  Vol.  xiii.,  1875,  p  77),  and  has  since  been  specially 
adapted  by  him  for  use  with  'homogeneous  immersion  '  objectives,  being  fitted 
to  the  microscope-stands  constructed  by  Zeiss;  but  not  being  found  easily  appli- 
cable to  Microscopes  of  the  ordinary  English  models,  it  has  not  been  taken  up  by 
Makers  of  this  country.  It  seems  to  the  Author,  however,  that  the  sZzdmg-plate, 
l>y  which  any  degree  of  eccentricity  can  be  given  to  the  apertures  that  the  opti- 
cal combination  admits  of,  might,  in  combination  with  the  Iris-dia^jhragm  for 
limiting  the  angle  of  the  pencil,  be  advantageously  substituted  for  the  rotating 
diaphragm-plate. 


ACCESSORY  APPARATUS.  105 

tion  of  sucli  obliquity  to  the  optic  axis  of  the  Microscope,  that  even  its 
axial  ray  shall  fall  upon  the  object-slide  at  a  very  low  inclination — as  in 
the  Ross-Zentmayer  Microscopes  (§§  59,  72),  and  in  the  arrangements  of 
Messrs.  Beck  (§  75)  and  Mr.  Svvift  (§  68).  It  is  considered  by  Mr.  Wen- 
ham  that  there  is  no  better  method  of  utilizing  this  arrangrement,  than 
by  making  the  Sub-stage  carry  an  ordinary  Objective  of  about  1-inch 
focus,  and  throwing  its  pencil  upon  a  hemispherical  lens  of  half  an 
inch  diameter,  the  plane  side  of  which  has  a  film  of  glycerine  interposed 
between  itself  and  the  object-slide.  The  lens  may  either  be  held  in  this 
position  by  its  own  adhesion,  or  it  may  be  so  fitted  into  a  thin  stage, 
that  its  plain  surface  shall  lie  flush  with  the  surface  of  the  object- plat- 
form. This  (as  also  the  Disk-Illuminator  to  be  next  described)  may  be 
made  to  work  well  with  any  form  of  Students'  Microscope,  which,  like 
Wale's  (§  60),  has  a  thin  stage  and  a  mirror  so  swung  as  to  be  capable  of 
reflecting  rays  of  great  obliquity. — For  the  illumination  of  objects  by  a 
line  of  light  thrown  upon  them  very  obliquely,  Mr.  Wenham  has  devised 
the  simple  Illuminator  shown  in  Fig.  76.  This  consists  of  a  semi-circu- 
lar disk  of  glass  (somewhat  resembling  the  half  of 
a  button)  of  half  an  inch  in  diameter,  the  sides  of 
which  are  flattened,  while  the  circular  edge  is 
rounded  and  well  polished  to  a  transverse  radius 
of  1-lOth  of  an  inch.  This  concentrates  the  light 
thrown  upon  any  part  of  its  circumference,  upon 
an  object  mounted  on  a  slide  of  the  usual  thick- 
ness,   with   whose   under   side  it  is  brought  into  Wenham's  Disk-niuminator. 

immersion-contact  by  the  intervention  of  either 

water,  glycerine,  or  a  more  refractive  oil.  As  it  should  be  so  fitted 
to  the  Microscope  as  to  illuminate  the  objects  from  any  azimuth,  it  should 
have  its  flat  sides  grasped  in  a  clip,  which  may  either  be  mounted  on  the 
Sub-stage,  or  attached  to  under  side  of  the  Stage — in  either  case  having 
its  diametric  section  brought  up  to  the  under  surface  of  the  object-slide. 
By  giving  rotation  to  the  object,  the  illuminator  remaining  fixed,  the 
illuminating  beam  may  be  made  to  cross  the  former  in  any  direction  that 
is  fitted  to  bring  out  its  markings.  With  this  simple  Illuminator,  even 
Amphipleura pellitcida  may  be  resolved  without  the  aid  of  a  Condenser, 
the  mirror  alone  sufficing. ^ — Another  simple  and  effective  appliance  for 
the  same  purpose,  is  the  Woodward  Prism:  a  small  obtuse-angled  triangle 
of  glass,  whose  long  face  must  be  brought  into  immersion-contact  with 
the  object-slide  by  a  film  of  interposed  glycerine.  Originally  devised  as  a 
right-angled  prism,  it  was  suited  only  for  the  illumination  of  objects  seen 
under  immersion  Objectives  of  widest  angular  aperture;  but  by  reducing  its 
oblique  angles  to  less  than  45°,  so  as  to  open-out  the  two  equal  sides,  it 
may  be  adapted  to  Objectives  of  much  smaller  aperture.  In  using  it, 
the  light  is  made  to  enter  one  of  the  oblique  facets  perpendicularly  to  its 
surface;  and  by  looking  in  the  like  direction  through  the  other  side  of 
the  prism,  the  observer  can  see  when  the  face  of  the  object  is  best  illumi- 
nated, by  the  rays  reflected  on  it  from  the  inner  surface  of  that  facet. — 
This  prism  can  be  made  to  hang  to  the  under  surface  of  the  object-slide 
by  the  film  of  interposed  glycerine;  but  as  it  is  very  apt  to  slip  when  the 
microscope  is  inclined,  and  as  its  full  advantage  can  only  be  obtained 
when  the  object  is  made  to  rotate  so  as  to  meet  the  illuminating  beam  in 


^  For  the  mode  of  constructing  this  Illuminator,  see  Journ.  of  Roy.  Microsc. 
See,"  Vol.  iii.  (1880),  p.  246. 


106 


THE  MICROSCOPE  AND  ITS  REVELATIONS. 


every  azimuth,  it  should  be  mounted,  like  the  Disk-illuminator  just  de- 
scribed, in  an  independent  fitting.' 

102.  The  A  mici  PrUm,  which  causes  the  rays  to  be  at  once  reflected 
by  a  plane  surface  and  concentrated  by  lenticular  surfaces,  so  as  to  answer 
the  purpose  of  Mirror  and  Condenser  at  the  same  time,  is  much  approved 
by  many  who  have  used  it.  Such  a  Prism  may  be  either  mounted  on  a 
separate  base,  or  attached  to  some  part  of  the  Microscope-stand.  The 
mounting  shown  in  Fig.  77,  is  a  very  simple  and  convenient  one;  this 

consists  in  attaching  the  frame  of  the  prism 
•EnL.v:7  to  a  sliding  bar,  which  works  in  dovetail 

grooves  on  the  top  of  a  cap  that  may  be  set 
on  the  ^secondary  body'  beneath  the  stage; 
the  slide  serves  to  regulate  the  distance  of 
the  prism  from  the  axis  of  the  microscope, 
and  consequently  the  obliquity  of  the  illum- 
ination; whilst  its  distance  beneath  the  stage 
is  adjusted  by  the  rack-movement  of  cylindri.- 
cal  fitting.  In  this  manner,  an  illuminating 
pencil  of  almost  any  degree  of  obliquity  that 
Amici's  Prism.  is  permitted  by  the  construction  of  the  Stage 

may  be  readily  obtained ;  but  there  is  no  pro- 
vision for  the  correction  of  its  aberations.  In  order  to  use  this  oblique 
illumination  to  the  greatest  advantage,  either  the  prism  or  the  object 
should  be  made  to  rotate,  thus  causing  the  oblique  rays  to  fall  upon  the 
latter  from  every  azimuth  in  succession,  so  as  to  bring  out  all  its  markings 
(§  145). 

103.  Black- Ground  Illuminators. — When  the  rays  are  directed  with 
such  obliquity  as  not  to  be  received  into  the  Object-glass  at  all,  but  are 
suflBciently  retained  by  the  Object  to  render  it  (so  to  speak)  self-luminous, 
we  have  what  is  known  as  the  Black-ground  illuminatio7i.  For  low 
powers  whose  angular  aperture  is  small,  and  for  such  objects  as  do  not 
require  any  more  special  provision,  a  sufficiently  good  '  black-ground 
illumination  may  be  obtained  by  turning  the  concave  Mirror  as  far  as 
possible  out  of  the  axis  of  the  microscope,  especially  if  it  be  so  mounted 
as  to  be  capable  of  a  more  than  ordinary  degree  of  obliquity.  In  this 
manner  it  is  often  possible,  not  merely  to  bring  into  view  features  of 
structure  that  might  not  otherwise  be  distinguishable,  but  to  see  bodies  of 
extreme  transparence  (such,  for  instance,  as  very  minute  Animalcules) 
that  are  not  visible  when  the  field  is  flooded  (so  to  speak)  by  direct  light; 
these  presenting  the  beautiful  spectacle  of  phosphorescent  points  rapidly 
sailing  through  a  dark  ocean.  It  is  one  of  the  great  advantages  of  this 
kind  of  illumination,  that,  as  the  light  radiates  from  each  part  of  the 
object  as  its  proper  source,  instead  of  m^vQly  passing  through  it  from  a 
more  remote  source,  its  different  parts  are  seen  much  more  in  their  normal 
relations  to  one  another,  and  it  acquires  far  more  of  the  aspect  of  solidity. 
The  rationale  of  this  is  easily  made  apparent,  by  holding  up  a  glass 
vessel  with  a  figured  surface  in  front  of  a  lamp  or  a  window,  at  some  dis- 
tance from  the  eye,  so  that  it  is  seen  by  transmitted  light  alone:  for  the 
figures  of  its  two  surfaces  are  then  so  blended  together,  that  unless  their 
form  and  distribution  be  previously  known,  it  can  scarcely  be  said  with 
certainty  which  markings  belong  to  either.  If,  on  the  other  hand,  an 
opaque  body  be  so  placed  behind  the  vessel  that  no  rays  are  transmitted 


abid..  Vol.  i.  (1878).  p.  246. 


ACCESSORY  APPARATUS. 


107 


directly  through  it,  whilst  it  receives  adequate  illumination  from  the 
light  around,  its  form  is  clearly  discerned,  and  the  two  surfaces  are  dis- 
tinguished without  the  least  difficulty. 

104.  A  simple  method  of  obtaining  '  black-ground  '  illumination, 
which  works  well  with  objectives  of  low  power  and  small  angular  aper- 
ture, consists  in  fixing  into  the  top  of  a  short  tube  that  slides  into  the 
'  cylindrical  fitting'  usually  carried  beneath  the  stage  in  Educational  and 
Students'  Microscopes,  a  small  '  bull's  eye '  lens,  the  plane  surface  of 
which  (placed  uppermost)  has  its  central  portion  covered  by  a  black  spot. 
When  light  reflected  by  the  mirror  falls  on  the  lower  surface  of  this  Spot- 
Lens,  only  the  rays  that  fall  on  its  marginal  ring  are  allowed  to  pass; 
and  these,  owing  to  its  high  curvature,  are  so  strongly  refracted  inwards, 
as  to  cross  eacii  other  in  the  object  (when  the  lens  is  focussed  for  it),  and 
then  diverge  again  at  an  angle  sufficiently  wide  to  pass  beyond  the  mar- 
gin of  the  objective,  like  those  transmitted  by  the  Paraboloid  to  be  pres- 


ently  described  (Fig.  79,  F  G,  f  h).  Thus  the  field  is  left  dark;  whilst 
the  light  stopped  by  the  object  gives  it  a  luminosity  of  its  own.— The 
same  effect  is  gained  by  the  use  of  the  Webster  Condenser  (§  100)  with  a 
central  stop  placed  immediately  behind  the  lower  lens  or  upon  the  flat 
surface  of  the  upper.— Neither  of  the  foregoing  plans,  however,  will 
answer  well  for  objectives  of  high  power,  having  such  large  angles  of 
aperture  that  the  light  must  fall  very  obliquely  to  pass  beyond  them  alto- 
gether. Thus  if  the  pencil  formed  by  the  'spot-lens'  have  an  angle  of 
50°,  its  rays  will  enter  a  4-lOths  objective  of  60°,  and  the  field  will  not 
be  darkened. 

105.  A  greater  degree  of  obliquity,  suited  to  afford  '  black-ground '  il- 
lumination with  Objectives  of  larger  angular  aperture,  may  be  obtained 
by  the  use  ot  the  Parabolic  lUummator'  (Fig.  78);  which  consists  of  a 

Parabolic  Illuminator  was  first  devised  by  Mr.  Wenham,  who,  however, 
employed  a  Silver  speculum  for  the  purpose.  About  the  same  time,  Mr.  Shad- 
bolt  devised  an  Annular  Condenser  of  Glass  for  the  same  purpose  (see  Transact, 
of  Microsc.  Soc,"  Ser.  I  ,  Vol.  iii.,  1852,  pp.  85, 132).  The  two  principles  are  com- 
bined in  the  Glass  Paraboloid. 


108 


THE  MICROSCOPE  AND  ITS  REVELATIONS. 


Paraboloid  of  glass  that  reflects  to  its  focus  the  rays  which  fall  upon  its 
internal  surface*  A  diagrammatic  section  of  this  instrument,  showing 
the  course  of  tlie  rays  through  it,  is  given  in  Fig.  ?9,  the  shaded  portion 
representing  the  Paraboloid.  The  parallel  rays  r  r'  r'\  entering  its 
lower  surface  perpendicularly,  pass  on  until  they  meet  its  parabolic  sur- 
face, on  wliich  they  fall  at  such  an  angle  so  as  to  be  totally  reflected  by 
it(§  2)  and  are  all  directed  towards  its  focus,  F.  The  top  of  the  parabo- 
loid being  ground  out  into  a  spherical  curve  of  which  r  is  the  centre,  the 
rays  in  emerging  from  it  undergo  no  refraction,  since  each  falls  perpen- 
dicularly upon  the  part  of  the  surface  through  which  it  passes.  A  stop 
placed  at  s  prevents  any  of  the  rays  reflected  upwards  by  the  mirror  from 
]).issing  to  the  object,  which,  being  placed  at  F,  is  illuminated  by  the  rays 
reflected  into  it  from  all  sides  of  the  Paraboloid.  Those  rays  which  pass 
through  it  diverge  again  at  various  angles;  and  if  tlie  least  of  these, 
G  F  H,  be  greater  than  the  angle  of  aperture  of  the  Object-glass,  none  of 
them  can  enter  it.  The  stop  s,  is  attached  to  a  stem  of  wire,  which 
passes  vertically  through  the  Paraboloid  and  terminates  in  a  knob  beneath, 
as  shown  in  Fig.  78;  and  by  means  of  this  it  may  be  pushed  upwards  so 
as  to  cut  off  the  less  divergent  rays  in  their  passage  towards  the  object, 
thus  giving  a  black-ground  illumination  with  Objectives  of  an  angle  of 
aperture  nmch  wider  than  G  f  h. — In  using  the  Paraboloid  for  delicate 
objects,  the  rays  which  are  made  to  enter  it  should  be  parallel,  con- 
sequently the  'plane  Mirror  should  always  be  employed;  and  when, 
instead  of  the  parallel  rays  of  daylight,  we  are  obliged  to  use  the  diverg- 
ing rays  of  a  lamp,  these  should  be  rendered  as  parallel  as  possible,  pre- 
yiously  to  their  reflection  from  the  mirror,  by  the  interposition  of  the 
M)uirs  eye'  Condenser  (Fig.  87)  so  adjusted  as  to  produce  this  effect. 
There  are  many  cases,  however,  in  which  the  stronger  light  of  the  concave 
Mirror  is  preferable. — When  it  is  desired  that  the  light  should  fall  on  the 
object  from  one  side  only,  the  circular  opening  at  the  bottom  of  the  wide 
tube  (Fig.  78)  that  carries  the  Paraboloid,  may  be  fitted  with  a  diaphragm 
adapted  to  cover  all  but  a  certain  portion  of  it;  and  by  giving  rotation  to 
this  diaphragm,  rays  of  great  obliquity  may  be  made  to  fall  upon  the 
object  from  every  azimuth  in  succession.^ — A  small  glass  cone,  with  the 
apex  downwards,  and  the  base  somewhat  convex,  with  a  stop  in  the  cen- 
tre, is  fitted  by  MM.  Nachet  to  their  Microscopes  for  the  same  purpose; 
and  performs  very  effectively. 

lU6.  In  order  to  adapt  the  Paraboloid  for  black-ground  illumination 
under  Objectives  of  wide  angle  of  aperture,  Mr.  Wenham^  long  since 
constructed  ^  flat-topped  paraboloid,  fitted  to  rellect  only  rays  of  such 
extreme  obliquity,  that  they  would  not  pass  out  of  the  flat  surface  of  the 
l)araboloid  into  the  under  surface  of  the  slide,  unless  a  film  of  either 
water  or  of  some  liquid  of  higher  refractive  index  (such  as  turpentine, 
or  oil  of  cloves)  was  interposed  between  them.  When  thus  enabled  to 
enter  the  slide,  these  rays  pass  on  until  they  meet  the  cover,  from  which 
(in  the  case  of  dry-front  objectives)  they  are  reflected  downwards  upon 
the  surface  of  the  object,  giving  it  a  bright  illumination  on  a  perfectly 
dark  field.  The  special  value  of  this  instrument,  however,  not  being 
then  understood,  it  was  not   constructed  for  sale. — The  same  prin- 


^  By  the  use  of  such  a  diaphragm,  or  of  a  large  stop  with  an  eccentric  per- 
foration, Mr.  G.  Williams  has  succeeded  in  resolving  the  transverse  striae  of 
Amphipleura  pellucida  with  water-immersion  Objectives.  See  Journ.  of  Boy. 
Microsc.  Soc,"  Vol.  lii.  (1880),  p.  524. 

2    Transact,  of  Microsc.  Soc,"  N.  S.,  Vol.  iv.  (1856),  p.  59. 


ACCESSORY  APPARATUS. 


109 


ciple,  however,  haying  been  more  recently  taken  up  by  Dr.  Edmunds,  an 
Immersion  Paraboloid  specially  devised  by  him  for  use  with  immersion 
Objectives  of  large  aperture,  has  been  constructed  by  Messrs.  Powell  & 
Lealand,  with  results  so  satisfactory,  that  it  now  ranks  among  the 
Accessories  most  valued  by  such  as  habitually  work  with  Objectives  of 
that  highest  class.  ^ 

107.  Wenliam's  Reflex  Illuminator, — Another  very  ingenious  and 
valuable  illuminator  for  high  powers  has  been  devised  by  Mr.  Wenham,^ 
and  constructed  by  Messrs.  Eoss.  It  is  composed  of  a  glass  cylinder 
(Fig.  80,  a)  half-an-inch  long, 
and  four-tenths  of  an  inch  in 
diameter;  one  side  of  Avhich, 
starting  from  the  bottom  edge, 
is  worked  to  a  polished  face  at 
an  angle  of  64°  with  the  base. 
The  top  of  the  cylinder  is  polish- 
ed flat,  whilst  its  lower  surface 
is  convex,  being  polished  to  a 
radius  of  4-lOths  of  an  inch; 
close  beneath  this  last  is  set  a 
plano-convex  lens  of  1^  inch 
focus;  and  the  combination  is 
set  eccentrically  in  a  fitting,  i  ?*, 
adapted  to  be  received  into  the 
Sub-stage.  The  parallel  rays, 
/  /  /,  reflected  up  into  it  from 
the  mirror,  are  made  to  con- 
verge, by  the  convex  surfaces  at 
the  base  of  the  cylinder,  at  such 
an  angle,  that  if  their  course 
were  continued  through  glass 
they  would  meet  at  the  point 
A,  above  the  glass  slide  c ;  but 
by  impinging  on  the  inclined 
polished  surface,  they  are  reflect- 
ed to  the  flat  segmental  top, 
from  which  again  they  would  be 

reflected    obliquely    downwards  Wenham's  Reflex  illuminator. 

so  as  to   meet  m   the  ponit 

iy  but  for  its  being  brought  into  ^  immersion-contact '  with  the  under 
side  of  the  slide.  Passing  upwards  through  the  slide,  they  meet  in  a 
point,  g,  a  little  above  its  upper  surface,  in  the  optic  axis  of  the  Micro- 
scope, to  which  point  the  object  must  be  brought;  and  by  giving  rota- 
tion either  to  the  object  or  to  the  illuminator,  it  may  be  illuminexl  from 
every  azimuth.  For  convenience  of  centering,  a  black  half-cylinder  e,  is 
so  fixed  by  the  side  of  the  cylinder,  that  if  a  dot  upon  its  upper  surface 
be  brought  into  the  centre  of  the  field  of  view  of  a  low-power  objective, 
its  focus  g,  will  lie  in  the  optic  axis. — Some  skill  and  practice  are 
required  to  use  this  apparatus  to  advantage,  but  it  will  amply  repay  the 
trouble  of  mastering  its  difficulties.  It  is  best  suited  to  thin  flat  objects; 
with  those  that  are  thick  and  irregular,  distortion  is  unavoidable. 


^  Monthly  Journ.  of  Microsc.  Sci.,"  Vol.  xviii.,  p.  78. 
UUd,,  Vol.  vii.,  p.  239. 


110 


THE  MICROSCOPE  AND  ITS  REVELATIONS. 


Although  specially  designed  as  a  '  black-ground '  illuminator,  it  may  also 
be  made  useful  in  the  resolution  of  difficult  Test-objects  by  transmitted 
light,'  the  illuminator  being  lowered  until  a  colored  spectrum  appears  in 
the  field,  the  rays  of  which  bring  out  their  markings  with  remarkable 
distinctness. — For  use  with  either  of  these  arratigements  for  ^black- 
ground  '  illumination,  it  is  better  that  the  objects  should  be  mounted 
*  dry,'  especially  when  they  are  to  be  viewed  under  '  immersion '  objec-^ 
tives;  balsam-mounted  objects  being  thus  seen  better  with  dry-front 
objectives. 

108.  The  following  directions  are  given  by  Mr.  Schulze  (^^ English 
Mechanic,"  1877,  No.  661)  for  the  use  of  two  illuminators  last  described: 
— First,  rack  up  the  Sub-stage,  until  the  plane  top  of  the  illaminator 
is  level  with  the  stage;  centre  carefully;  put  a  drop  or  two  of  glycerine 
on  the  under  side  of  the  slide,  taking  care  that  no  air-bells  are  formed; 
and  place  the  slide  on  the  stage.  If,  now,  rays  parallel  to  the  optic  axis 
are  thrown  up  by  the  plane  mirror  or  rectangular  prism,  a  luminous  spot 
w^ill  appear  on  the  slide  if  an  object  lies  in  the  optic  axis.  Next  focus; 
and  by  adjusting  the  mirror  or  rectangular  prism  more  carefully,  the 
object  will  be  brilliantly  illuminated  by  very  oblique  rays  on  a  black 
ground.  ...  I  generally  use  one  of  How's  common  Microscope  lamps 
filled  with  good  paraffin  oil,  and  having  a  wick  half  an  inch  broad;  bat 
for  the  highest  powers  I  have  recourse  to  the  Dallinger  lamp  (§  131). 
After  I  have  obtained  the  best  results,  I  interpolate  a  bulTs-eye  Con- 
denser to  increase  the  light,  focussing  carefully  a  miniature  image  of  the 
flame  on  the  slide.  I  invariably  use  the  narrow  side  of  the  flame  turned 
towards  the  mirror  or  prism,  when  resolving  lined  tests.  It  is,  however, 
by  sunlight  that  the  performances  of  the  Immersion  Paraboloid  and 
Eeflex  Illuminator  seem  to  eclipse  any  resolution  that  can  be  obtained  by 
transmitted  light."  [This  was  written  before  Mr.  Schulze  had  found 
out  the  mode  of  working  these  instruments  already  noticed.]  In  regard 
to  the  relative  values  of  the  two  illuminators,  Mr.  Schulze  states  as  the 
result  of  careful  comparative  trials  of  them: — "  The  Paraboloid  is  a  trifle 
easier  managed,  gives  a  little  more  light  by  lamplight,  and  is  somewhat 
cheaper  than  the  Reflex  Illuminator.  Both  perform  equally  well  on  dark 
ground  by  sunlight;  but  the  Reflex  Illuminator  can  also  be  used  on 
balsamed  slides  and  with  immersion  lenses  for  the  examination  of  objects 
by  transmitted  very  oblique  white  light." 

109.  Liglit-Modifiers. — For  (1)  reducing  the  intensity  either  of  Solar- 
light  or  Lamp-light,  (2)  for  correcting  the  yellowness  of  the  latter,  and 
(3)  for  the  equable  diffusion  of  either  light  over  a  large  field,  it  is  often 
convenient  to  employ  interposed  media,  the  nature  of  which  must  be 
varied  according  to  the  particular  purpose  to  be  attained. — The  direct 
rays  of  the  Sun  are  very  little  employed  by  Microscopists,  except  for 
Photography  or  some  other  special  purpose.  But  when  recourse  is  had 
to  them  in  ordinary  Microscopy,  it  is  well  to  take  advantage  of  '  Rainey's 
Light-modifier,'  which  is  a  combination  of  one  thickness  of  dark-blue 
glass  free  from  any  tint  of  red,  another  of  very  pale  blue  w^itli  a  slight 
shade  of  green,  and  two  of  thick  white  plate-glass,  all  cemented  together 
by  Canada  balsam.  This  is  mounted  by  Messrs.  Powell  and  Lealand  on 
a  separate  stand;  and  may  be  used  with  Lamp-light  as  with  sunlight. — 
Some  observers  use  Lamp-chimneys  of  either  neutral-tint  or  bluish  glass 


^  See  Schulze  in  *'Journ.  Roy.  Microsc.  Soc,"  Vol.  i.  (1878),  p.  45;  and  Col. 
Dr.  Woodward  in  same  Vol.,  p.  248. 


ACCESSORY  APPARATUS. 


Ill 


for  the  purpose  of  moderating  the  glare  of  the  flame  or  of  correcting  its 
yellowness;  but  as  the  chimney  cannot  be  conveniently  changed  when- 
ever the  full  light  is  required,  the  Author  much  prefers  making  such 
night-modifiers^  a  part  of  the  Illuminating  apparatus  attached  to  the 
Microscope  itself:  and  this  may  be  done  in  different  modes,  according  to 
the  construction  ol  the  instrument.  Thus,  when  the  Webster  Condenser 
(§  100)  is  in  use,  it  may  be  furnished  with  three  caps  made  to  slide  upon 
its  upper  portion;  one  of  them  fitted  with  a  disk  of  blue-glass,  second 
with  one  of  neutral-tint  glass,  and  the  third  with  a  finely-ground  glass. 
And  in  Swift's  Combination  Sub-stago  (§  112)  similar  disks  maybe  made 
to  drop  into  the  openings  of  the  rotating  plate;  so  that  one  may  readily 
be  changed  for  another,  or,  if  all  three  be  placed  in  the  plate  at  once,  an 
object  may  be  examined  under  any  one  of  them  by  merely  rotating  the 
plate.  Every  ordinary  Diaphragm-plate  (§  98)  ought  to  have  its  largest 
aperture  fitted,  by  means  of  a  projecting  shoulder,  to  carry  such  a  set  of 
disks. — The  three  arms  on  which  the  rotating  Selenites  are  attached  to 
the  Sub-stage  of  Messrs.  Beck's  First-class  Microscope  (Fig.  82),  may  be 
fitted  with  similar  disks,  each  of  which  may  then  be  used  either  sepa- 
rately or  in  combination  with  one  or  both  of  the  others. — Every  '  Light- 
modifier'  should  be  so  constructed  and  worked,  that  the  light  should  be 
made  as  nearly  as  possible  to  resemble  that  of  a  bright  white  cloud.  For 
this  purpose  a  Avhite-cloud  Eeflector  may  be  easily  made — either  flat,  by 
casting  a  Plaster  of  Paris  disk  upon  the  plane  surface  of  the  mirror — or 
concave,  by  casting  it  on  the  surface  of  a  glass  globe;  the  light  reflected 
from  the  surface  of  the  plaster  requiring  to  be  condensed  for  the  illum- 
ination of  small  objects. — Very  pleasant  white-cloud  effects  may  be 
obtained  by  methods  adopted  by  Mr.  Slack.  For  large  objects,  viewed 
with  powers  of  1^  to  4  inches,  he  places  under  the  stage  a  tube  holding 
a  large  disk  (1|-  inch  diameter)  of  ground  glass,  the  ground  surface  being 
protected  by  a  plain  glass  cover  over  it.  By  this  means  the  peculiar  tint 
of  the  freshly  ground  surface  is  permanently  retained.  For  2-3ds  and 
half-inch  powers  he  employs  a  glass  slide  carrying  a  disk  or  square  of 
thin  paper,  saturated  with  spermaceti,  and  protected  from  dirt  by  a  thin 
glass  cover  that  adheres  to  it.  This  slide,  disk  downwards,  is  placed 
under  the  object.  Under  still  higher  powers,  some  objects  may  be  very 
conveniently  illuminated  by  a  small  bull's-eye  finely  ground  on  its  flat 
surface,  and  fixed  with  its  convex  face  downwards  in  a  tube  that  slides 
into  the  Sub-stage  fitting. 

110.  Polarizing  Apparatus, — In  order  to  examine  transparent  objects 
by  Polarized  Light,  it  is  necessary  to  employ  some  means  of  polarizing 
the  rays  before  they  pass  through  the  object,  and  to  apply  to  them,  in 
some  part  of  their  course  between  the  object  and  the  eye,  an  analyzing 
medium.  These  two  requirements  may  be  provided  for  in  different 
modes.  polarizer  m^^^      either  a  bundle  of  plates  of  thin  glass, 

used  in  place  of  the  mirror,  and  polarizing  the  rays  by  reflection;  or  it 
may  be  a  ^single  image ^  or  ^  NicoP  prism  of  Iceland  Spar,  which  is  so 
constructed  as  to  transmit  only  one  of  the  two  rays  into  which  a  beam  of 
ordinary  light  is  made  to  divaricate  by  passing  through  this  substance. 
Of  these  two  methods,  the  'Nicer  prism  is  the  one  generally  preferred, 
the  objection  to  the  reflecting  polarizer  being  that  it  cannot  be  made  to 
rotate.  This  polarizing  prism  is  usually  fixed  in  a  tube  (Fig.  81,  a,  a), 
furnished  with  a  large  milled-head,  c,  at  the  bottom,  by  which  it  is  made 
to  rotate  in  a  collar,  5,  that  screws  into  the  Sub-stage  fitting.  For  the 
analyzer  a  second  'Nicor  prism  is  usually  employed;  and  this,  fixed  in  a 


112 


THE  MICROSCOPE  AND  ITS  REVELATIONS. 


short  tube,  may  be  fitted  either  into  a  collar  interposed  between  the 
lower  end  of  the  body  and  the  Objective,  or  into  a  cap  placed  over  the 
Eye-piece  (Fig.  81,  b),  in  the  stead  of  the  ordinary  eye-piece  cap.  ^  The 
former  arrangement,  which  is  specially  adapted  for  use  with  the  Binocu- 
lar Microscope,  has  the  advantage  of  not  limiting  the  field,  but  it  stops  a 

good  deal  of  light;  while 
*  in  the  latter,  the  image 

A  ^       is  brighter,  but  a  good 

deal  of  the  margin  of  the 
field  is  cut  off.  In  the 
Harley  Binocular  (§  68) 
the  analyzing  prism  is  fit- 
ted into  a  slide  below  the 
Wenham  prism,  which  is> 
drawn  out  when  thepolari- 
scope  is  not  in  use;  while 
in  Swift's  Challenge  Bi- 

A,  Fitting  of  Polarizing  B,  Fitting  of  Analyzing  n  hp  n  1  ^i  r   n  <5i  m  i  1  ji  r     1 1  rl  p  i  «5 

Prism  in  Sub-stage.  Prism  above  Eye-piece.  ^^^^^^^^^  ^  SimUdl   Sliue  IS 

fitted  into  the  body  above 
the  Wenham  prism.  In  these  arrangements,  such  advantage  as  is  obtainable 
by  the  rotation  of  the  analyzing  prism  is  of  course  foregone;  and  the  same 
sacrifice  is  made,  when,  in  the  Stephenson  Binocular  (§  36),  the  Iceland 
spar  analyzer  is  replaced  by  a  refiector. — The  Polarizing  apparatus  may  be 
worked  in  combination  either  with  the  Achromatic  Condenser  (by  which 
means  it  may  be  used  with  high  power  Objectives),  or  with  either  of  the 
^black-ground'  Illuminators  (§§  104,  105),  which  show  many  objects — 
such  as  the  horny  polyparies  of  Zoophytes — gorgeously  projected  in  colors 
upon  a  dark  field. 

111.  For  bringing  out  certain  effects  of  Color  by  the  use  of  Polarized 
Light  (Chap,  xxii.),  it  is  desirable  to  interpose  a  plate  of  Selenite  be- 
tween the  polarizer  and  the  object;  and  it  is  advantageous  that  this 
should  be  made  to  revolve."  A  very  convenient  mode  of  effecting  this,  is 
to  mount  the  Selenite  plate  in  a  revolving  collar,  which  fits  into  the 
upper  end  of  the  tube  that  receives  the  Polarizing  prism.  In  order  to 
obtain  the  greatest  variety  of  coloration  with  different  objects,  films  of 
Selenite  of  different  thickness  should  be  employed;  and  this  may  be  ac- 
complished by  substituting  one  for  another  in  the  revolving  collar.  A 
still  greater  variety  may  be  obtained  by  mounting  three  films,  which 
separately  give  three  different  colors,  in  collars  revolving  in  a  frame  re- 
sembling that  in  which  hand-magnifiers  are  usually  mounted;  this  frame 
being  fitted  into  the  Sub-stage  in  such  a  manner,  that  either  a  single 
Selenite,  or  any  combination  of  two  Selenites,  or  ail  three  together,  may 
be  brought  into  the  optic  axis  above  the  polarizing  prism  (Fig.  82).  As 
many  as  thirteen  different  tints  may  thus  be  obtained. — When  the  con- 
struction of  the  Microscope  does  not  readily  admit  of  the  connection  of  the 
Selenite  plate  with  the  Polarizing  prism,  it  is  convenient  to  make  use  of 
a  plate  of  brass  (Fig.  83)  somewhat  larger  than  the  glass  slides  in  which 
objects  are  ordinarily  mounted,  with  a  ledge  near  one  edge  for  the  slide 
to  rest  against,  and  a  large  circular  aperture  into  which  a  glass  is  fitted, 
having  a  film  of  Selenite  cemented  to  it;  this  '  Selenite  stage'  or  object- 
carrier  being  laid  upon  the  Stage  of  the  Microscope,  the  slide  containing 
the  object  is  placed  upon  it;  and,  by  an  ingenious  modification  contrived 
by  Dr.  Leeson,  the  ring  into  which  the  Selenite  plate  is  fitted  being  made 
movable,  one  plate  may  be  substituted  for  another,  whilst  rotation  may 


ACCESSORY  APPARATUS.  113 

be  given  to  the  ring  by  means  of  a  tangent-screw  fitted  into  the  brass- 
plate. — The  variety  of  tints  given  by  a  Selenite-film  nnder  Polarized 
light,  is  so  greatly  increased  by  the  interposition  of  a  rotating  film  of 
Mica,  that  two  Selenites — red  and  Uue — with  a  Mica-film,  are  found  to 
give  the  entire  series  of  colors  obtainable  from  any  number  of  Selenite- 
films,  either  separately  or  in  combination  with  each  other.    The  Revolv- 


Darker's  Selenites,  as  fitted  by 
Messrs.  Beck. 

ing  Mica-Selenite  Stage  (Fig.  84)  devised  by  Mr.  Blankly,  and  made  by 
Mr.  Swift,  furnishes  a  very  simple  and  effective  means  of  obtaining  these 
beautiful  effects;  the  Mica  film  being  set  in  a  diaphragm  which  can  be 
made  to  rotate  by  applying  the  finger  at  the  front  edge  of  the  stage; 
whilst  the  two  Selenites  are  so  placed  in  a  slide,  that  either  of  them  can 
be  brought  under  the  aperture  as  desired. 

Fig.  84. 


Blankley's  Revolving  Mica-Selenite  Stage. 

112.  Swiff s  Combination  Sui-stage. — In  this  ingenious  piece  of  ap- 
paratus (Fig.  85)  are  combined  the  advantages  of  (1)  an  Achromatic 
Condenser,  a,  centred  by  two  milled-headed  screws,  c  c,  and  having  an 
angle  of  140",  which  fits  it  for  use  with  Objectives  of  very  wide  angular 
aperture,  whilst,  by  removing  the  upper  combination,  it  is  made  to  suit 
lower  powers;  (2)  a  contracting  Diaphragm  worked  by  the  lever  b;  (3)  a 
revolving  Diaphragm,  E,  with  four  apertures,  into  which  can  be  fitted  ; 
either  {a)  a  series  of  three  central  stops,  giving  a  Black-ground  illumina-' 
tion  scarcely  inferior  to  that  of  the  paraboloid,  and  capable  of  being  used 
with  the  small  angled  l-5th,  {h)  tinted  or  ground-glass  Moderators,  or 
{c)  two  Selenite-films  for  the  Polarizing  apparatus;  (4)  a  Polarizing  prism, 
F,  mounted  on  an  eccentric  arm,  so  as  to  be  brought  under  the  axis  of 
the  condenser  when  not  in  use,  and  thrown  out  when  not  wanted;  and 
8 


lU 


THE  MICROSCOPE  AND  ITS  REVELATIONS. 


(5)  an  upper  arm  carrying  two  revolving  cells  geared  together  by  fine 
teeth  (one  of  them  shown  at  d,  while  the  other  is  under  the  condenser), 
SO  that  a  revolving  motion  may  be  given  to  either  by  acting  on  the  other; 
one  of  these  cells  carries  a  plate  of  Mica,  the  revolution  of  which  over  the 

selenite-films  gives  a  great  variety 
of  color- tints  with  Polarized  light; 
while  the  other  serves  to  receive 
oblique-light  disks,  to  which  rota- 
tion can  be  given  by  the  same 
means. — The  special  advantage  of 
this  Condenser  Hps  in  its  having 
the  polarizing  prism,  the  selenite- 
and  mica-films,  the  black-ground 
and  oblique-light  stops,  and  the 
moderator,  all  brought  close  under 
the  back  lens  of  the  Achromatic; 
whilst  it  combines  in  itself  all  the 
most  important  applianctes  which 
the  Sub-stage  of  Secondary  body 
of  First-class  Microscopes  is  able  to 
afford.  It  may  be  specially  recom- 
mended to  such  as  make  much  use 
of  Polarized  light. 

113.  Illuminators  for  Opaque 
Objects, — All  objects  through 
which  sufficient  light  cannot  be 
transmitted  to  enable  them  to  be 
viewed  in  the  modes  already  de- 
scribed, require  to  be  illuminated 
by  rays,  which,  being  thrown  upon 
the  surface  under  examination, 
shall  be  reflected  from  it  into  the 
Microscope;  and  this  mode  of  view- 
ing them  may  often  be  advantage- 
ously adopted  in  regard  to  semi- 
transparent  or  even  transparent 
objects,  for  the  sake  of  the  diverse 
aspects  it  affords.  Among  the  va- 
rious methods  devised  for  this  purpose,  the  one  most  generally  adopted  con- 
sists in  the  use  of  a  Condensing  Lens  (Fig.  86),  either  attached  to  the 
Microscope,  or  mounted  upon  a  separate  stand,  by  which  the  rays  pro- 
ceeding from  a  lamp  or  from  a  bright  sky  are  made  to  converge  upon  the 
object. — For  the  efficient  illumination  of  large  opaque  objects,  however, 
it  is  desirable  to  employ  a  BulVs  eye  Condenser  (which  is  a  plano-convex 
lens  of  short  focus,  two  or  three  inches  in  diameter),  mounted  upon  a 
separate  stand,  in  such  a  manner  as  to  allow  of  being  placed  in  a  great 
variety  of  positions.  The  mounting  shown  in  Fig.  87,  is  one  of  the  best 
that  can  be  adopted:  the  frame  which  carries  the  lens  is  borne  at  the  bot- 
tom upon  a  swivel  joint,  which  allows  it  to  be  turned  in  any  azimuth; 
whilst  it  may  be  inclined  at  any  angle  to  the  horizon,  by  the  revolution  of 
the  horizontal  tube  to  which  it  is  attached,  around  the  other  horizgntal 
tube  which  projects  from  the  stem;  by  the  sliding  of  one  of  these  tubes 
within  the  other,  again,  the  horizontal  arm  may  be  lengthened  or  short- 
ened; the  lens  may  be  secured  in  any  position  (as  its  weight  is  apt  to  drag 


Swift's  Combination  Sub-stage. 


ACCESSORY  APPARATUS. 


115 


it  down  when  it  is  inclined,  unless  the  tubes  may  bo  made  to  work,  the 
one  into  the  other,  more  stiffly  than  is  convenient)  by  means  of  a  tio-ht- 
ening  collar  milled  at  its  edges;  and  finally  the  horizontal  arm  is  attached 
to  a  sprung  socket,  which  slides  up  and  down  upon  a  vertical  stem.  The 
optical  effect  of  such  a  *  bull's-eye'  differs  according  to  the  side  of  it 
turned  towards  the  light,  and  the  condition  of  the  rays  which  fall  upon  it. 
The  position  of  least  spherical  aberration  is  when  its  convex  side  is  turned 
towards  parallel  or  towards  the  least  diverging  rays:  consequently,  when 
used  by  Daylight,  its  plane  surface  should  be  turned  towards  the  ohject; 
and  the  same  position  should  be  given  to  it  when  it  is  used  for  procuring 
converging  rays  from  a  lamp,  this  being  placed  four  or  five  times  farther 

IFia.  87. 


Condensing  Lens.  Bull's-eye  Condenser. 


off  on  one  side  than  the  object  is  on  the  other.  But  it  may  also  be  em- 
ployed for  the  purpose  of  reducing  the  diverging  rays  of  the  Lamp  to  par- 
allelism, for  use  either  with  the  Paraboloid  (§  105)  or  with  the  Parabolic 
speculum  to  be  presently  described;  and  the  plane  side  is  then  to  be 
turned  towards  the  lamp,  which  must  be  placed  at  such  a  distance  from 
the  '  bull's-eye,'  that  the  rays  which  have  passed  through  the  latter  shall 
form  a  luminous  circle  equal  to  it  in  size,  at  whatever  distance  from  the 
lens  the  screen  may  be  held.  For  viewing  minute  objects,  under  high 
powers,  the  smaller  Condensing  lens  may  be  used  to  obtain  a  further  con- 
centration of  the  rays  already  brought  into  convergence  by  the  ^bull's- 
eye.' — An  ingenious  and  effective  mode  of  using  the  ^bull's-eye'  con- 
denser, for  the  illumination  of  very  minute  objects  under  high-power 


116 


THE  MICROSCOPE  AND  1T8  KEVELATIONS. 


Objectives,  has  been  devised  by  Mr.  James  Smith.  The  Microscope 
being  in  position  for  observation,  the  lamp  should  be  placed  either  in  the 
front  or  at  the  side  (as  most  convenient),  so  that  its  flame,  turned  edge- 
ways to  the  stao^e,  should  be  at  a  somewhat  lower  level,  and  at  a  distance 
of  about  three  inches.  The  bull's-eye  should  be  placed  between  the 
stage  and  the  lamp,  with  its  plane  surface  uppermost,  and  with  its  con- 
vex surface  a  little  above  the  stage.  The  light  entering  its  convex  sur- 
face near  the  margin  turned  towards  the  lamp,  falls  on  its  plane  surface 
at  an  angle  so  oblique  as  to  be  almost  totally  reflected  towards  the  oppo- 
site margin  of  the  convex  surface,  through  which  it  passes  to  the  object, 
a  little  above  the  plane  of  the  stage,  on  which  it  should  cast  a  sharp  and 
brilliant  wedge  of  light.  The  adjustment  is  best  made  by  first  placing  a 
slip  of  white  card  on  the  stage,  and  when  this  is  well  illuminated,  substi- 
tuting the  object-slide  for  it;  making  the  final  adjustment  while  the 
object  is  being  viewed  under  the  Microscope.  No  difficulty  is  experienced 
in  getting  good  results  with  powers  of  from  200  to  400  diameters;  but 
high  powers  require  careful  manipulation.  Mr.  Smith  states,  that  he  has 
succeeded  in  illuminating  by  this  simple  method,  minute  objects  (such  as 


Beck's  Parabolic  Speculum.  Crouch's  Adapter  for  Parobolic 

Speculum. 


Diatoms  and  scales  of  Lejnaopiera),  very  brilliantly  and  clearly,  upon  a 
dark  field,  under  an  immersion  l-16th  inch  Objective.  But  he  considers 
that  it  answers  better  for  objectives  of  moderate  than  of  very  wide  angu- 
lar aperture.  ^ 

114.  The  Illumination  of  Opaque  objects  may  be  effected  by  reflection 
as  well  as  by  refraction;  and  the  most  convenient  as  well  as  most  efficient 
instrument  yet  devised  for  this  purpose  is  the  Parabolic  Speculum  of 
Mr.  E.  Beck  (Fig.  88),  which  is  attached  to  a  spring-clip  that  fits  upon 
the  Objectives  {%  inch,  \\  inch,  1  inch,  2-3ds  inch)  to  which  it  is  espe- 
cially suited,  and  is  slid  up  or  down,  or  turned  round  its  axis,  when  the 
object  has  been  brought  into  focus,  until  the  most  suitable  illumination 
has  been  obtained.  The  ordinary  rays  of  diffused  Daylight,  which  may 
be  considered  as  falling  in  a  parallel  direction  on  the  Speculum  turned 
towards  the  window  to  receive  them,  are  reflected  upon  a  small  object  in 
its  focus,  so  as  to  illuminate  it  sufficiently  brightly  for  most  purposes; 


^  See    Journ.  Roy.  Micro.c.  Soc,"  Vol.  iii.  (1880),  p.  398. 


ACCESSORY  APPARATUS. 


117 


but  a  much  stronger  light  may  be  concentrated  on  it,  when  the  Speculum 
receives  its  rays  from  a  lamp  placed  near  the  opposite  side  of  the  stage,  a 
^bulFs-eye^  being  interposed  to  give  parallelism  to  the  rays.  For  the 
sake  of  Microscopists  who  may  desire  to  use  this  admirable  instrument 
with  Objectives  to  which  it  has  not  been  specially  fitted,  an  adapter  is 
made  by  Mr.  Crouch,  consisting  of  a  collar  (Fig.  89,  a)  interposed  between 
the  lower  end  of  the  body  of  the  Microscope  and  the  objective;  on  this  is 
fitted  the  ringB,  which  turns  easily  round  it,  and  carries  the  horizontal 
arm  c  c,  jointed  at  each  end;  whilst  the  stem  D,  which  can  be  lengthened 
or  shortened  at  pleasure,  hanging  from  this,  carries  at  its  lower  end  the 
Speculum  f  attached  to  it  by  the  ball-and-socket  joint  e.  By  this  arrange- 
ment the  Parabolic  Speculum  may  be  used  not  only  with  the  objectives 
already  named,  but  also  with  those  of  one-half  or  4-lOths  inch  focus,  if 
these  do  not  approach  the  object  so  nearly  as  to  interfere  with  the  reflec- 
tion of  the  illuminating  rays  from  the  Speculum. 

115.  LieberMilm. — A  mode  of  illuminating  opaque  objects  by  a  small 
concave  Speculum  reflecting  directly  down  upon  them  the  light  reflected 


Diagram  of  Lieberkuhn 

up  to  it  from  the  Mirror,  was  formerly  much  in  use,  but  is  now  compara- 
tively seldom  employed.  This  concave  Speculum,  termed  a  '  Lieber- 
kuhn' from  the  celebrated  Microscopist  who  invented  it,  is  made  to  fit 
upon  the  end  of  the  Objective,  having  a  perforation  in  its  centre  for  the 
])assage  of  the  rays  from  the  object  to  the  lens;  and  in  order  that  it  may 
receive  its  light  from  a  mirror  beneath  (Fig.  90,  a),  the  object  must  be 
so  mounted  as  only  to  stop-out  the  central  portion  of  the  rays  that  are 
reflected  upwards.  The  curvature  of  the  Speculum  is  so  adapted  to 
the  focus  of  the  Objective,  that,  when  the  latter  is  duly  adjusted,  the 
rsys  reflected  up  to  it  from  the  mirror  shall  be  made  to  converge  strongly 
upon  the  part  of  the  object  that  is  in  focus:  a  separate  speculum  is  conse- 
quently required  for  every  objective.  The  disadvantages  of  this  mode  of 
illumination  are  chiefly  these: — first,  that  by  sending  the  light  down  upon 
the  object  almost  perpendicularly,  there  is  scarcely  any  shadow,  so  that 
the  inequalities  of  its  surface  and  any  minute  markings  which  it  might 
present,  are  but  faintly  or  not  at  all  seen;  second,  that  the  size  of  the  ob- 


118 


THE  MICROSCOPE  AND  ITS  REVELATIONS. 


ject  must  be  limited  by  that  of  the  speculum,  so  as  to  allow  the  rays  to 
pass  to  its  marginal  portion;  and  third,  that  a  special  mode  of  mounting 
is  required,  to  allow  the  light  to  be  reflected  from  the  mirror  around  the 
margin  of  the  object.  The  first  objection  may  be  in  some  degree  removed 
by  turning  the  mirror  considerably  out  of  the  axis,  so  as  to  reflect  its 
light  obliquely  upon  the  Lieberkiihn,  which  will  then  send  it  down 
obliquely  upon  the  object  (Fig.  90,  b);  or  by  covering  one  side  of  the 
Lieberkiihn  by  a  diaphragm,  which  should  be  made  capable  of  rotation, 
so  that  light  may  be  reflected  from  the  uncovered  portion  in  every  azi- 
muth: the  illumination,  however,  will  in  neither  case  be  so  good  as  that 
which  is  afforded  with  powers  up  to  2-3ds  inch,  by  the  Parabolic  Spec- 
ulum just  described.  The  mounting  of  Opaque  objects  in  wooden  slides 
(Fig.  124),  which  affords  in  many  cases  the  most  convenient  means  of 
preserving  them,  completely  prevents  the  employment  of  the  Lieberkiihn 
in  the  examination  of  them;  and  they  must  be  set  for  this  purpose  either 
upon  disks  which  afford  them  no  protection,  or  in  cells  (§  169)  with  a 
blackened  background.  The  cases  wherein  the  Lieberkiihn  is  most  use- 
ful, are  those  in  which  it  is  desired  to  examine  small  opaque  objects, 
such  as  can  be  held  in  the  Stage-Forceps  (§  118)  or  mounted  on  small 
disks  (§  119),  or  laid  upon  a  slip  of  glass,  with  objectives  of  half-inch 
focus  or  less;  since  a  stronger  light  can  be  thus  concentrated  upon  them, 
than  can  be  easily  obtained  by  side-illumination.  In  every  such  case,  a 
bhick  background  must  be  provided,  of  such  a  size  as  to  fill  the  field,  so 
that  no  light  shall  come  to  the  eye  direct  from  the  mirror,  and  yet  not 
large  enough  to  create  any  unnecessary  obstruction  to  the  passage  of  the 
rays  from  the  mirror  to  the  speculum.  With  each  Lieberkiihn  is  com- 
monly j)rovided  a  blackened  stop  of  appropriate  size,  having  a  well-like 
cavity,  and  mounted  upon  a  pin  which  fits  into  a  support  connected 
with  the  under  side  of  the  stage;  but  though  this  ^dark  well^  serves  to 
throw  out  a  few  objects  with  peculiar  force,  yet,  for  all  ordinary  pur- 
poses, a  spot  of  black  paper  or  black  varnish  will  answer  the  required 
purpose  very  effectually,  this  spot  being  either  made  on  the  underside 
of  the  cell  which  contains  the  object,  or  upon  a  separate  slip  of  glass  laid 
upon  the  stage  beneath  this. 

116.  Vertical  Illuniination  for  High  Powers, — ^Various  attempts  have 
been  made  by  Mr.  Wenham  and  others  to  view  opaque  objects  under 
powers  too  high  for  the  advantageous  use  of  the  Lieberkiihn,  by  employ- 
ing the  Objective  itself  as  the  illuminator,  light  being  transmitted  into  it 
downwards  from  above.  By  Prof.  H.  L.  Smith,  of  Geneva  College,  U. 
S.,  a  pencil  of  light  admitted  from  a  lateral  aperture  above  the  objective, 
was  reflected  downwards  upon  the  object  through  its  lenses,  by*  means  of 
a  small  silver  speculum  placed  on  one  side  of  its  axis  and  cutting  off  a 
portion  of  its  aperture.  By  Messrs,  Powell  and  Lealand,  a  piece  of  plane 
glass  was  placed  at  an  angle  of  45°  across  a  tube  placed  like  an  adapter 
between  the  Objective  and  the  body  of  the  Microscope;  and  whilst  a  pencil 
of  light,  entering  at  the  side  aperture  and  striking  against  this  inclined 
surface,  is  reflected  by  it  downwards  through  the  objective  on  to  the 
object,  the  rays  proceeding  upwards  from  the  object  pass  upwards  (with 
some  loss  by  reflection)  through  the  plane  glass  into  the  body  of  the 
Microscope.  For  this  fixed  plate  of  glass,  Mr.  E.  Beck  substituted  a 
disk  of  thin  glass  attached  to  a  milled-head  (Fig.  91,  b),  by  the  rotation 
of  which  its  angle  may  be  exactly  adjusted;  and  this  is  introduced  by  a 
slot  (shown  at  Fig.  91,  a)  into  the  interior  of  an  adapter  that  is  inter- 
posed between  the  objective  (c,  d)  and  the  nose  {c)  of  the  Microscope. 


4 


ACCESSORY  APPARATUS.  119 

The  light  which  enters  at  the  lateral  aperture  (a,  a)  falling  upon  the 
oblique  surface  of  the  disk  (c,  I),  is  reflected  downwards,  and  is  concen- 
trated by  the  lenses  of  the  Objective  upon  the  object  beneath.  The 
lateral  aperture  may  be  provided  with  a  diaphragm,  having  a  series  of 
apertures,  for  diminishing  the  false  light  to  which  this  method  is  liable; 
or  a  screen  with  a  small  aperture  may  be  placed  at  any  distance  between 
the  lamp  and  the  Illuminator,  that  is  found  to  produce  the  best  effects. 
In  using  this  illuminator,  the  lamp  should  be  placed  at  a  distance  of 
about  8  inches  from  the  aperture;  and  when  the  proper  adjustments 
have  been  made,  the  image  of  the  flame  should  be  seen  upon  the  object. 
The  illumination  of  the  entire  field,  or  the  direction  of  the  light  more  or 
less  to  either  side  of  it,  can  easily  be  managed  by  the  interposition  of  a 
small  Condensing  lens  placed  at  about  the  distance  of  its  own  focus  from 
the  lamp.  The  Objects  viewed  by  this  mode  of  illumination  with  dry- 
front  objectives,  are  best  uncovered;  since,  if  they  are  covered  with  thin 
glass,  so  large  a  proportion  of  the  light  sent  down  upon  them  is  reflected 
from  the  cover  (especially  when  Objectives  of  large  angle  of  aperture 


A  c 


Beck's  Vertical  Illuminator. 


are  employed)  that  very  little  is  seen  of  the  objects  beneath,  unless  their 
reflective  power  is  very  high.  With  immersion  objectives,  however, 
covered  objects  may  be  used;  and  the  author  has  seen  a  more  perfect 
resolution  of  difficult  tests  by  this  mode  of  viewing  them  (first  suggested 
by  Mr.  Morehouse,  of  Wayland,  New  York)  than  by  any  other.' — 
Another  method  of  Vertical  Illumination  long  since  devised  by  Mr. 
Tolles  has  recently  been  brought  into  notice  by  Prof.  W.  A.  Eogers,  of 
Boston,  U.  S.  It  consists  in  the  introduction  of  a  small  rectangular 
prism,  resembling  that  of  Nachet's  Binocular  (a.  Fig.  27),  at  a  short  dis- 
tance behind  the  front  combination  of  the  Objective;  so  that  parallel  rays 
entering  its  vertical  end-surface,  pass  on  between  the  parallel  horizontal 
surfaces,  until  they  meet  the  inclined  surface  by  which  they  are  reflected 
downwards.    In  passing  through  the  front  combination  of  the  objective. 


Journ.  of  Roy.  Microsc.  Soc,"  Vol.  ii.  (1879),  pp.  194,  266. 


120 


THE  MICROSCOPE  AND  ITS  REVELATIONS. 


they  are  deflected  towards  its  axis;  but  as  their  angle  of  convergence  is 
less  than  the  angle  of  divergence  of  the  rays  proceeding  from  the  object, 
the  reflected  rays  will  not  meet  in  the  focal  point  of  the  lens,  but  will  be 
so  distributed  as  to  illuminate  a  sufficient  area.  By  altering  the  extent 
to  which  the  prism  is  pushed  in,  or  by  lifting  or  depressing  its  outer  end 
by  means  of  a  milled-head  screw,  the  field  of  illumina- 
j^  c)2^  tion  can  be  regulated.  The  working  of  this  prism  with 
immersion  objectives  is  stated  by  Mr.  ToUes  to  be  pecu- 
liarly satisfactory.^ 

117.  Stephenson's  Safety  Stage. — In  examining  ob- 
jects with  those  higher  powers  which  focus  extremely 
close  to  the  covering  glass,  the  slightest  inadvertence 
is  likely  to  lead  to  a  fracture  of  the  glass,  and  perhaps 
to  the  destruction  of  a  valuable  slide.  This  is  a  serious 
matter  with  Moller's  Diatom  Type  Slide,  or  Nobert's 
Test  Lines,  or  with  many  others  that  are  expensive  or 
perhaps  impossible  to  replace.  To  remove  this  source 
of  danger,  Mr.  Stephenson  contrived  the  safety  stage," 
shown  in  Fig.  92.  The  frame  on  which  the  slide  car- 
Safety-stage.  I'yii^g  the  object  rcsts,  is  hinged  at  its  upper  part,  and 
kept  in  its  true  position  by  slight  springs,  which  give 
way  directly  the  slide  is  pressed  by  the  objective.  It  is  found  that  springs 
firm  enough  to  insure  the  steadiness  required  for  high  powers,  may  yet 
be  sufficiently  flexible  to  give  way  before  very  thin  glass  is  endangered, 
and  a  glance  at  the  stage  shows  if  it  is  made  to  deviate  from  the  nor- 
mal position  in  which  its  upper  and  lower  edges  are  parallel,— (See  also 
§  54.) 

Section  2.  Apparatus  for  the  Presentation  of  Objects, 

118.  Stage- Forceps  and  Vice, — For  bringing  under  the  Object-glass 
in  different  positions  such  small  opaque  objects  as  can  be  conveniently 
held  in  a  pair  of  forceps,  the  Stage- Forceps  (Fig.  93)  supplied  with  most 
^^cj^^  Microscopes    afford  a  ready 

means.  These  are  mounted  by 
means  of  a  joint  upon  a  pin, 
which  fits  into  a  hole  either  in 
the  corner  of  the  Stage  itself  or 
in  the  object-platform;  the  ob- 
stage-Forceps.  j^^^      inserted  by  pressing  the 

pin  that  projects  from  one  of  the  blades,  whereby  it  is  separated  from  the 
other;  and  the  blades  close  again  by  their  own  elasticity,  so  as  to  retain 
the  object  when  the  pressure  is  withdrawn.  By  sliding  the  wire  stem 
which  bears  the  Forceps  through  its  socket,  and  by  moving  that  socket 
vertically  upon  its  joint,  and  the  joint  horizontally  upon  the  pin,  the 
object  may  be  brought  into  the  field  precisely  in  the  position  required; 
and  it  may  be  turned  round  and  round,  so  that  all  sides  of  it  may  be 
examined,  by  simply  giving  a  twisting  movement  to  the  wire  stem.  The 
other  extremity  of  the  stem  often  bears  a  small  brass  box  filled  with  cork, 
and  perforated  with  holes  in  its  side;  this  affords  a  secure  hold  to 
commmon  pins,  to  the  heads  of  which  small  objects  can  be  attached  by 
gum,  or  to  which  disks  of  card,  etc.,  may  be  attached,  whereon  objects 
are  mounted  for  being  viewed  with  the  Lieberkiihn  (§  115).  This 


'  •*  Journ.  of  Roy.  Microsc.  Soc,"  Vol.  iii.,  pp.  526,  754. 


ACCESSORY  APPARATUS. 


121 


method  of  mounting  was  formerly  much  in  vogue,  but  has  been  less 
employed  of  late,  since  the  Lieberkiihn  has  fallen  into  comparative  dis- 
use.— The  Stage  Vice,  as  made  by  Mr.  Ross  for  Mr.  Slack,  was  contrived 
for  the  purpose  of  holding  small  hard  bodies,  such  as  Minerals,  apt  to  be 
jerked  out  by  the  angular  motion  of  the  blades  of  the  forceps,  or  very 
delicate  substances  that  will  not  bear  rough  compression.  In  this  appa- 
ratus the  blades  meet  horizontally,  and  their  movements  can  be  regulated 
to  a  nicety  with  a  fine  screw.  The  Stage  Vice  fits  into  a  plate,  as  is  the 
case  with  Beck's  disk-holder.  Fig.  94. 

119.  For  the  examination  of  objects  which  cannot  be  conveniently 
held  in  the  stage-forceps,  but  which  can  be  temporarily  or  permanently 
attached  to  disks,  no  means  is  comparable  to  the  Disk-holder  of  Mr.  E. 
Beck  (Fig.  94)  in  regard  to  the  facility  it  affords  for  presenting  them  in 
every  variety  of  position.  The  object  being  attached  by  gum  (having  a 
small  quantity  of  glycerine  mixed  with  it)  or  by  gold-size,  to  the  surface 
of  a  small  blackened  metallic  Disk,  this  is  fitted  by  a  short  stem  project- 
ing from  its  under  surface  into  a  cylindrical  holder;  and  the  holder  carry- 
ing the  disk  can  be  made  to  rotate  around  a  vertical  axis  by  turning  the 
milled-head  on  the  right,  which  acts  on  it  by  means  of  a  small  chain  that 
works  through  the  horizontal  tubular  stem;  whilst  it  can  be  made  to  in- 
cline to  one  side  or  to  the  other,  until  its  plane  becomes  vertical,  by  turn- 
ing the  whole  movement  on  the 

horizontal  axis  of  its  cylindrical  Tlik^i» 
socket.^  The  supporting  plate  be- 
ing perforated  by  a  large  aperture, 
the  object  may  be  illuminated  by 
the  Lieberkiihn  if  desired.  The 
disks  are  inserted  into  the  holder, 
or  are  removed  from  it,  by  a  pair 
of  Forceps  constructed  for  the 
purpose;  and  they  may  be  safely  Beck's  Disk-holder, 

put  away,  by  inserting  their  stems  ^i,  •  4. 

into  a  plate  perforated  with  holes.  Several  such  plates,  with  inter- 
vening  guards  to  prevent  them  from  coming  into  too  close  apposition, 
may  be  packed  into  a  small  box.  To  the  value  of  this  little  piece  of 
apparatus  the  Author  can  bear  the  strongest  testimony  from  his  own 
experience,  having  found  his  study  of  the  Foraminifera  greatly  tacili- 
tated  by  it.— A  less  costly  substitute,  however,  which  answers  suthciontly 
well  for  general  purposes,  is  found  m  the  Object-holder  of  Mr.  Morris 
(Fig.  95),  which  consists  of  a  supporting  plate  that  carries  a  ball-and- 
socket  joint  in  its  centre,  into  the  ball  of  which  can  be  fitted  by  a  taper- 
ing stem  either  a  holder  for  small  cardboard  disks,  or  a  larger  holder 
suitable  for  carrying  an  ordinary  slide.  By  the  free  play  ot  the  ball- 
and-socket  joint  in  different  directions,  the  object  may  either  be  made  to 
rotate,  or  may  be  so  tilted  as  to  be  viewed  obliquely  or  almost  laterally. 
This  instrument  can,  of  course,  be  used  only  by  side  illumination;  and 
in  order  to  turn  it  to  the  best  account,  the  objects  to  be  viewed  by  it  must 
be  mounted  on  special  disks;  but  it  has  an  advantage  over  the  preceding, 
in  being  applicable  also  to  objects  mounted  m  ordinary  slides.— 1  he 
same  purpose  is  answered,  in  the  Ross  Zentmayer  Microscopes  (^^  &y, 
72),  and  in  the  Improved  Beck  Microscope  (§  65),  by  turning  the  stage 
round  its  horizontal  axis,  so  that  an  object  mounted  on  a  slide  may  be 

1  A  small  pair  of  Forceps  adapted  to  take  up  minute  objects  may  be  fitted  into 
the  cylindrical  holder,  in  place  of  a  disk. 


122 


THE  MICROSCOPE  AND  ITS  REVELATIONS. 


viewed  at  any  desired  angle  or  inclination,  when  it  has  been  brought  into 
the  most  suitable  azimuth  by  the  rotating  of  the  stage  round  its  vertical 
axis. 

120.  Glass  Stage-plate, — Every  microscope  should  be  furnished  with  a 
piece  of  Plate-glass,  about  4  in.  by  1-^  in.,  to  one  margin  of  which  a  nar- 
row strip  of  glass  is  cemented,  so  as  to  form  a  ledge.  This  is  extremely 
useful,  both  for  laying  objects  upon  (the  ledge  preventing  them — to- 
gether with  their  covers,  if  used — from  sliding  down  when  the  Miscro- 
scope  is  inclined),  and  for  preserving  the  stage  from  injury  by  the  spill- 


Fig:  95. 


Morris's  Object-holder. 


ing  of  sea-water  or  otner  saline  or  corrosive  liquids,  when  such  are  in 
use.  Such  a  plate  not  only  serves  for  the  examination  of  transparent, 
but  also  of  opaque  objects;  for  if  the  Condensing-lens  be  so  adjusted  as 
to  throw  a  side-light  upon  an  object  laid  upon  it,  either  the  Diaphragm- 
plate  or  a  slip  of  black-paper  will  afford  a  dark  back-ground;  whilst  ob- 
jects mounted  on  the  small  black  disks  suitable  to  the  Lieberkuhn  may 
conveniently  rest  on  it,  instead  of  being  held  in  the  Stage-forceps. 

121.  Growing  Slide, — A  number  of  contrivances  have  been  devised  of 
late  years,  for  the  purpose  of  watching  the  life-histories  of  minute  aquatic 
organisms,  and  of  '  cultivating '  such  as  develop  and  multiply  themselves 
in  particular  fluids.  One  of  the  simplest  and  most  effective,  that  of  Mr. 
Botterill,  represented  in  Fig.  96, — consists  of  a  slip  of  ebonite,  three 
inches  by  one,  with  a  central  aperture  of  3-4ths  of  an  inch  at  its  under 
side;  this  aperture  is  reduced  by  a  projecting  shoulder,  whereon  is  ce- 


Botterill's  Growing-Slide. 

mented  a  disk  of  thin  glass,  which  thus  forms  the  bottom  of  a  cell  hol- 
lowed in  the  thickness  of  the  ebonite  slide.  On  each  side  of  this  central 
cell,  a  small  lateral  cell  communicating  with  it  and  about  l-4th  inch  in 
diameter,  is  drilled-out  to  the  same  depth;  this  serves  for  the  reception 
of  a  supply  of  water  or  other  fluid,  which  is  imparted,  as  required,  to  the 


ACCESSORY  APPARATUS. 


123 


IFiG 


central  ^ growing^  cell,  which  is  completed  by  placing  a  thin-glass  cover 
over  the  objects  introduced  into  it,  with  the  interposition  of  a  ring  of 
thin  paper,  or  (if  a  greater  thickness  be  required)  of  a  ring  of  cardboard 
or  vulcanite.  If  the  fluid  be  introduced 
into  one  of  the  lateral  cells,  and  be  drawn- 
off  from  the  others — either  by  the  use, 
from  time  to  time,  of  the  small  glass 
syringe  to  be  hereafter  described  (§  127), 
or  by  threads  so  arranged  as  to  produce 
a  continuous  drip  into  one  and  fro7n  the 
other — a  constantly  renewed  supply  is 
furnished  to  the  central  cell,  which  it 
enters  on  one  side,  and  leaves  on  the 
other,  by  capillary  attraction.  ^ — Dr.  Mad-  Maddox's  Growing-siide. 

dox^s  Groioing- Slide  will  be  understood  from  the  annexed  sketch.  The 
shaded  parts  are  pieces  of  tinfoil  fastened  with  shellac  glue  to  a  glass 
slide.  The  minute  fungi  or  spores  to  be  grown  are  placed  on  a  glass 
cover  large  enough  to  cover  the  tinfoil,  with  a  droplet  of  the  fluid  re- 
quired. This,  after  examination  to  see  that  no  extraneous  matter  is 
introduced,  is  placed  over  the  tinfoil,  and  the  edges  fastened  with  wax 
softened  with  oil,  leaving  free  the  spaces  x  x  for  entrance  of  air.  Grow- 
ing-slides of  this  description  could  be  made  cheaply  with  thin  glass 
instead  of  tinfoil.^ — For  an  account  of  a  more  elaborate  apparatus  devised 
by  Messrs.  Dallinger  and  Drysdale  for  the  prosecution  of  their  admirable 
researches  hereafter  to  be  noticed  (Chap,  xi.),  the  reader  is  referred  to 
^he  description  and  figures  given  by  them  in  the  Monthly  Microscopi- 
cal  Journal,"  Vol.  xi.,  1874,  p.  97. 

122.  Aquatic  Box, — The  Live-Box  or  Animalcule-cage  (Fig.  98,  a) 
consists  of  a  short  piece  of  wide  brass  tube,  fixed  perpendicular  into  an 
aperture  of  its  own  diameter  in  a  flat-plate  of  brass,  and  closed-in  at  its 
top  by  the  object- tablet,  a  disk  of  glass  with  bevelled  edges  (b);  ovet 
this  box  there  slides  a  cover,  consisting  of  another  piece  of  brass  tube  hav- 
ing a  disk  of  thin  glass  fixed  into  its  top.  The  cover  being  taken  of,  a 
drop  of  the  liquid  to  be  examined,  or  any  thin  object  which  can  be 
most  advantageously  looked-at  in  fluid,  is  placed  upon  the  lower  plate; 
the  cover  is  then  slipped  over  it,  and  is  pressed  down  until  the  drop  of 
liquid  be  spread  out,  or  the  object  be  flattened,  to  the  degree  most 
convenient  for  observation.  If  the  glass  disk  which  forms  the  lid  be  ce- 
mented or  burnished  into  the  brass  ring  which  carries  it,  a  small  hole 
should  be  left  for  the  escape  of  air  or  superfluous  fluid;  and  this  may  be 


^  For  descriptions  of  other  forms  of  Growing-Slide,  see  Transact,  of  Microsc. 
Society/'  Vol.  xiv.,  N.S.,  p.  34,  and  *' Quart.  Journal  of  Microsc.  Science,"  N.S., 
Vol,  vii  ,  p.  11. 

^  See  his  paper  on  Cultivation  of  Microscopic  Fungi,  in  Monthly  Microscopi- 
cal Journ.,"  Vol.  iii.  (1870),  p.  14. — Dr.  Maddox  recommends  the  following  fluid  as 
sufficiently  hygrometric  to  keep  the  spores  moist,  and  as  adapted  to  Fungoid 


growths: — 

Dextrine   2  grains. 

Phosphate  of  Soda  and  Ammonia   2 

Saturated  Solution  of  Acetate  Potash   12  drops. 

Grape  Sugar   16  grains. 

Freshly  distilled  water   .  1  oz. 


The  water  is  to  be  boiled  in  a  large  test-tube  or  beaker  for  15  minutes,  and 
covered  whilst  boiling  and  cooling;  when  settled,  it  should  be  poured  into  per- 
fectly clean  2-drachm  stoppered  bottles,  and  kept  for  use. 


124 


THE  MICROSCOPE  AND  ITS  REVELATIONS. 


closed  np  with  a  morsel  of  wax,  if  it  be  desired  to  prevent  the  included 
fliiid  from  evaporating.  But  as  it  is  desirable  that  the  cover-glass  should 
]  0  thin  enough  to  allow  a  l-4th  or  a  l-6th  inch  Objective  to  be  em- 
[)loyed,  and  as  such  thin  glass  is  extremely  apt  to  be  broken,  it  is  a  much 
better  plan  to  furnish  the  brass  cover  with  a  screw-cap,  which  holds  the  glass 
disk  with  sufficient  firmness,  but  permits  it  to  be  readily  replaced.    It  is 

always  desirable,  if  possible,  to  pre- 
vent the  liquid  from  spreading  to  the 
edge  of  the  disk,  since  any  objects  it 
may  contain  are  very  apt  in  such  a  case 
to  be  lost  under  the  opaque  ring  of  the 
cover:  this  is  to  be  avoided  by  limit- 
ing the  quantity  of  liquid  introduced, 
by  laying  it  upon  the  centre  of  the 
lower  plate,  and  by  pressing  down 
the  cover  with  great  caution,  so  as  to 
flatten  the  drop  equally  on  all  side  , 
stopping  short  when  it  is  spreading 
too  close  to  the  margin.  If  the  Live- 
box  be  well  constructed,  and  the  glass 
disks  be  quite  flat,  they  will  come  in- 
to such  close  contact,  that  objects  of 
extreme  thinness  may  be  compressed 
between  them;  and  it  may  thus  be 

Aquatic  Box  or  Animalcule-Cage,  as  seen  in     made,  with  a  little  practice,  to  SCrVC 

perspective  at  A,  and  in  secDion  at  B  and  c.  the  purposc  of  a  Compressor  (§  125). 
In  its  ordinary  form,  however,  the  elevation  of  the  object-tablet  above 
the  stage  prevents  the  Live-box  from  being  used  with  the  Achromatic 
Condenser  or  Paraboloid:  but  another  form  is  made  by  Mr.  Swift,  in 
which  the  object-tablet  is  fixed  at  the  bottom  of  the  tube,  flush  with  the 
surface  of  the  plate  (as  shown  at  c);  and  as  the  covering  disk  is  fixed  to 
the  'bottom  of  the  cover-tube,  and  thus  slides  inside  the  box-tube,  the  ob- 
ject can  be  illuminated  by  any  of  the  means  applicable  to  objects  con- 
tained in  ordinary  flat  cells  (§  123).  The  only  disadvantage  of  this  con- 
struction is  that  the  cover-disk  must     fixed  in  the  tube  which  carries  it. 

123.  Infusoria,  minute  Algse,  etc.,  however,  can  be  well  seen  by  plac- 
ing a  drop  of  the  water  containing  them,  on  an  ordinary  slide,  and  laying  a 
thin  piece  of  covering-glass  on  the  top.  And  objects  of  somewhat  greater 
thickness  can  be  examined  by  placing  a  loop  or  ring  of  fine  cotton-thread 
upon  an  ordinary  slide,  to  keep  the  covering-glass  a  small  distance  from 
it;  and  the  object  to  be  examined  being  placed  on  the  slide  with  a  drop 
of  water,  the  covering-glass  is  gently  pressed  down  till  it  touches  the 
ring.  Still  thicker  objects  may  be  viewed  in  the  various  forms  of  '  cells  ' 
hereafter  to  be  described  (§§  171-3);  and  as,  when  the  cells  are  filled 
with  fluid,  their  glass  covers  will  adhere  by  capillary  attraction,  provided 
the  superfluous  moisture  that  surrounds  their  edges  be  removed  by  blot- 
ting-paper, they  will  remain  in  place  when  the  Microscope  is  inclined. — 
An  Annular  Cell,  that  may  be  used  either  as  a  ^live-box'  or  as  a  ^grow- 
ing-slide,' has  lately  been  devised  by  Mr.  Weber  (U.  S.).  It  is  a  slip  of 
plate-glass  of  the  usual  size  and  ordinary  thickness,  out  of  which  a  cir- 
cular ^cell'  of  3-4ths  inch  diameter  is  ground,  in  such  a  manner  that 
its  bottom  is  convex  instead  of  concave,  its  shallowest  part  being  in  the 
centre,  and  the  deepest  round  the  margin.  A  small  drop  of  the  fluid 
to  be  examined  being  placed  upon  the  central  convexity  (the  highest 


ACCESSORY  APPARATUS. 


125 


part  of  which  should  be  almost  flush  with  the  general  surface  of  the 
plate),  and  the  thin  glass-cover  being  placed  upon  is,  the  drop  spreads 
itself  out  in  a  thin  film,  without  finding  its  way  into  the  deep  furrow 
around  it;  and  thus  itholds-on  the  covering-glass  by  capillary  attraction, 
while  the  furrow  serves  as  an  air  chamber.  If  the  cover  be  cemented, 
down  by  a  ring  of  gold-size  or  dammar,  so  that  the  evaporation  of  the 
fluid  IS  prevented,  either  Animal  or  Vegetable  life  may  thus  be  main- 
tained for  some  days,  or,  if  the  two  should  be  balance  d  (as  in  an  Aqua- 
rium), for  some  weeks. ^ — An  improvement  has  been  devised  by  Dr.  Ed- 
monds in  the  form  of  this  Annular  Cell;  which  he  also  makes  to  serve 
as  a  ^gas-chamber'  for  the  introduction  of  gases  or  vapors  into  the  Annu- 
lar space.  The  central  prominence  is  shaped  as  a  truncated  paraboloid; 
and  while,  by  focussing  in  the  object  a  2-inch  objective  used  as  a  conden- 
ser, a  bright  field  is  obtained,  this  may  be  exchanged  for  a  dark  field  by 
])utting  the  condenser  out  of  focus  (so  that  its  light  is  thrown  on  the 
sides  of  the  paraboloid),  and  by  gumming  a  black  disk  on  the  centre  of 
its  under  surface.  A  straight  groove  being  cut  in  the  slide,  parallel  to 
its  long  side,  and  tangentially  to  the  annular  groove  which  it  should 
equal  in  depth,  two  fine  glass  tubes  are  cemented  in  it;  one  of  them, 
which  is  left  projecting  beyond  the  end  of  the  slide,  being  connected 
with  a  slender  elastic  tube  through  which  gases  or  vapors  may  be  pro- 
jected into  the  annular  space,  while  the  other  serves  to  convey  them 
away.^ 

124.  Zoopliyte  Trougli. — For  the  examination  of  larger  aquatic  Ani- 
mals or  Plants  under  low  or  moderate  powers,  recourse  may  be  advanta- 
geously had  either  to  the  original  Zoophyte-trough  of  Mr.  Lister  (which 
is  still  kept  on  sale  by  most  Makers),  or  to  a  form  lately  devised  by  Mr. 
Botterill,    which  has  several 

advantages  over  the  older  one.  Fig.  99.. 

This  consists  of  two  plates  of 
vulcanite,  a  back  and  a  front, 
shaped  as  in  Fig.  99,  connected 
together  by  three  brass  screws; 
these,  being  fixed  in  the  back 
plates,  pass  through  the  front, 
where  their  projecting  ends  are 
furnished  with  small  milled- 

heads.      Between  these  plates  Botteriirs  Zoophyte  Trough. 

are  two  rectangular  plates  of 

glass,  cut  to  such  a  length  as  to  lie  between  the  two  side-screws  of  the 
vulcanite  plates,  and  having  such  a  breadth  that  while  their  lower 
edges  rest  on  the  bottom-screw,  their  upper  are  flush  with  the  top  of  the 
vulcanite  disks.  The  glass  plates  are  kept  apart  by  a  half-ring  of  vul- 
canized india-rubber,  of  such  a  diameter  as  to  lie  just  outside  of  the  semi- 
circular margin  of  the  vulcanite  plates;  and  they  thus  form  the  sides  and 
bottom  of  a  trough,  which  is  made  water-tight  by  a  moderate  pressure 
exerted  by  turning  the  milled-heads.  The  space  between  the  two  glass 
plates  may  be  varied  by  using  half -rings  of  different  thicknesses;  whilst, 
if  it  be  desired  to  use  a  higher  power  than  will  work  through  ordinary 
glass,  a  front  plate  of  tliin  glass  may  be  substituted.— One  great  advan- 
tage of  this  arrangement  is  the  facility  with  which  the  pieces  composing 

^    Journ.  Roy.  Microsc.  Society,"  Vol.  ii.  (1879),  p.  55. 

2  Ihid,,  Vol.  iii.  (1880),  p.  585.— This  Parabolized  Oas-Slide  is  made  by  Messrs, 
Beck. 


126 


THE  MICROSCOPE  AND  ITS  REVELATIONS. 


it  may  be  taken  apart,  either  for  cleaning  or  for  the  repair  of  a  fracture 
— an  accident  to  which  the  use  of  thin  glass  of  course  renders  it  specially 
liable. 

125.  Compressor. — The  purpose  of  this  "nstrument  is  to  apply  a 
gradual  pressure  to  objects  whose  structure  ..an  only  be  made  out  when 
they  are  thinned  by  extension,  while  their  organization  is  so  delicate  as 
to  be  confused  or  altogeiner  destroyed  by  the  slightest  excess  of  pressure. 
For  the  examination  of  such,  an  instrument  in  which  the  degree  of 
compression  can  be  regulated  with  precision  is  almost  indispensable.  The 
Compressorium  represented  in  i'ig.  100  was  originally  devised  by  Schick 
of  Berlin;  whilst  its  details  were  modified  by  M.  de  Quatrefages,  who 

constantly  employed  it  in 
his  elaborate  and  most  suc- 
cessful researches  on  the  or- 
ganization of  the  Marine 
Worms.  Being,  however, 
deficient  in  any  provisions 
for  securing  the  parallelism 
of  the  approximated  sur- 
faces, it  has  been  superseded 
that  view. — In  Bosses  Improved 
upper  plate  d  is  attached  to 


Schick's  Compressor. 


,oy  other  forms  devised  expressly  with 
Compressor^  shown  in  Fig.  101,  the 

p  slide  that  works  between  grooves  in  the  vertical  piece  c,  so  that, 
when  raised  or  lowered  by  the  milled-head,  it  always  maintains  its 


parallelism  to  the  lower  plate  A. 


Pig.  101. 


The  thin  glass  carried  by  the  upper 
plate  D  (which  can  be  turned  aside  on 
a  swivel  joint,  as  shown  in  the  lower 
figure)  is  a  square  that  slides  into 
grooves  on  its  under  side,  so  as  to  be 
easily  replaced  if  broken.  The  glass 
to  which  it  is  opposed  is  a  circular 
disk  lodged  in  a  shallow  socket  in  plate 
B,  which  is  received  into  a  part  of  the 
lower  plate  a  that  is  sunk  below  the 
rest.  The  plate  b  carrying  the  lower 
glass  can  be  drawn  out  (as  shown  in 
the  lower  figure)  and  laid  upon  the 
Dissecting  Microscope,  to  be  replaced 
in  the  Compressorium  after  the  object 
has  been  prepared  for  compression. 
The  only  drawback  to  the  use  of  this 
instrument  lies  in  the  inconvenience  of 
using  it  in  the  reversed  position  so  as 
to  look  at  the  object  from  its  under 
side. — This  reversion  is  provided  for  in 
the  two  forms  of  the  instrument  made 
by  Messrs.  Beck,  which  are  shown  in 
Figs.  1C2,  104.  In  both,  the  upper 
and  the  lower  glasses  are  fixed,  upon  a  plan  devised  by  Mr.  Slack,  by 
means  of  flat-headed  screws,  two  to  each  glass  (Fig.  103,  a),  the  heads 
fitting  into  holes  of  the  opposite  frame,  so  as  to  permit  the  close  approxi- 
mation of  the  two  glass  surfaces.  In  their  Parallel  Plate  Compressor 
(Fig.  102)  the  constant  parallelism  of  the  two  plates  is  secured  by  the  two 
parallel  bars,  a,  a;  while  the  degree  of  their  approximation  and  pressure 


Ross's  Improved  Compressor, 


ACCESSORY  APPARATUS. 


127 


is  regulated  by  the  screw  h,  which  works  out  of  centre  in  a  conical  hole 
of  the  lower  frame,  so  that,  the  further  it  is  introduced,  the  more  closely 
the  two  frames,  with  their  glasses,  are  approximated.  This  pattern  works 
equally  well  whichever  side  is  uppermost.  In  the  Reversible  Cell  Coin- 
pressor  of  the  same  makers  (Figs.  103  B,  J  04)  the  upper  glass  is  held  down 
by  a  ring  a,  which  screws-on  to  that  which  bears  the  lower  one,  giving 
any  degree  of  pressure  that  may  be  required.  When  screwed  together, 
they  form  a  cell  that  fits  into  the  plate  Z>,  and  is  attached  to  it  by  the 
milled-head  c;  by  unscrewing  which  the  cell  can  be  instantly  detached 
and  replaced  in  a  reverse  position. — In  all  these  Compressors,  it  is  easy 
to  vary  the  thickness  of  the  glass  within  convenient  limits;  and  the  ob- 


FiG.  105. 

Fig.  102.  ADC 


Dipping  Tubes.  Glass  Syringe. 


server  should  be  always  provided  with  a  stock  of  glass  slips  and  disks  of 
the  requisite  sizes  and  of  different  thicknesses,  suitable  to  the  kind  of 
investigation  he  may  be  prosecuting.  As  thin  glasses,  when  used  for 
compression,  are  very  liable  to  fracture,  the  power  of  immediately  re- 
placing them  without  the  employment  of  cement  (as  in  Mr.  Slack's  con- 
struction) is  a  great  convenience. 

126.  Dipping  Tubes. — In  every  operation  in  which  small  quantities 
of  liquid,  or  small  objects  contained  in  liquid,  have  to  be  dealt  with  by 
the  Microscopist,  he  will  find  it  a  very  great  convenience  to  be  provided 
with  a  set  of  Tubes  of  the  forms  represented  in  Fig.  105,  but  of  some- 


128 


THE  MICROSCOPE  AND  ITS  REVELATIONS. 


what  larger  dimensions.  These  were  formerly  designated  as  ^fishing 
tubes;' the  purpose  for  which  they  were  originally  devised  having  been 
the  fishing-out  of  Water-fleas,  aquatic  Insect-larvae,  the  larger  Animal- 
cules, or  other  living  objects  distinguishable  either  by  the  unaided  eye  or 
by  the  assistance  of  a  magnifying-glass,  from  the  vessels  that  may  contain 
them.  But  they  are  equally  applicable,  of  course,  to  the  selection  of 
minute  Plants;  and  they  may  be  turned  to  many  other  no  less  useful  pur-v 
poses,  some  of  which  will  be  specified  hereafter. — When  it  is  desired  to 
secure  an  object  which  can  be  seen  either  with  the  eye  alone  or  with  a 
magnifying-glass,  one  of  these  tubes  is  passed  down  into  the  liquid,  its 
upper  orifice  having  been  previously  closed  by  the  forefinger,  until  its 
lower  orifice  is  immediately  above  the  object;  the  finger  being  then  re- 
moved, the  liquid  suddenly  rises  into  the  tube,  probably  carrying  the 
object  up  with  it;  and  if  this  is  seen  to  be  the  case,  by  putting  the  finger 
again  on  the  top  of  the  tube,  its  contents  remain  in  it  when  the  tube  is 
lifted  out,  and  may  be  deposited  on  a  slip  of  glass,  or  on  the  lower  disk 
of  the  Aquatic-box,  or,  if  too  copious  for  either  receptacle,  may  be  dis- 
charged into  a  large  glass  cell  (Fig.  120).  In  thus  fishing  in  jars  for  any 
but  minute  objects,  it  will  be  generally  found  convenient  to  employ  the 
open-mouthed  tube  c;  those  with  smaller  orifices,  B,  c,  being  employed 
for  ^fishing'  for  Animalcules,  etc.,  in  small  bottles  or  tubes,  or  for 
selecting  minute  objects  from  the  cell  into  which  the  water  taken  up  by 
the  tube  a  has  been  discharged.  It  will  be  found  very  convenient  to 
have  the  tops  of  these  last  blown  into  small  funnels,  which  shall  be  cov- 
ered with  thin  sheet  India-rubber;  for  their  action  (like  that  of  the  stop- 
per of  the  Dropping-bottle,  Fig.  138)  can  then  be  regulated  with  the 
greatest  nicety  by  the  pressure  of  the  finger. 

127.  Glass  Sifringe. — In  dealing  with  minute  Aquatic  objects,  and  in 
a  great  variety  of  other  manipulations,  a  small  Glass  Syringe  of  the  pat- 
tern represented  in  Fig.  106,  and  of  about  double  the  dimensions  will  be 
found  extremely  convenient.  When  this  is  firmly  held  between  the 
fore  and  middle  fingers,  and  the  thumb  is  inserted  into  the  ring  at  the 
summit  of  the  piston-rod,  such  complete  command  is  gained  over  the 
piston,  that  its  motion  may  be  regulated  with  the  greatest  nicety:  and 
thus  minute  quantities  of  fluid  may  be  removed  or  added,  in  the  various 
operations  which  have  to  be  performed  in  the  preparation  and  mounting 
of  Objects  (Chap,  v.);  or  any  minute  object  may  be  selected  (by  the  aid 
of  the  simple  Microscope,  if  necessary)  from  amongst  a  number  in  the 
same  drop,  and  transferred  to  a  separate  slip.  A  set  of  such  Syringes, 
with  points  drawn  to  different  degrees  of  fineness,  and  bent  to  different 
curvatures,  will  be  found  to  be  among  the  most  useful  '  tools  '  that  the 
working  Microscopist  can  have  at  his  command. 


riG;.lQ7. 


Forceps. 


128.  Forceps,~kxioi\\QT  instrument  so  indispensable  to  the  Micro- 
scopist as  to  be  commonly  considered  an  appendage  to  the  Microscope,  is 
the  Forceps  for  taking  up  minute  objects;  many  forms  of  this  have  been 
devised,  of  which  one  of  the  most  convenient  is  represented  in  Fig.  107, 


ACCESSORY  APPARATUS. 


129 


of  something  less  than  the  actual  size.  As  the  forceps,  in  Marine  re- 
searches, have  continually  to  be  plunged  into  sea-water,  it  is  better  that 
they  should  be  made  of  brass  or  of  German  silver  than  of  steel,  since  the 
latter  rusts  far  more  readily;  and  as  they  are  not  intended  (like  Dissecting- 
f creeps)  to  take  a  firm  grasp  of  the  object,  but  merely  to  hold  it,  they 
may  be  made  very  light,  and  their  spring-portion  slender.  As  it  is  es- 
sential, however,  to  their  utility,  that  their  points  should  meet  accurately, 
it  is  well  that  one  of  the  blades  should  be  furnished  with  a  guide-pin 
passing  through  a  hole  in  the  other. 

The  foregoing  constitute,  it  is  believed,  all  the  most  important  pieces 
of  Apparatus  which  can  be  considered  in  the  light  of  Accessories  to  the 
Microscope.  Those  which  have  been  contrived  to  afford  facilities  for  the 
preparation  and  mounting  of  Objects,  will  be  described  in  a  future  chap- 
ter (Chap.  v.).  And  the  simple  and  efficient  substitute  which  the  Author 
has  been  accustomed  to  use  for  the  Frog-Plate  thought  essential  by 
many  Microscopists,  will  be  described  in  Chap.  xx.  under  the  head  of 
Circulation  of  the  Blood. 
9 


130 


THE  MICKOSCOPE  AND  ITS  KEVELATIONS. 


CHAPTEE  IV. 
MANAGEMENT  OF  THE  MICROSCOPE. 

129.  Tahle. — The  Table  on  which  the  Microscope  is  placed  when  in 
use,  should  be  one  whose  size  enables  it  also  to  receive  the  various  appur- 
tenances which  the  observer  finds  it  convenient  to  have  within  his  reach, 
and  whose  steadiness  is  such  as  to  allow  of  his  arms  being  rested  upon  it 
without  any  yielding  ;  it  should,  moreover,  be  so  framed,  as  to  be  as  free 
as  possible  from  any  tendency  to  transmit  the  vibrations  of  the  building 
or  floor  whereon  it  stands.  The  working  Microscopist  will  find  it  a  matter 
of  great  convenience  to  have  a  Table  specially  set  apart  for  his  use,  fur- 
nished with  drawers,  in  whicli  are  contained  the  various  Accessories  he 
may  require  for  the  preparation  and  mounting  of  objects.  If  he  should 
desire  to  carry  about  with  him  all  the  apparatus  he  may  need  for  the 
prosecution  of  his  investigations  in  different  localities,  and  for  the 
mounting  of  his  preparations  on  the  spot,  he  will  find  it  very  convenient 
to  provide  himself  with  a  small  Cabinet,  fitted  with  drawers  in  which 
every  requisite  can  be  securely  packed,  and  of  such  a  height,  that,  when 
laid  upon  an  ordinary  table,  it  may  bring  up  the  Quekett  or  other  Dis- 
secting Microscope  placed  upon  it  to  the  position  most  convenient  for  use.* 
— If  the  Microscope  be  one  which  is  not  very  readily  taken  out  from  and 
put  back  into  its  case,  it  is  very  convenient  to  cover  it  with  a  large  bell- 
glass  ;  which  may  be  so  suspended  from  the  ceiling,  by  a  cord  carrying  a 
counterpoise  at  its  other  end,  as  to  be  raised  or  lowered  with  the  least 
possible  trouble,  and  to  be  entirely  out  of  the  way  when  the  Microscope 
is  in  use.  Similar  but  smaller  bell-glasses  (wine-glasses  whose  stems  have 
been  broken  answer  very  well)  are  also  useful  for  the  protection  of  objects 
which  are  in  course  of  being  examined  or  prepared,  and  which  it  is  desir- 
able to  seclude  from  dust. — For  the  purpose  of  Demonstration  in  the 
Lecture-room,  a  small  traversing  platform  may  be  constructed  to  run 
easily  upon  rollers,  and  to  carry  the  Microscope  and  Lamp  securely 
clamped  down  upon  it,  so  as  to  be  passed  from  one  observer  to  another. 
For  Demonstration  to  a  small  party  sitting  round  a  circular  table,  it  is 
convenient  to  employ  a  -shaped  platform,  the  vertical  angle  of  which 
is  pivoted  to  a  weight  placed  in  the  centre  of  the  table,  whilst  the  angles 

*  The  dimensions  of  the  Cabinet  which  the  Author  has  had  constructed  for 
himself  (its  size  being  so  adapted  to  that  of  the  box  of  his  Crouch's  Binocular 
that  the  two  are  received  into  the  same  travelling-case)  are  14  inches  long,  7  inches 
broad,  and  4^  inches  high.  In  the  middle  there  are  five  shallow  drawers,  5  inches 
broad,  containing  dissecting  apparatus,  large  flat  cells,  glass-covers,  syringes,  etc.  ; 
on  one  side  are  two  drawers,  each  3^  inches  broad,  the  upper  one,  containing  slides, 
cells,  etc.,  rather  more  than  one  inch  deep  inside,  the  lower,  for  larger  pieces  of 
apparatus,  2  inches  deep  ;  on  the  other  side  is  a  single  drawer  of  the  same  breadth 
and  3:^  inches  deep,  for  bottles  containing  solutions,  cements,  etc. 


MANAGEMENT  OF  THE  MICROSCOPE. 


131 


at  the  base  are  supported  upon  castors,  so  that  the  platform  may  run 
round  to  each  observer  in  succession.  Or  the  table  itself,  if  not  too  large, 
may  rotate  (like  a  dumb-waiter)  upon  its  central  pillar,  as  made  by 
Messrs.  Beck. 

130.  Light, — Whatever  maybe  the  purposes  to  which  the  Microscope 
is  applied,  it  is  a  matter  of  the  first  importance  to  secure  a  pure  and  ade- 
quate Illumination.  For  the  examination  of  the  greater  proportion  of 
objects,  good  daylight  is  to  be  preferred  to  any  other  kind  of  light ;  but 
good  lamplight  is  preferable  to  bad  daylight,  especially  for  the  illumina- 
tion of  opaque  objects.  When  daylight  is  employed,  the  Microscope 
should  be  placed  near  a  window,  whose  aspect  should  be  (as  nearly  as  may 
be  convenient)  opposite  to  the  side  on  which  the  sun  is  shining;  for  the  light 
of  the  sun  reflected  from  a  bright  cloud  is  that  which  the  experienced 
Microscopist  will  almost  always  prefer,  the  rays  proceeding  from  a  cloud- 
less blue  sky  being  by  no  means  so  well-fitted  for  his  purpose,  and  the 
dull  lurid  reflection  of  a  dark  cloud  being  the  worst  of  all.  The  direct 
light  of  the  sun  is  far  too  powerful  to  be  ordinarily  used  with  advantage, 
unless  its  intensity  be  moderated,  either  by  reflection  from  a  plaster  of 
Paris  mirror,  or  by  jDassage through  some  ^Modifier*  (§109)  ;  it  is,  how- 
ever, occasionally  used  by  some  observers  to  work  out  intricate  markings 
or  fine  color,  and  may  sometimes  be  of  advantage  for  these  purposes,  but 
without  great  care  would  be  a  fertile  source  of  error. — The  young  Micro- 
scopist is  earnestly  recommended  to  make  as  much  use  of  daylight  as 
possible ;  not  only  because,  in  a  large  number  of  cases,  the  view^of  the 
object  which  it  affords  is  more  satisfactory  than  that  which  can  be  ootained 
by  any  kind  of  lamplight,  but  also  because  it  is  much  less  trying  to  the 
eyes.  So  great,  indeed,  is  the  difference  between  the  two  in  this  respect, 
that  there  are  many  who  find  themselves  unable  to  carry  on  their  obser- 
vations for  any  length  of  time  by  lamplight,  al- 
though they  experience  neither  fatigue  nor  strain 
from  many  hours'  continuous  work  by  daylight. 
Even  ordinary  daylight  may  be  considerably  im- 
proved by  the  interposition  of  a  glass  globe  of 
about  six  inches  in  diameter,  filled  with  water;  and 
this  may  also  bo  advantageously  used  for  the  illu- 
mination of  transparent  objects  by  lamp-light,  if 
the  water  be  very  slightly  tinged  with  ammonio- 
sulphate  of  copper,  which  takes  off  the  yellow 
glare. 

131.  Lamps, — When  recourse  is  had  to  Artifi- 
cial light,  it  is  essential,  not  only  that  it  should  be 
of  good  quality,  but  that  the  arrangement  for  fur- 
nishing it  should  be  suitable  to  the  special  wants 
of  the  Microscopist.  The  most  useful  light  for 
ordinary  use  is  that  furnished  by  the  steady  and 
constant  flame  of  a  flat- wicked  Lamp,  fed  with  one 
of  the  best  varieties  of  Paraffin  oil.  This  (with  its 
chimney-shade)  should  be  so  mounted  on  a  stem 
rising  from  a  secure  base,  as  to  be  capable  of  ad- 
justment to  any  height  above  the  table;  and  on  the  same  stem  should  also 
slide  a  telescope-arm  having  a  bulFs  eye  condenser  attached  to  it  by  a 
ball-and-socket  joint,  in  such  a  manner  as  to  be  adjustable  in  any  posi- 
tion in  regard  to  the  flame,  and  at  the  same  time  to  be  carried  upwards 
or  downwards  with  the^lamp — an  arrangement  originally  devised  by  Mr. 


132 


THE  MICROSCOPE  AND  ITS  REVELATIONS. 


Beckett  (Fig.  108).  It  is  preferable,  however,  to  surround  the  glass 
chimney  by  a  cylinder  of  porcelain,  having  a  large  aperture  on  one  side 
for  the  passage  of  the  light;  and  this  may  be  advantageously  blackened 
on  the  outside,  contracted  above  into  a  cone,  and  furnished  with  a  shade 
over  the  aperture  (as  in  Mr.  Swift's  construction,  Fig.  109),  so  that  as 
little  light  as  possible  may  enter  the  eye  of  the  observer,  except  that 
which  proceeds  from  the  object.  The  lamp  should  be  so  hung  as  to  be 
capable  of  being  rotated  on  its  own  vertical  axis;  so  that  either  the  whole 
breadth  of  the  flame,  or  its  edge  only,  may  be  turned  towards  the  mir- 
ror or  condenser,  according  as  diffused  or  concentrated  light  is  required. 
In  Mr.  Swift's  Lamp  (Fig.  110),  the  Bull's-eye  is  mounted  on  a  separate 


Tia.  m  Tig,  lio. 


Chimney  and  Shade  of  Swift's  Swift's  Microscope -Lamp. 

Microscope-Lamp. 


stem,  capable  both  of  vertical  elevation  and  of  horizontal  adjustment, 
which  rises  from  one  end  of  an  arm  that  is  pivoted  beneath  the  base  of 
the  brass  cylinder  that  carries  the  lamp;  and  from  the  other  end  of  this 
arm  there  rises  a  second  stem,  carrying  a  speculum,  from  which  addi- 
tional light  may  be  reflected  when  desired.  By  rotating  this  arm  on  its 
pivot,  the  speculum  and  condenser  are  shifted  together,  so  as  to  direct 
the  full  power  of  the  flame  wherever  it  may  be  required;  an  arrangement 
especially  convenient  for  the  illumination  of  opaque  objects. — As  it  ]6 
often  found  extremely  difficult  to  obtain  an  exact  centering  of  the  illumi- 


MANAGEMENT  OF  THE  MICROSCOPE. 


133 


nating  beam,  when  yery  high  powers  are  employed,  by  mere  hand-shif t- 
ings  of  the  lamp  and  its  condenser,  Messrs.  Dallinger  and  Drysdale,  in 
the  admirable  investigations  of  whose  results  a  summary  will  be  given 
hereafter  (Chap,  xi.),  have  found  great  advantage  from  the  use  of  a 
Lamp  mounted  on  a  base  to  which  a  traversing  horizontal  movement  can 
be  given  in  any  direction  by  rectangular  screws,  and  furnished  with  an 
upright  standard  carrying  two  racks,  on  which  the  lamp  itself  and  the 
bull's-eye  condenser  can  be  separately  raised  or  lowered  by  milled-head 
pinions.  By  this  more  exact  method  of  adjustment,  the  observer  is  able, 
after  a  little  experience  in  its  use,  to  secure  that  most  perfect  position  of 
the  flame  and  condenser,  which  ordinary  hand-adjustment  might  not  suc- 
ceed in  attaining  until  after  a  great  expenditure  of  time  and  patience.^ 

132.  Position  of  the  Light, — When  the  Microscope  is  used  by  day- 
light, it  will  usually  be  found  most  convenient  to  place  it  in  such  a  man- 
ner that  the  light  shall  be  at  the  left  hand  of  the  observer.  It  is  most 
important  that  no  light  should  enter  his  eye,  save  that  which  comes  to  it 
through  the  Microscope;  and  the  access  of  direct  light  can  scarcely  be 
avoided,  when  he  sits  with  his  face  to  the  light.  Of  the  two  sides,  it  is 
more  convenient  to  have  the  light  on  the  left;  first,  because  it  is  not  in- 
terfered with  by  the  right  hand,  when  this  is  employed  in  giving  the 
requisite  direction  to  the  mirrr,  or  in  adjusting  the  illuminating  appara- 
tus; and,  secondly,  because,  as  most  persons  in  using  a  Monocular  Micro- 
scope employ  the  right  eye  rather  than  the  left,  the  projection  of  the 
nose  serves  to  cut  off  those  lateral  rays,  which,  when  the  light  comes  from 
the  right  side,  glance  between  the  eye  and  the  eye-piece.  The  side-shades 
fitted  by  Mr.  Collins  to  the  eye-pieces  of  his  Harley  Binocular  (Fig.  49) 
may  be  advantageously  employed  with  every  instrument  of  that  class. — 
When  Artificial  light  is  employed,  the  same  general  precautions  should  be 
taken.  The  Lamp  should  always  be  placed  on  the  left  side,  unless  some 
special  reason  exist  for  placing  it  otherwise;  and  if  the  Object  under  ex- 
amination be  transparent,  the  lamp  should  be  placed  at  a  distance  from 
the  eye  about  midway  between  that  of  the  stage  and  that  of  the  mirror. 
In  the  examination  of  objects  of  the  greatest  delicacy  and  difficulty, 
however,  in  which  it  is  important  to  get  rid  of  the  reflection  from  the 
front  surface  of  the  Mirror,  a  rectangular  Prism  should  be  substituted  for 
it,  when  the  conditions  of  the  observation  necessitate  the  use  of  the 
Microscope  in  the  vertical  position;  but  when  the  instrument  can  be 
inclined,  the  Lamp  may  be  most  advantageously  placed  in  the  axis  of  the 
Achromatic  Condenser  or  other  Illuminator,  so  that  its  light  may  be 
transmitted  to  the  object  without  intermediate  reflection.  If,  on  the 
other  hand,  the  Object  be  opaque,  the  Lamp  should  be  at  a  distance  about 
midway  behind  the  eye  and  the  stage;  so  that  its  light  may  fall  on  the 
object  at  an  angle  of  about  45°  with  the  axis  of  the  Microscope. — The 
passage  of  direct  rays  from  the  flame  to  the  eye  should  be  guarded  against 
by  the  interposition  of  the  lamp-shade;  and  no  more  light  should  be 
diffused  through  the  apartment,  than  is  absolutely  necessary  for  other 
purposes.  If  observations  of  a  very  delicate  nature  are  being  made,  it  is 
desirable,  alike  by  daylight  and  by  lamplight,  to  exclude  all  lateral  rays 
from  the  eye  as  completely  as  possible;  and  this  may  be  readily  accom- 


*  See  Monthly  Microsc.  Jour.,"  Vol.  xv.  (1876),  p.  165.— As  the  directions 
given  by  these  excellent  observers  for  centering  the  illuminating  beam  are  too 
long  for  citation,  such  as  desire  to  profit  by  their  experience  must  learn  its  results 
from  their  own  account  of  them. 


134 


THE  MICROSCOPE  AND  ITS  REVELATIONS. 


plished  by  means  of  a  shade  made  like  the  upper  part  of  a  Mask,  and 
lined  witii  black  cloth  or  velvet,  which  should  be  fixe(i  on  the  ocular  end 
of  the  Microscope. 

133.  Care  of  the  Eyes. — Although  most  Microscopists  who  habitually 
work  with  the  Monocular  microscope  acquire  a  habit  of  employing  only 
one  eye  (generally  the  right),  yet  it  will  be  decidedly  advantageous  to  the 
beginner  that  he  should  learn  to  use  either  eye  indifferently;  since  by 
employing  and  resting  each  alternately,  he  may  work  much  longer 
without  incurring  unpleasant  or  injurious  fatigue,  than  when  he  always 
employs  the  same. — Whether  or  not  he  do  this,  he  will  find  it  of  great 
importance  to  acquire  the  habit  of  keeping  open  the  unemployed  eye. 
This,  to  such  as  are  unaccustomed  to  it,  seems  at  first  very  embarrassing, 
on  account  of  the  interference  with  the  microscopic  image,  which  is 
occasioned  by  the  picture  of  surrounding  objects  formed  upon  the  retina 
of  the  second  eye;  but  the  habit  of  restricting  the  attention  to  that 
impression  only  which  is  received  through  the  microscopic  eye,  may 
generally  be  soon  acquired;  and  when  it  has  once  been  formed,  all  diffi- 
culty ceases.    Those  who  find  it  unusually  difficult  to  acquire  this  habit, 
may  do  well  to  learn  it  in  the  first  instance  with  the  assistance  of  the 
shade  just  described;  the  employment  of  which  will  permit  the  second  eye 
to  be  kept  open  without  any  confusion. — So  much  advantage,  however, 
is  derived  from  the  use  of  the  Binocular  arrangement,  either  stereoscopic 
or  non-stereoscopic,  that  the  Author  would  strongly  recommend  its  use 
to  every  observer,  save  in  cases  of  exceptional  difficulty.    There  can  be 
no  doubt  that  the  habitual  use  of  the  Microscope,  for  many  hours 
together,  especially  by  lamp-light,  and  with  high  magnifying  powers,  has 
a  great  tendency  to  injure  the  sight.    Every  Microscopist  who  thus 
occupies  himself,  therefore,  will  do  well,  as  he  values  his  eyes,  not  merely 
to  adopt  the  various  precautionary  measures  already  specified,  but  rigor- 
ously to  keep  to  the  simple  rule  of  not  continuinn  to  observe  any  longer 
than  he  can  do  so  without  fatigue.^ 

134.  Care  of  the  Microscope, — Before  the  Microscope  is  brought  into 
use,  the  cleanliness  and  dryness  of  its  glasses  ought  to  be  ascertained.  If 
dust  or  moisture  should  have  settled  on  the  Mirror,  this  can  be  readily 
wiped  off.  If  any  spots  should  show  themselves  on  the  field  of  view, 
when  it  is  illuminated  by  the  mirror,  these  are  probably  due  to  parti- 
cles adherent  to  one  of  the  lenses  of  the  Eye-piece:  and  this  maybe 
determined  by  turning  the  eye-piece  round,  which  will  cause  the  spots 
also  to  rotate,  if  their  source  lies  in  it.  It  may  very  probably  be  sufficient 
to  wipe  the  upper  surface  of  the  eye-glass  (by  removing  its  cap),  and  the 
lower  surface  of  the  field-glass;  but  if,  after  this  has  been  done,  the  spots 
should  still  present  themselves,  it  will  be  necessary  to  unscrew  the  lenses 
from  their  sockets,  and  to  wipe  their  inner  surfaces;  taking  care  to  screw 
them  firmly  into  their  places  again,  and  not  to  confuse  the  lenses  of 
different  eye-pieces.    Sometimes  the  eye-glass  is  obscured  by  dust  of 

^  The  Author  attributes  to  his  rigorous  observance  of  the  above  rule  his  entire 
freedom  from  any  injurious  affection  of  his  visual  organs,  notwithstanding  that, 
of  the  wh  le  amount  of  Microscopic  study  which  he  has  prosecuted  for  forty-five 
years  past,  a  large  proportion  has  been  necessarily  carried  on  by  Artificial  light, 
most  of  his  daylight  hours  having  been  occupied  in  other  ways.  He  has  found 
the  length  of  time  during  which  he  can  '  microscopize '  without  the  sense  of 
fatigue,  to  vary  greatly  at  different  periods,  half-an-hour's  work  being  sometimes 
sufficient  to  induce  discomfort,  whilst  on  other  occasions  none  has  been  left  by 
three  or  four  hours'  almost  continuous  use  of  the  instrument — his  power  of  visual 
endurance  being  usually  in  relation  to  the  vigor  of  his  general  system. 


MANAGEMENT  OF  THE  MICROSCOPE. 


135 


ex^iT.me  fineness,  wliieli  may  be  carried  off  by  a  smart  puff  of  breath;  the 
vapor  which  then  remains  upon  the  surface  being  readily  dissipated  by 
rapidly  moving  the  glass  backwards  and  forwards  a  few  times  through 
the  air.  And  it  is  always  desirable  to  try  this  plan  in  the  first  instance; 
since,  however  soft  the  substance  with  which  the  glasses  are  wiped,  their 
polish  is  impaired  in  the  end  by  the  too  frequent  repetition  of  the  pro- 
cess. The  best  material  for  wiping  glass  is  a  piece  of  soft  wash-leather, 
from  which  the  dust  it  generally  contains  has  been  well  beaten  out. — If 
the  Object-glasses  be  carefully  handled,  and  kept  in  their  boxes  when  not 
in  use,  they  will  not  be  likely  to  require  cleansing.  One  of  the  chief 
dangers,  however,  to  which  they  are  liable  in  the  hands  of  an  inexperi- 
enced Microscopist,  arises  from  the  neglect  of  precaution  in  using  them 
with  fiuids;  which,  when  allowed  to  come  in  contact  with  the  surface  of 
the  outer  glass,  should  be  wiped  off  as  soon  as  loossible.  In  screwing  and 
unscrewing  them,  great  care  should  be  taken  to  keep  the  glasses  at  a 
distance  from  the  surface  of  the  hands;  since  they  are  liable  not  only  to 
be  soiled  by  actual  contact,  but  to  be  dimmed  by  the  vaporous  exhalation 
from  skin  which  they  do  not  touch.  This  dimness  will  be  best  dissipated 
by  moving  the  glass  quickly  through  the  air.  It  will  sometimes  be  found, 
on  holding  an  Object-glass  to  the  light,  that  particles  either  of  ordinary 
dust,  or  more  often  of  the  black  coating  of  the  interior  of  the  Microscope, 
have  settled  upon  the  surface  of  its  back-lens;  these  are  best  removed  by 
a  clean  and  dry  camel's-hair  pencil.  If  any  cloudiness  or  dust  should  still 
present  itself  in  an  object-glass,  after  its  front  and  back  surfaces  have 
been  carefully  cleansed,  it  should  be  sent  to  the  maker  (if  it  be  of  English 
manufacture)  to  be  taken  to  pieces,  as  the  amateur  will  seldom  succeed 
in  doing  this  without  injury  to  the  work;  the  foreign  combinations, 
however,  being  usually  put  together  in  a  simpler  manner,  may  be  readily 
unscrewed,  cleansed,  and  screwed  together  again.  Not  unfrequently  an 
objective  is  rendered  dim  by  the  cracking  of  the  cement  by  which  the 
lenses  are  united,  or  by  the  insinuation  of  moisture  between  them;  this 
last  defect  occasionally  arises  from  a  fault  in  the  quality  of  the  glass, 
which  is  technically  said  to  *  sweat.'  In  neither  of  these  cases  has  the 
Microscopist  any  resource,  save  in  an  Optician  experienced  in  this  kind 
of  work;  since  his  own  attempts  to  remedy  the  defect  are  pretty  sure  to 
be  attended  with  more  injury  than  benefit. 

135.  General  Arrangement  of  the  Microscope  for  Use, — The  inclined 
position  of  the  instrument,  already  so  frequently  referred  to,  is  that  in 
Avhich  observation  by  it  may  be  so  much  more  advantageously  carried-on 
than  in  any  other,  that  recourse  should  always  be  had  to  it,  unless  par- 
ticular circumstances  render  it  unsuitable.  The  precise  inclination  that 
may  prove  to  be  most  convenient  w^ill  depend  upon  the  ^  build '  of  the 
Microscrope,  upon  the  height  of  the  observer's  seat  as  compared  with  that 
of  the  table  on  which  the  instrument  rests,  and  lastly,  upon  the  sitting 
height  of  the  individual;  and  it  must  be  determined  in  each  case  by  his 
own  experience  of  what  suits  him  best — that  which  he  finds  most  com- 
fortaUe  being  that  in  which  he  will  be  able  not  only  to  work  the  longest, 
but  to  see  most  distinctly. — The  selection  of  the  Objectives  and  Eye- 
pieces to  be  employed  must  be  entirely  determined  by  the  character  of 
the  object.  Large  objects  presenting  no  minute  structural  features  should 
always  be  examined  in  the  first  instance  by  iYiQ  lowest  powers,  whereby  a 
general  view  of  their  nature  is  obtained;  and  since,  with  lenses  of  com- 
paratively long  focus  and  small  angle  of  aperture,  the  precision  of  the 
focal  adjustment  is  not  of  so  much  consequence  as  it  is  with  the  higher 


136 


THE  MICROSCOPE  AND  ITS  REVELATIONS. 


powers,  not  only  those  parts  can  be  seen  which  are  exactly  in  focus,  but 
those  also  can  be  tolerably  well  distinguished  which  are  not  precisely  in 
that  plane,  but  are  a  little  nearer  or  more  remote.  Wlien  the  general 
aspect  of  an  object  has  been  sufficiently  examined  through  low  powers, 
its  details  may  be  scrutinized  under  a  higher  amplification;  and  this  will 
be  required  in  the  first  instance,  if  the  object  be  so  minute  that  little  or 
nothing  can  be  made  out  respecting  it  save  when  a  very  enlarged  image 
is  formed.  The  power  needed  in  each  particular  case  can  only  be  learned 
by  experience;  that  which  is  most  suitable  for  the  several  classes  of  ob- 
jects hereafter  to  be  described,  will  be  specified  under  each  head.  In  the 
general  examination  of  the  larger  class  of  objects,  the  range  of  power  that 
is  afforded  by  Zeiss's  Adjustable  Low-power  Objective  (§  159,  I.)  will 
often  be  found  useful;  whilst  for  the  ready  exchange  of  a  low  power  for  a 
higher  one,  great  convenience  is  afforded  by  the  Nose-piece  (§  96). 

136.  When  the  Microscopist  wishes  to  augment  his  magnifying  power, 
he  has  a  choice  between  the  employment  of  an  Objective  of  shorter  focus 
and  the  use  of  a  deeper  Eye-piece.  If  he  possess  a  complete  series  of 
Objectives,  he  will  frequently  find  it  best  to  substitute  one  of  these  for 
another  without  changing  the  Eye-piece  for  a  deeper  one;  but  if  his 
^powers'  be  separated  by  wide  intervals,  he  will  be  able  to  break  the 
abruptness  of  the  increase  in  amplification  which  they  produce,  by  using 
each  Objective  first  with  the  shallower  and  then  with  the  deeper  Eye- 
piece. Thus,  if  a  Microscope  be  provided  only  with  fwo  Objectives  of 
1-inch  and  l-4th  inch  focus  respectively,  and  with  two  Eye-pieces,  one 
nearly  double  the  power  of  the  other,  such  a  range  as  the  following  may 
be  obtained — 60,  90,  240,  360  diameters;  or,  with  two  Objectives  of 
somewhat  shorter  focus,  and  with  deeper  Eye-pieces  (as  in  some  French 
and  German  instruments) — 88,  176,  350,  700  diameters.  In  the  exami- 
nation of  large  Opaque  objects  having  uneven  surfaces,  it  is  generally 
preferable  to  increase  the  power  by  the  Eye-piece  rather  than  by  the 
Objective;  thus  a  more  satisfactory  view  of  such  objects  may  usually  be 
obtained  with  a  3-inch  or  2-inch  Objective  and  the  B  Eye-piece,  than 
with  a  l|-inch  or  1-inch  Objective  and  the  A  Eye-piece.  The  reason  of 
this  is,  that  in  virtue  of  their  smaller  Angle  of  Aperture,  the  Objectives 
first  named  have  a  much  greater  amount  of  ^penetrating  power' or 
^ focal  depth'  than  the  latter  (§  158,  i.);  and  in  the  case  just  specified 
this  quality  is  of  the  first  importance.  The  use  of  the  Draw-tube  (§  83) 
enables  the  Microscopist  still  further  to  vary  the  magnifying  power  of  his 
instrument,  and  thus  to  obtain  almost  any  exact  number  of  diameters  he 
may  desire,  within  the  limits  to  which  he  is  restricted  by  the  focal  length 
of  his  01)jectives.  The  advantage  to  be  derived,  however,  either  from 
^deep  Eye-piecing'  or  from  the  use  of  the  Draw-tube,  will  mainly  depend 
upon  the  quality  of  the  Object-glass.  For,  if  it  be  imperfectly  corrected, 
its  errors  are  so  much  exaggerated,  that  more  is  lost  in  definition  than  is 
gained  in  amplification;  whilst,  if  its  apertures  be  small,  the  loss  of  light 
is  an  equally  serious  drawback.  On  the  other  hand,  an  Objective  of 
perfect  correction  and  adequate  angle  of  aperture  will  sustain  this  treat- 
ment with  so  little  impairment  in  the  perfection  of  its  image,  that  » 
magnifying  power  may  be  obtained  by  its  use,  such  as,  with  an  inferior 
instrument,  can  only  be  derived  from  an  Objective  of  much  shorter 
focus  combined  with  a  shallow  Eye-piece. — The  author  thinks  it  a  great 
mistake,  however,  to  attempt  to  make  an  Objective  of  medium  power 
ordinarily  do  the  work  on  which  an  Objective  of  high  power  should 
properly  be  employed.    For  not  only  can  it  not  be  brought  up  to  this 


MANAGEMENT  OF  THE  MICROSCOPE. 


137 


without  such  an  increase  of  its  angle  of  aperture  as  unfits  it  for  its  own 
proper  work,  but  the  "deep  eye-piecing'  required  cannot  be  had  recourse 
to  habitually  without  exposing  the  eyes  to  severe  overstrain.  The  advan- 
tage of  loio  Eye-pieces  and  deej:)  Objectives,  as  compared  with  deep  Eye- 
pieces and  loiv  Objectives,  has  been  very  well  ])ut  by  likening  it  to  the 
comfort  of  reading  large  print  without  spectacles,  or  with  spectacles 
suited  to  the  sight,  and  reading  small  print  with  a  magnify ing-glass. 

137.  In  making  the  Focal  Adjustment,  when  low  powers  are  used,  it 
will  scarcely  be  necessary  to  employ  any  but  the  coarse  adjustment,  or 
^  quick  motion;'  provided  that  the  rack  be  well  cut,  the  pinion  work  in 
it  smoothly  and  easily,  without  either  ^spring,'  ^loss  of  time,'  or  Uwist,' 
and  the  milled-head  be  large  enough  to  give  the  requisite  leverage.  All 
these  are  requisites  which  should  be  found  in  every  well-constructed 
instrument;  and  its  possession  of  them  should  be  tested,  like  its  freedom 
from  vibration,  by  the  use  of  high  powers,  since  a  really  good  coarse 
adjustment  should  enable  the  observer  to  ^ focus'  an  Objective  of  l-8th 
inch  with  precision. — What  is  meant  by  ^spring'  is  the  alteration  which 
may  often  be  observed  to  take  place  on  the  withdrawal  of  the  hand;  the 
object  which  has  been  brought  precisely  into  focus,  and  which  so  remains 
as  long  as  the  milled-head  is  between  the  fingers,  becoming  indistinct 
when  the  milled-head  is  let  go.  The  source  of  this  fault  may  lie  either 
in  the  rack-movement  itself,  or  in  the  general  framing  of  the  instrument, 
which  is  so  weak  as  to  allow  of  displacement  by  the  mere  weight  or  pres- 
sure of  the  hand:  should  the  latter  be  the  case,  the  ^spring'  may  be  in  a 
great  degree  prevented  by  carefully  abstaining  from  hearing  on  the 
milled-head,  which  should  be  simply  rotcHed  between  the  fingers. — By 
Hoss  of  time '  is  meant  the  want  of  sufficient  readiness  in  the  action  of 
the  pinion  upon  the  rack,  so  that  the  milled-head  may  be  moved  slightly 
in  either  direction  without  affecting  the  body;  thus  occasioning  a  great 
diminution  in  the  sensitiveness  of  the  adjustment.  This  fault  may 
sometimes  be  detected  in  Microscopes  of  the  best  original  construction, 
which  have  gradually  worked  loose  owing  to  the  constancy  with  which 
they  have  been  in  employment;  and  it  may  often  be  corrected  by  tight- 
ening the  screws  that  bring  the  pinion  to  bear  against  the  rack. — And  by 
Hwist' it  is  intended  to  express  that  apparent  movement  of  the  object 
across  the  field,  which  results  from  a  real  displacement  of  the  axis  of  the 
body  to  one  side  or  the  other,  owing  to  a  want  of  correct  fitting  in  the 
working  parts.  ^  As  this  last  fault  depends  entirely  on  bad  original 
workmanship,  there  is  no  remedy  for  it;  but  it  is  one  which  most 
seriously  interferes  with  the  convenient  use  of  the  instrument,  however 
excellent  may  be  -its  optical  performance. — In  the  use  of  the  coarse 
adjustment  with  an  Objective  of  short  focus,  extreme  care  is  necessary  to 
avoid  bringing  it  down  upon  the  object,  to  the  injury  of  one  or  both;  for 
although  the  spring  with  which  the  tube  for  the  reception  of  the  object- 
glass  is  furnished,  whenever  the  ^fine  adjustment'  is  immediately  applied 
to  this,  takes  off  the  violence  of  the  crushing  action,  yet  such  an  action, 
even  when  thus  moderated,  can  scarcely  fail  to  damage  or  disturb  the 
object,  and  may  do  great  mischief  to  the  lenses.  Where  the  fine  adjust- 
ment is  otherwise  provided  for,  still  greater  care  is  of  course  required, 

^  In  testing  either  the  '  coarse'  or  the  *  fine '  adjustment  for  '  twist/  care  should 
be  taken  that  the  light  reflected  from  the  mirror  is  axial  not  oblique ;  since,  if 
the  illuminating  rays  are  inclined  to  the  optic  axis,  the  object,  when  thrown  out 
of  focus,  will  appear  to  vanish  laterally,  which  it  does  not  do  (provided  the 
adjustments  work  well)  when  illuminated  axially. 


138 


THE  MICROSCOPE  AND  ITS  REVELATIONS. 


unless  a  spring  ^safety-tube'  be  provided,  into  which  the  Objectives  are 
screwed. — It  is  here,  perhaps,  well  to  notice,  for  the  guidance  of  the 
young  Microscopist,  that  the  actual  distance  between  the  Objective  and 
the  object,  when  a  distinct  image  is  formed,  is  always  considerably  less 
than  the  nominal  focal  length  of  the  objective. — One  more  precaution  it 
may  be  well  to  specify;  namely,  that  either  in  changing  one  object  for 
another,  or  in  substituting  one  Objective  for  another,  save  when  powers 
of  such  focal  length  are  employed  as  to  remove  all  likelihood  of  injury, 
the  Body  should  have  its  distance  from  the  Stage  increased  by  the 
^coarse  adjustment.'  This  precaution  is  absolutely  necessary  when 
Objectives  of  short  focus  are  in  use,  to  avoid  injury  either  to  the  lenses 
or  to  the  object;  and  when  it  is  habitually  practised  with  regard  to  these, 
it  becomes  so  much  like  an  ^acquired  instinct,'  as  to  be  almost  invariably 
practised  in  other  cases. 

138.  In  obtaining  an  exact  Focal  Adjustment  with  Objectives  of  less 
than  half-an-inch  focus,  it  will  be  generally  found  convenient  to  employ 
t\iQ  fine  adjustjnent  or  ^slow  motion;'  and  as  recourse  will  frequently  be 
had  to  its  assistance  for  other  purposes  also,  it  is  very  important  that  it 
should  be  well  constructed  and  in  good  working  order.  The  points  to  be 
l^articularly  looked  to  in  testing  it,  are  for  the  most  part  the  same  with 
those  already  noticed  in  relation  to  the  coarse  movement.  It  should 
Avork  smoothly  and  equably,  ]oroducing  that  graduated  alteration  of  the 
distance  of  the  Objective  from  the  object  which  it  is  its  special  duty  to 
effect,  without  any  jerking  or  irregularity.  It  should  be  so  sensitive, 
that  any  movement  of  the  milled-head  should  at  once  make  its  action 
apparent  by  an  alteration  in  the  distinctness  of  the  image,  when  high 
powers  are  employed,  without  any  ^oss  of  time.'  ^  And  its  action  should 
not  give  rise  to  any  twisting  or  displacing  movement  of  the  image,  which 
ought  not  to  be  in  the  least  degree  disturbed  by  any  number  of  rotations 
of  the  milled-head,  still  less  by  a  rotation  through  only  a  few  degrees. — 
One  great  use  of  this  adjustment  consists  in  bringing  into  view  different 
strata  of  the  object,  and  this  in  such  a  gradual  manner  that  their  con- 
nection with  one  another  shall  be  made  apparent.  AVhether  an  Opaque 
or  a  Transparent  object  be  under  examination,  only  that  part  which  is 
exactly  in  focus  can  be  perfectly  discerned  under  any  power;  and  when 
high  powers  of  large  angular  aperture  are  employed,  this  is  the  only  part 
that  can  be  seen  at  all.  A  minute  alteration  of  the  focus  often  causes  so 
different  a  set  of  appearances  to  be  presented,  that,  if  this  alteration  be 
made  abruptly,  the  relation  of  each  to  its  predecessors  can  scarcely  be 
even  guessed  at;  and  the  gradual  transition  from  the  one  to  the  other, 
which  the  ^slow  motion'  alone  affords,  is  therefore  necessary  to  the 
correct  interpretation  of  either.  To  take  a  very  simple  case: — The 
transparent  body  of  a  certain  animal  being  traversed  by  vessels  lying  in 
different  planes,  one  set  of  these  vessels  is  brought  into  view  by  one 
adjustment,  another  set  by  ^focussing'  to  a  different  plane;  and  the 
connection  of  the  two  sets  of  vessels,  which  may  be  the  point  of  most 
importance  in  the  whole  anatomy  of  the  animal,  may  be  entirely  over- 
looked for  want  of  a  ^fine  adjustment,' whose  graduated  action  shall 
enable  one  to  be  traced  continuously  into  the  other.    What  is  true  even 

1  It  will  sometimes  happen  that  the  *  slow  motion '  will  seem  not  to  act, 
merely  because  it  has  been  so  habitually  worked  in  one  direction  rather  than  the 
other,  that  its  screw  has  been  turned  too  far.  In  that  case,  nothing  more  is 
required  for  its  restoration  to  good  working  order,  than  turning  the  screw  in  the 
other  direction,  until  it  shall  have  reached  about  the  middle  of  its  range  of  action. 


MANAGEMENT  OF  THE  MICROSCOPE. 


139 


of  low  and  medium  powers,  is  of  course  true  to  a  still  greater  degree  as 
to  high  powers;  for  although  the  ^ quick  motion^  may  enable  the 
observer  to  bring  any  stratum  of  the  object  into  accurate  focus,  it  is 
impossible  for  him  by  its  means  to  secure  that  transitional  ^focussing' 
which  is  often  much  more  instructive  than  an  exact  adjustment  at  any 
one  point.  A  clearer  idea  of  the  nature  of  a  doubtful  structure  is,  in 
fact,  often  derived  from  what  is  caught  sight  of  in  the  act  of  changing 
the  focus,  than  by  the  most  attentive  study  and  comparison  of  the  differ- 
ent vrews  obtained  by  any  number  of  separate  ^focussings.'  The  experi-' 
enced  Microscopist,  therefore,  whilst  examining  an  object  of  almost  any 
description,  constantly  keeps  his  finger  on  the  milled-head  of  the  '  slow 
motion,^  and  watches  the  effect  produced  by  its  revolution  upon  every 
feature  which  he  distinguishes;  never  leaving  off  until  he  be  satisfied 
that  he  has  scrutinized  not  only  the  entire  surface,  but  the  entire  thick- 
ness of  the  object.  It  will  often  happen  that,  where  different  structural 
features  present  themselves  on  different  planes,  it  will  be  difficult  or  even 
impossible  to  determine  with  the  Monocular  microscope  which  of  them  is 
the  nearer  and  which  the  more  remote,  unless  it  be  ascertained  by  the  use 
of  the  ^slow  motion,*  when  they  are  successively  brought  into  focus, 
whether  the  Objective  has  been  moved  towards  or  away  from  the  object.^ 
Even  this,  however,  will  not  always  succeed  in  certain  of  the  most  diffi- 
cult cases,  in  which  the  difference  of  level  is  so  slight  as  to  be  almost 
inappreciable;  as,  for  instance,  in  the  case  of  the  markings  on  the  silice- 
ous valves  of  the  Diatoms  (Fig.  166). 

139.  When  Objectives  of  short  focus  and  of  wide  angular  aperture  are 
in  use,  something  more  is  necessary  (save  in  the  case  of  '  homogeneous- 
immersion*  lenses,  §  20),  than  exact  focal  adjustment;  this  being  the 
adjustment  of  the  Objective  itself, 
which  is  required  to  neutralize  the 
disturbing  effect  of  the  glass  cover 
upon  the  course  of  the  rays  pro- 
ceeding from  the  object  (§  17), — 
unless  (as  in  the  Objectives  now 
commonly  made  for  Students' 
Microscopes)  they  are  construct- 
ed for  working  only  with  cover- 
glasses  of  a  certain  standard  thick-  cncQvcrctlf 
ness.  For  such  adjustment,  it  'toemu 
will  be  recollected,  a  power  of  al- 
tering the  distance  between  the 
front  pair  and  the  remainder  of 
the  combination  is  required;  and 
this  power  is  obtained  in  the  fol- 
lowing manner: — The  front  pair 
of  lenses  is  fixed  into  a  tube  (Fig. 
Ill,  a),  which  slides  over  an 
interior  tube  (b)  by  which  the 

other  two  pairs  are  held;  and  it  is  drawn  up  or  down  by  means  of  a  col- 
lar (c),  which  works  in  a  furrow  cut  in  the  inner  tube,  and  upon  a  screw- 
thread  cut  in  the  outer,  so  that  its  revolution  in  the  plane  to  which  it 
is  fixed  by  the  one  tube  gives  a  vertical  movement  to  the  other.    In  one 


Section  of  Adjusting  Object-Glass. 


^  It  is  in  objects  of  this  kind  that  the  great  advantage  of  the  Stereoscopic 
Binocular  arrangement  makes  itself  most  felt  (§§  30-40). 


14:0 


THE  MICROSCOPE  AND  ITS  REVELATIONS. 


part  of  the  outer  tube  an  oblong  slit  is  made,  as  seen  at  into  which 
projects  a  small  tongue  screwed  on  the  inner  tube;  at  the  side  of  the 
former  two  horizontal  lines  are  engraved,  one  pointing  to  the  word  ^  un- 
covered/ the  other  to  the  word  ^covered;'  whilst  the  latter  is  crossed  by 
a  horizontal  mark,  which  is  brought  to  coincide  with  either  of  the  two 
lines  by  the  rotation  of  the  screw-collar,  whereby  the  outer  tube  is  moved 
up  or  down.  When  the  mark  has  been  made  to  point  to  the  line  ^uncov- 
ered/ it  indicates  that  the  distance  of  the  lenses  of  the  object-glass  is 
such  as  to  make  it  suitable  for  viewing  an  object  without  any  interference 
from  thin  glass;  when,  on  the  other  hand,  the  mark  has  been  brought  by 
the  revolution  of  the  screw-collar  into  coincidence  with  the  line  ^cov- 
ered,' it  indicates  that  the  front  lens  has  been  brought  into  sach  proxi- 
mity with  the  other  two,  as  to  produce  a  ^positive  aberration^  in  the 
Objective,  fitted  to  neutralize  the  '  negative  aberration  ^  produced  by  the 
interposition  of  a  glass  cover  of  extremest  thickness.  But  unless  this  cor- 
rection be  made,  with  the  greatest  precision,  to  the  thickness  of  the  par- 
ticular cover  in  use,  the  enlargement  of  the  Angle  of  Aperture,  to  which 
Opticians  have  of  late  applied  themselves  with  such  remarkabe  success, 
becomes  worse  than  useless;  being  a  source  of  diminished  instead  of  in- 
creased distinctness  in  the  details  of  the  object,  which  are  far  better  seen 
with  an  Objective  of  greatly  inferior  aperture,  possessing  no  special  ad- 
justment for  the  thickness  of  the  glass.  The  following  general  rule  is 
given  by  Mr.  Wenham  for  securing  the  most  efficient  performance  of  an 
Object-glass  with  any  ordinary  object: — "  Select  any  dark  speck  or  oj)aque 
portion  of  the  object,  and  bring  the  outline  into  perfect  focus;  then  lay 
the  finger  on  the  milled-head  of  the  fine  motion,  and  move  it  briskly 
backwards  and  forwards  in  both  directions  from  the  first  position.  Ob- 
serve the  expansion  of  the  dark  outline  of  the  object,  both  when  within 
and  when  without  the  focus.  If  the  greater  expansion,  or  coma,  is  when 
the  object  is  without  the  focus,  or  farthest  from  the  Objective,  the  lenses 
must  be  placed  farther  asunder,  or  towards  the  mark  ^  uncovered.^  If 
the  greater  coma  is  when  the  object  is  tuithin  the  focus,  or  nearest  to  the 
Objective,  the  lenses  must  be  brought  closer  together,  or  towards  the 
mark  ^  covered.^  When  the  object-glass  is  in  proper  adjustment,  the  ex- 
pansion of  the  outline  is  exactly  the  same  both  within  and  without  the 
focus.''  A  different  indication,  however,  is  afforded  by  such  '  test-ob- 
jects '  as  present  (like  the  Podura-scale  and  the  Diatomaceae)  a  set  of  dis- 
tinct dots  or  other  markings.  For  '^if  the  dots  have  a  tendency  to  run 
into  lines  when  the  object  is  placed  witliout  the  focus,  the  glasses  must 
be  brought  closer  together;  on  the  contrary,  if  the  lines  appear  when  the 
object  is  within  the  focal  point,  the  lenses  must  be  farther  separated."^ 
AVhen  the  Angle  of  Aperture  is  very  wide,  the  difference  in  the  aspect  of 
any  severe  test  under  different  adjustments  becomes  at  once  evident; 
markings  which  are  very  distinct  when  the  correction  has  been  exactly 
made,  disappearing  almost  instantaneously  when  the  screw-collar  is 
turned  a  little  way  round.  2 

1  See    Quart.  Journ.  of  Microsc.  Science,"  Vol.  ii.  (1854),  p.  138. 

2  Mr.  "Wenham  remarks  (loo.  cit.),  not  without  justice,  upon  the  difficulty  of 
making  this  adjustment  even  in  the  objectives  of  our  best  Opticians;  and  he  states 
that  he  has  himself  succeeded  much  better  by  making  the  outer  tube  the  fixture, 
and  by  making  the  tube  that  carries  the  other  pairs  slide  within  this;  the  motion 
being  given  by  the  action  of  an  incHned  slit  in  the  revolving  collar  upon  a  pin 
that  passes  through  a  longitudinal  slit  in  the  outer  tube,  to  be  attached  to  the 
inner. — The  admirable  Objectives  in  the  first-class  American  Opticians,  are  (the 
Author  believes)  always  constructed  so  that  the  adjustment  is  effected  by  the 
movement  of  the  hack  combinations,  as  long  since  recommended  by  Mr.  Wenham. 


MANAGEMENT  OF  THE  MICROSCOPE. 


141 


140.  Although  the  most  perfect  correction  required  for  each  particu- 
lar object  (which  depends  not  merely  upon  the  thickness  of  its  glass 
cover,  but  upon  that  of  the  fluid  or  balsam  in  Avhich  it  may  be  mounted) 
can  only  be  found  by  experimental  trial,  yet  for  all  ordinary  purposes, 
the  following  simple  method,  first  devided  by  Mr.  Powell,  will  suffice. 
The  object-glass,  adjusted  to  ^uncoyered,'  is  to  be  ^focussed'  to  the  ob- 
ject; the  screw-collar  is  next  to  be  turned  until  the  surface  of  the  glass 
cover  comes  into  focus,  as  may  be  perceived  by  the  spots  or  striae  by 
which  it  may  be  marked;  the  object  is  then  to  be  again  brought  into  fo- 
cus by  the  '  slow  motion.^  The  edge  of  the  screw-collar  being  graduated, 
the  particular  adjustment  which  any  object  may  have  been  found  to  re- 
quire, and  of  which  a  record  has  been  kept,  may  be  made  again  without 
any  difficulty. — By  Messrs.  Smith  and  Beck,  however,  who  first  intro- 
duced this  graduation,  a  further  use  is  made  of  it.  By  experiments  such 
as  those  described  in  the  last  paragraph,  the  correct  adjustment  is  first 
found  for  any  particular  object,  and  the  number  of  divisions  observed 
through  which  the  screw-collar  must  be  moved  in  order  to  bring  it  back 
to  0^,  the  position  suitable  for  an  uncovered  object.  The  thickness  of 
the  glass  cover  must  then  be  measured  by  means  of  the  ^  slow  motion  '; 
this  is  done  by  bringing  into  exact  focus,  first  the  object  itself,  and  then 
the  surface  of  the  glass  cover,  and  by  observing  the  number  of  divisions 
through  which  the  milled-head  (which  is  itself  graduated)  has  passed  in 
making  this  change.  A  definite  ratio  between  that  thickness  of  glass,  and 
the  correction  required  in  that  particular  Objective,  is  thus  established; 
and  this  serves  as  the  guide  to  the  requisite  correction  for  any  other 
thickness,  which  has  been  determined  in  like  manner  by  the  ^slow  mo- 
tion.^ Thus,  supposing  a  particular  thickness  of  glass  to  be  measured 
by  12  divisions  of  the  milled-head  of  the  ^slow  motion,^  and  the  most 
perfect  performance  of  the  Objective  to  be  obtained  by  moving  the  screw- 
collar  through  8  divisions,  then  a  thickness  of  glass  measured  by  9  divi- 
sions of  the  milled-head  would  require  the  screw-collar  to  be  adjusted  to 
6  divisions  in  order  to  obtain  the  best  effect.  The  ratio  between  the  two 
sets  of  divisions  is  by  no  means  the  same  for  different  combinations;  and  it 
ought  to  be  determined  for  each  Objective  by  its  maker,  who  will  generally 
be  the  fittest  judge  of  the  best  ^ points^  of  his  lenses;  but  when  this  ratio 
has  been  once  ascertained,  the  adjustment  for  any  thickness  of  glass  with 
which  the  object  may  happen  to  be  covered,  is  readily  made  by  the  Micro- 
scopist  himself.  Although  this  method  appears  somewhat  more  complex 
than  that  of  Mr.  Powell,  yet  it  is  more  perfect;  and  when  the  ratio  be- 
tween the  two  sets  of  divisions  has  been  once  determined,  the  adjustment 
does  not  really  involve  more  trouble. — Another  use  is  made  of  this  ad- 
justment by  Messrs.  Smith  and  Beck;  namely,  to  correct  the  disturbance 
in  the  performance  of  Objectives,  which  is  made  by  the  increase  of  dis- 
tance between  the  Objective  and  the  Eye-piece,  occasioned  by  the  use  of 
the  Draw-tube  (§  83).  Accordingly  they  mark  a  scale  of  inches  on  the 
Draw-tube  (which  is  useful  for  many  other  purposes),  and  direct  that  for 
every  inch  the  body  is  lengthened,  the  screw-collar  of  the  Objective  shall 
be  moved  through  "^a  certain  number  of  divisions. 

141.  Arrangement  for  Transparent  Objects. — If  the  Object  be  already 
'  mounted  ^  in  a  slide,  nothing  more  is  necessary,  in  order  to  bring  it  into 
the  right  position  for  viewing  it,  than  to  lay  the  slide  upon  the  Object- 
platform  of  the  Stage,  and  so  to  support  it  by  means  of  the  spring-clips, 
sliding-ledge,  or  other  contrivance,  that  the  part  to  be  viewed  is,  as 
nearly  as  can  be  guessed,  in  the  centre  of  the  aperture  of  the  stage,  and 


142 


THE  MICROSCOPE  AND  ITS  REVELATIONS. 


therefore  in  a  line  with  the  axis  of  the  body.  If  the  object  be  not 
^mounted/  and  be  of  such  a  kind  that  it  is  best  seen  dry,  it  may  be  sim- 
ply laid  upon  the  glass  Stage-plate  (§  120),  the  ledge  of  which  will  pre- 
vent it  from  slipping  off  when  the  Microscope  is  inclined;  and  a  plate 
of  thin  glass  may  be  laid  over  it  for  its  protection,  if  its  delicacy  should 
seem  to  render  this  desirable.  If,  again,  it  be  disposed  to  curl  up,  so 
that  a  slight  pressure  is  needed  to  flatten  or  extend  it,  recourse  maybe  had 
to  the  use  of  the  Aquatic  Box  (§  122 )  or  the  Compressor  (§  125),  without  the 
introduction  of  any  liquid  between  the  surfaces  of  glass.  In  a  very  large 
proportion  of  cases,  however,  either  the  objects  to  be  examined  are  al- 
ready floating  in  fluid  or  it  is  preferable  to  examine  them  in  fluid,  on 
account  of  the  greater  distinctness  with  which  they  may  be  seen.  If  such 
objects  be  minute,  and  the  quantity  of  liquid  be  small,  the  drop  is  sim- 
ply to  be  laid  on  a  slip  of  glass,  and  covered  with  a  plate  of  thin  glass;  if 
the  object  or  the  quantity  of  liquid  be  larger,  it  will  be  better  to  place  it 
in  a  concave  slide  or  cell;  whilst,  if  the  object  have  dimensions  which 
render  even  this  inconvenient,  the  Zoophyte  Trough  (§  124)  will  afford 
the  best  means  for  its  examination. — In  the  case  of  minute  living  ani- 
mals, whose  movements  it  is  desired  limit  (so  as  to  keep  them  within  the 
field  of  view)  without  restraining  them  by  compression,  the  Author  has 
found  the  following  plan  extremely  convenient.  The  drop  of  water 
taken  up  with  the  animal  by  the  Dipping- tube  being  allowed  to  fall  into 
a  concave  slide  (Fig.  122),  the  whole  of  the  superfluous  water  may  be  re- 
moved by  the  Syringe  (§  127),  only  just  as  much  being  left  as  will  keep 
the  animal  alive.  If  the  animal  be  very  minute,  it  is  convenient  to  effect 
this  withdrawal  by  placing  the  slide  on  the  stage  of  the  Dissecting  Mi- 
croscope (§  44),  and  working  the  Syringe  under  the  magnifier;  and  it  will 
be  found  after  a  little  practice,  that  the  complete  command  which  the 
operator  has  over  the  movements  of  the  piston,  as  well  as  over  the  place 
of  the  point  of  the  syringe,  enables  him  to  remove  every  drop  of  super* 
fluous  water  without  drawing  the  animal  into  the  syringe.  When,  on 
the  other  hand,  it  is  desired  to  isolate  a  particular  animal  from  a  number 
of  others,  the  syringe  may  be  conveniently  used,  after  the  same  fashion, 
to  draw  it  up  and  transfer  it  to  another  slide;  care  being,  of  course, 
taken  that  the  syringe  so  employed  has  a  sufficient  aperture  to  receive  it 
freely.  —  If  it  be  wished  to  have  recourse  to  compression,  for  the  expansion 
or  flattening  of  the  object,  this  may  be  made  upon  the  ordinary  slide,  by 
pressing  down  the  thin-glass  cover  with  a  pointed  stick;  and  this  method 
which  allow  the  pressure  to  be  applied  at  the  spot  where  it  is  most  re- 
quired, will  generally  be  found  preferable  for  delicate  portions  of  tis- 
sue which  are  easily  spread  out,  and  which,  in  fact,  require  little  other 
compression  than  is  afforded  by  the  weight  of  the  glass  cover,  and  by  the 
capillary  attraction  which  draws  it  into  proximity  with  the  slide  beneath. 
A  firmer  and  more  enduring  pressure  may  be  exerted  by  the  dexterous 
management  of  a  well-constructed  Aquatic  Box;  and  this  method  is  pecu- 
liarly valuable  for  confining  the  movements  of  minute  animals,  so  as  to 
keep  them  at  rest  under  the  field  of  the  microscope,  without  killing 
them.  It  is  where  a  firm  but  graduated  pressure  is  required,  for  the 
flattening-out  of  the  bodies  of  thin  semi-transparent  animals,  without 
the  necessity  of  removing  them  from  the  field  of  the  Microscope,  that 
the  Compressor  is  most  useful. 

142.  In  whatever  way  the  Object  is  submitted  to  examination,  it  must 
be  first  brought  approximately  into  position,  and  supported  there,  just 
as  if  it  were  in  a  mounted  Slide.    The  precise  mode  of  effecting  this  will 


MANAGEMENT  OF  THE  MICROSCOPE. 


143 


differ,  according  to  the  particular  plan  of  the  instrument  employed- 
thus,  in  some  it  is  only  the  ledge  itself  that  slides  along  the  stage*  in 
others  it  is  a  carriage  of  some  kind,  whereon  the  object-slide  rests;  in 
others,  again,  it  is  the  entire  platform  itself  that  moves  upon  a  fixed 
plane  beneath.  Having  guided  his  object,  as  nearly  as  he  can  do  by  the 
unassisted  eye,  into  its  proper  place,  the  Microscopist  then  brings  his 
light  (whether  natural  or  artificial)  to  bear  upon  it,  by  turning  the 
Mirror  in  such  a  direction  as  to  reflect  upon  its  under  surface  the  rays 
which  are  received  by  itself  from  the  sky  or  the  lamp.  The  concave 
mirror  is  that  which  should  always  be  first  employed,  the  plane  being 
reserved  for  special  purposes;  and  it  should  bring  the  rays  to  convergence 
in  or  near  the  plane  in  which  the  object  lies  (Fig.  112).  The  distance 
at  which  it  should  be  ordinarily  set  beneath  the  stage,  is  that  at  which  it 
brings  parallel  rays  to  a  focus;  but  this  distance  should  be  capable  of 
elongation,  by  the  lengthening  of  the  stem  to  which  the  mirror  is 
attached,  since  the  rays  diverging  from  a  lamp  at  a  short  distance  are 
not  so  soon  brought  to  a  focus.  The  correct  focal  adjustment  of  the 
Mirror  may  be  judged  by  its  formation  of  images  of  window-bars,  chim- 
neys, etc.,  upon  any  semi-transparent  medium  placed  in  the  plane  of  the 
object.  It  is  only,  however,  when  small  objects  are  being  viewed  under 
high  magnifying  powers,  that  such  a  concentration  of  the  light  reflected 
by  the  Mirror  is  either  necessary  or  desirable;  for,  with  large  objects 
seen  under  low  powers,  the  field  would  not  in  this  mode  be  equally 
illuminated.  The  diffusion 
of  the  light  over  a  larger  area 
may  be  secured,  either  by 
shifting  the  Mirror  so  much 
above  or  so  much  below  its 
previous  position,  that  the 
pencil  will  fall  upon  the  object 
whilst  still  converging,  or 
after  it  has  met  and  diverged; 
or,  on  the  other  hand,  by  the 
interposition  of  a  disk  of 
Ground-glass  in  the  course  of 
the  converging  pencil, — this 
method  which  is  peculiarly 
appropriate  to  lamp-light,  be- 
ing very  easily  had  recourse 
to,  if  the  diaphragm-plate 
have  had  its  larger  aperture 

fitted   to  receive  such  a  disk   Arrangement  of  Microscope  for  Transparent  Objects. 

(§  98).    The  eye  being  now 

applied  to  the  Eye-piece,  and  the  body  being  ^focussed,^  the  object  is  to 
be  brought  into  the  exact  position  required  by  the  use  of  the  transversing 
movement,  if  the  stage  be  provided  with  it;  if  not,  by  the  use  of  the 
two  hands,  one  moving  the  object-slide  from  side  to  side,  the  other 
pushing  the  ledge,  fork,  or  holder  that  carries  it,  either  forwards  or 
backwards  as  may  be  required. — It  is  always  to  be  remembered,  in 
making  such  adjustments  by  the  direct  use  of  the  hands,  that,  owing  to 
the  inverting  action  of  the  Microscope,  the  motion  to  be  given  to  the 
object,  whether  lateral  or  vertical,  must  be  precisely  opposed  to  that 
which  its  image  seems  to  require,  save  when  Erectors  (§§  84,  86)  are 
employed.    When  the  object  has  been  thus  brought  fully  into  view,  the 


144:  THE  MICROSCOPE  AND  ITS  REVELATIONS. 

Mirror  may  require  a  more  accurate  adjustment.  What  should  be 
aimed-at  is  the  diffusion  of  a  clear  and  equable  light  over  the  entire  field; 
and  the  observer  should  not  be  satisfied  until  he  has  attained  this  end. 
If  the  field  should  be  darker  on  one  side  than  on  the  other,  the  Mirror 
should  be  slightly  turned  in  such  a  direction  as  to  throw  more  light  upon 
that  side;  perhaps  in  so  doing,  the  light  may  be  withdrawn  from  some 
part  previously  illuminated;  and  it  may  thus  be  found  that  the  pencil  is 
not  large  enough  to  light  up  the  entire  field.  This  may  be  owing  to  one 
of  three  causes:  either  the  cone  of  rays  may  be  received  by  the  object  too 
near  to  its  focal  apex,  the  remedy  for  which  lies  in  an  alteration  in  the  dis- 
tance of  the  Mirror  from  the  stage;  or,  from  the  very  oblique  position  of 
the  mirror,  the  cone  is  too  much  narrowed  across  one  of  its  diameters, 
and  the  remedy  must  be  sought  in  a  change  in  the  position  either  of  the 
Microscope  or  of  the  Lamp,  so  that  the  face  of  the  Mirror  may  not  be 
turned  so  much  away  from  the  axis  of  vision;  or,  again,  from  the  centre 
of  the  Mirror  being  out  of  the  optic  axis  of  the  instrument,  so  that  the 
illuminating  cone  is  projected  obliquely, — an  error  which  can  be  rectified 
without  the  least  difficulty.  If  the  cone  of  rays  should  come  to  a  focus 
in  the  object,  the  field  is  not  unlikely  to  be  crossed  (in  the  day-time)  by 
window-bars  or  chimneys,  or  (at  night)  the  form  of  the  lamp-flame  may 
be  distinguished  upon  it;  the  former  must  be  got  rid  of  by  a  slight 
change  in  the  inclination  of  the  Mirror;  and  if  the  latter  cannot  be  dis- 
sipated in  the  same  way,  the  lamp  should  be  brought  a  little  nearer. 

143.  The  equable  illumination  of  the  entire  field  having  been  thus 
obtained,  the  quantity  of  light  to  be  admitted  should  be  regulated  by  the 
Diaphragm-plate  (§  98).  This  must  depend  very  much  upon  the  nature 
of  the  object,  and  upon  the  intensity  of  the  light.  Generally  speaking, 
the  more  transparent  the  object,  the  less  light  does  it  need  for  its  most 
perfect  disjolay;  and  its  most  delicate  markings  are  frequently  only  made 
visible,  when  the  major  part  of  the  cone  of  rays  has  been  cut  off.  Thus 
the  movement  of  the  cilia — those  minute  vibratile  filaments  with  which 
almost  every  Animal  is  provided  in  some  part  of  its  organism,  and  which 
many  of  the  humbler  Plants  also  possess  in  the  early  stages  of  their  exist- 
ence— can  only  be  discerned  in  many  instances  when  the  light  is  admitted 
through  the  smallest  aperture.  On  the  other  hand,  the  less  transparent 
objects  usually  require  the  stronger  illumination  which  is  afforded  by  a 
wider  cone  of  rays;  and  there  are  some  (such  as  semi-transparent  sections 
of  Fossil  Teeth)  which,  even  when  viewed  with  low  powers,  are  better 
seen  with  the  intenser  light  afforded  by  the  Achromatic  Condenser. — 
In  every  case  in  which  the  object  presents  any  considerable  obstruction 
to  the  passage  of  the  rays  through  it,  great  care  should  be  taken  to  pro- 
tect it  entirely  from  incident  light;  since  this  extremely  weakens  the 
effect  of  that  which  is  received  into  the  Microscope  by  transmission.  It 
is  by  daylight  that  this  interference  is  most  likely  to  occur;  since,  if  the 
precautions  already  given  (§  132)  respecting  the  use  of  lamp-light  be 
observed,  no  great  amount  of  light  can  fall  upon  the  upper  surface  of  the 
object.  The  observer  will  be  warned  that  such  an  effect  is  being  pro- 
duced, by  perceiving  that  there  is  a  want  not  only  of  brightness  but  of 
clearness  in  the  image,  the  field  being  veiled,  as  it  were,  by  a  kind  of 
thm  vapor;  and  he  may  at  once  satisfy  himself  of  the  cause,  by  inter- 
posing his  hand  between  the  stage  and  the  source  of  light,  when  the 
immediate  increase  of  brilliancy  and  distinctness  will  reveal  to  him  the 
source  of  the  previous  deficiency  in  both.  Nothing  more  is  necessary  for 
its  permanent  avoidance,  than  the  interposition  of  an  opaque  screen 


MANAGEMENT  OF  THE  MICROSCOPE. 


146 


(blackened  on  the  side  towards  the  stage)  between  the  window  and  the 
object;  care  being  of  course  taken  that  the  screen  does  not  interfere  with 
the  passage  of  light  to  the  mirror.  Such  a  screen  may  be  easily  shaped 
and  adapted  either  to  be  carried  by  the  stage  itself,  or  by  the  stand  for 
the  condenser;  but  it  is  seldom  employed  by  Microscopists,  as  it  inter- 
feres with  access  to  the  left  side  of  the  stage;  and  the  interposition  of 
the  hand,  so  often  as  it  may  be  needed,  is  more  frequently  had  recourse 
to  in  preference,  as  the  more  convenient  expedient.  The  young  Micro- 
scopist  who  may  be  examining  transparent  objects  by  daylight,  is  recom- 
mended never  to  omit  ascertaining  whether  the  view  which  he  may 
obtain  of  them  is  in  any  degree  thus  marred  by  incident  light. 

144.  Although  the  illumination  afforded  by  the  Mirror  alone  is  quite 
adequate  for  a  very  large  proportion  of  the  purposes  for  which  the  Micro- 
scope may  be  profitably  employed  (nothing  else  having  been  used  by 
many  of  those  who  have  made  most  valuable  contributions  to  Science  by 
means  of  this  instrument),  yet,  when  high  magnifying  powers  are 
employed,  and  sometimes  even  when  but  a  very  moderate  amplification 
is  needed,  great  advantage  is  gained  from  the  use  of  a  Condenser.  The 
form  which  has  been  described  under  the  name  of  the  Webster  Condenser 
(§  100)  answers  so  well  for  most  purposes,  and  may  in  addition  be  so 
easily  converted  into  a  ^ black  ground^  Illuminator,  that  the  working 
Microscopist  will  find  it  convenient  to  keep  it  always  in  place;  substitut- 
ing an  Achroinatic  Condenser  of  greater  power  (§  99)  only  when  specially 
needed.  Special  care  is  needed  in  the  use  of  this  last,  both  as  to  the 
coincidence  of  its  optic  axis  with  that  of  the  Microscope  itself,  and  as  to 
its  focal  distance  from  the  object.  The  centering  may  be  most  readily 
accomplished  by  so  adjusting  the  distance  of  the  Condenser  from  the 
Stage  (by  the  rack-and-pinion  action  or  the  sliding  movement  with  which 
it  is  always  provided),  that  a  sharp  circle  of  light  shall  be  thrown  on  any 
semi-transparent  medium  laid  upon  it;  then,  on  this  being  viewed 
through  the  Microsope  with  an  Objective  of  sufficiently  low  power  to  take 
in  the  whole  of  it,  if  this  circle  be  not  found  concentric  with  the  field  of 
view,  the  axis  of  the  Condenser  must  be  altered  by  means  of  the  milled- 
head  tangent-screws  with  which  it  is  provided.  Or  a  cap  with  a  minute 
central  aperture  may  be  fitted  on  the  top  of  the  Condenser,  and  this 
aperture  centered  in  the  field  of  an  objective  of  medium  power.  Or, 
again,  a  diaphragm  with  a  very  minute  central  perforation  may  be  placed 
at  a  little  distance  beneath  the  Achromatic  Condenser,  and  the  image  of 
this  may  be  brought  into  the  centre  of  the  field  of  a  l-4th  objective, 
which  is  the  best  arrangement  when  it  is  to  be  used  with  very  high  powers. 
The  focal  adjustment  of  the  Condenser,  on  the  other  hand,  must  be  made 
under  the  Objective  which  is  to  be  employed  in  the  examination  of  the 
object,  by  turning  the  Mirror  in  such  a  manner  as  to  throw  upon  the 
visual  image  of  the  object  (previously  brought  into  the  focus  of  the  Micro- 
scope) an  image  of  a  chimney  or  a  window-bar,  if  daylight  be  employed, 
or  of  the  top,  bottom,  or  edge  of  the  lamp-flame,  if  lamp-light  be  in  use; 
the  focus  of  the  condenser  should  then  be  so  adjusted  as  to  render  the 
view  of  this  as  distinct  as  possible;  and  the  direction  of  the  Mirror  should 
then  be  sufficiently  changed  to  displace  the  image,  and  to  substitute  for 
it  the  clearest  light  that  can  be  obtained.  It  will  generally  be  found,  how- 
ever, that  although  such  an  exact  focussing  gives  the  most  perfect  results 
by  Daylight,  yet  that  by  Lamp-light  the  best  illumination  is  obtained 
when  the  Condenser  is  removed  to  a  somewhat  greater  distance  from  the 
object,  than  that  at  which  it  gives  a  distinct  image  of  the  lamp.  In  every 
10 


146 


THE  MICROSCOPE   AND  ITS  REVELATIONS. 


case,  indeed,  in  which  it  is  desired  to  ascertain  the  effect  of  variety  in 
the  method  of  illumination,  the  effects  of  alterations  in  the  distance  of 
the  condenser  from  the  object  should  be  tried;  as  it  will  often  happen 
that  delicate  markings  become  visible  when  the  condenser  is  a  little  out 
of  focus,  which  cannot  be  distinguished  when  it  is  precisely  in,  focus. 
The  regulation  of  the  amount  of  light  transmitted  through  the  object  is 
often  of  the  very  first  importance;  and  no  means  of  accomplishing  this 
is  so  convenient  as  a  Graduating  or  Iris  Diaphragm  (§  98).  For  some 
objects  of  great  transparence,  the  White-Cloud  illumination  (§  109)  may 
be  had  recourse  to  with  advantage. 

145.  There  are  many  Transparent  Objects,  however,  whose  pecu- 
liar features  can  only  be  distinctly  made  out,  when  they  are  viewed  by 
light  transmitted  through  them  obliquely  instead  of  axially;  and  this  is 
especially  the  case  with  such  as  have  their  surfaces  marked  by  very  deli- 
cate and  closely-approximated  furrows,  the  direction  of  the  oblique  rays 
being  then  a  matter  of  primary  importance.  Thus,  suppose  that  an 
object  be  marked  by  longitudinal  striae  too  delicate  to  be  seen  by  ordinary 
direct  light;  the  oblique  light  most  fitted  to  bring  them  into  view  will  be  that 

^  proceeding  in  either  of  the  directions  c  or  D;  that  which 

falls  upon  it  in  the  directions  A  and  B  tending  to  obscure 
the  striae  rather  than  to  disclose  them.    But  if  the  striae 
^  ^  should  be  due  to  furrows  or  prominences  which  have  one 

side  inclined  and  the  other  side  abrupt,  they  will  not  be 
B  brought  into  view  indifferently  by  light  from  C  or  from  D, 

but  will  be  shown  best  by  that  which  makes  the  strongest  shadow; 
hence,  if  there  be  a  projecting  ridge,  with  an  abrupt  side  looking 
towards  c,  it  will  be  best  seen  by  light  from  d;  whilst  if  there  be  a 
furrow  with  a  steep  bank  on  the  side  of  c,  it  will  be  by  light  from  that 
side  that  it  will  be  best  displayed.  But  it  is  not  at  all  unfrequent  for 
the  longitudinal  striae  to  be  crossed  by  others;  and  these  transverse  striae 
will  usually  be  best  seen  by  the  light  that  is  least  favorable  for  the 
longitudinal;  so  that,  in  order  to  bring  them  into  distinct  view,  either 
the  illuminating  pencil  or  the  object  must  be  moved  a  quarter  round. 
The  simplest  mode  of  obtaining  this  end,  is  to  make  the  Mirror  capable 
of  being  turned  into  such  a  position  as  to  reflect  light  into  the  object 
from  one  side  and  at  a  very  oblique  angle  (which  is  best  done  by  the 
Zentmayer  arrangement);  and  to  give  the  Stage  a  rotatory  movement,  so 
that  the  object  may  be  presented  to  that  light  under  every  azimuth. 

146.  For  objects  of  greater  difficulty,  however,  it  is  better  to  have 
recourse  to  the  Accessories  already  described  (§§  101-108),  which  are 
specially  provided  to  furnish  oblique  illumination  in  the  most  effectual 
manner.  A  good  example  of  the  variety  of  appearances  which  the  same 
object  may  exhibit,  when  illuminated  from  different  azimuths,  and  with 
slight  changes  of  focussing,  is  shown  in  Fig.  113,  which  represents  por- 
tions of  a  valve  of  Pleurosigma  formosum  as  seen  under  a  power  of  1300 
diameters;  the  markings  shown  at  A,  B,  and  c,  being  brought-out  by  ollique 
light  in  different  directions,  which,  however,  when  carefully  used,  does 
not  produce  these  erroneous  aspects;  whilst  at  D,  is  shown  the  effect  of 
axial  illumination  with  the  Achromatic  Condenser. — It  cannot  be  too 
strongly  impressed  on  the  young  Microscopist,  however,  that  the  special 
value  of  very  oblique  illumination  is  limited  to  the  resolution  of  '  test- 
objects;'  and  that  for  the  ordinary  purposes  of  scientific  study  and 
research,  axial  illumination  is  generally  preferable.  As  in  regard  to  the 
qualities  of  Objectives  (§  55),  so  in  respect  to  Illumination,  may 


MANAGEMENT  OF  THE  MICROSCOPE. 


147 


Fig, 


Biological 


ifc  be  confidently  asserted  that  tlie  solution  of  the  most  difficult  Biological 
problems  to  which  the  Microscope  has  been  yet  applied,  lias  been  attained 
by  arrangements  by  no  means  the  most  favorable  to  the  discernment 
of  the  markings  on  Diatom-valyes  or  the  lines  on  Robert's  test-plate;  and 
that,  converseTy,  the  arrangements  specially 
effective  for  the  ^resolutions  of  the  most 
difficult  U7iecl  ^  tests  ^  have  not,  as  yet,  been 
shown  to  have  much  value  in  "  "  " 
investigation  (§  158). 

147.  There  are  many  kinds  of  Transpa- 
rent objects — especially  such  as  either  con- 
sist of  thin  plates,  disks,  or  spicules  of 
Siliceous  or  Calcareous  matter,  or  contain 
such  bodies — which  are  peculiarly  well  seen 
under  the  Black-ground  illumination  (§§ 
104,  105);  for  not  only  does  the  brilliant 
luminosity  which  they  then  present,  in 
contrast  with  the  dark  ground  behind  them, 
show  their  forms  'to  extraordinary  advan- 
tage; but  this  mode  of  illumination  imparts 
to  them  an  appearance  of  solidity  which 
they  do  not  exhibit  by  ordinary  transmitted 
light  (§  103);  and  it  also  frequently  brings 
out  surface-markings  which  are  not  other- 
wise distinguishable.  Hence  when  any  ob- 
ject is  under  examination  that  can  be  sup- 
posed to  be  a  good  subject  for  this  method, 
the  trial  of  it  should  never  be  omitted.  For 
low  powers,  the  use  of  the  Spot-lens  or  the 
Webster  Condenser  with  the  central  stop 
will  be  found  suflSciently  satisfactory;  for 
the  higher,  the  Paraboloid  or  the  Eeflex  Il- 
luminator should  be  employed. — Similar 
general  remarks  may  be  made  respecting 
the  examination  of  objects  by  Polarized  y^^^^      Pieurosigma  formosum, 

light  (S  110).     Some  OI  the  most  strikms^  with  portions  a,  b,  c,  d,  showing  diverse 

effects  of  this  kind  of  illumination  are  "^'"'^ 

produced  upon  bodies  whose  particles  have  a  crystalline  aggregation; 
and  hence  it  may  often  be  employed  with  great  advantage  to  bring 
such  bodies  into  view,  when  they  would  not  otherwise  be  distinguished: 
thus,  for  example,  the  r aphides  of  Plants  are  much  more  clearly  made 
out  by  its  means,  in  the  midst  of  the  tissues,  than  they  can  be  by  any 
other.  But  the  peculiar  effects  of  Polarized  light  are  also  exerted  upon  a 
great  number  of  other  Organized  substances,  both  animal  and  vegetable; 
and  it  often  reveals  differences  in  the  arrangement  or  in  the  relative  den- 
sity of  their  component  particles,  the  existence  of  which  would  not  other- 
wise have  been  suspected;  hence  the  Microscopist  will  do  well  to  have 
recourse  to  it,  whenever  he  may  have  the  least  suspicion  that  its  use  can 
give  him  an  additional  power  of  discrimination. 

148.  Arrangement  for  Opaque  Objects, — There  are  many  objects  of 
the  most  interesting  character,  the  opacity  of  which  entirely  forbids  the 
transmission  of  light  through  them,  and  of  which,  therefore,  the  surfaces 
only  can  be  viewed  by  means  of  the  incident  rays  which  they  reflect. 
These  are,  for  the  most  part,  objects  of  comparatively  large  dimensions. 


148 


THE  MICROSCOPE  AND  ITS  REVELATIONS. 


for  which  a  low  magnifying  power  suffices;  and  it  is  specially  important, 
in  the  examination  of  such  objects,  not  to  use  a  lens  of  shorter  focus  than  is 
absolutely  necessary  for  discerning  the  details  of  the  structure;  since,  the 
longer  the  focus  of  the  Objective  employed,  the  less  is  the  indistinctness 
produced  by  inequalities  of  the  surface,  and  the  larger,  too,  may  be  its 
aperture,  so  as  to  admit  a  greater  quantity  of  light,  to  the  great  improve- 
ment of  the  brightness  of  the  image.  Objectives  of  long  focus  are  espe- 
cially required  in  Microscopes  that  are  to  be  used  for  Educational  purposes;^ 
and  an  endless  variety  of  ^  common  objects '  suitable  to  these  may  be  found 
by  such  as  will  take  the  trouble  to  search  for  them. — The  mode  of  bringing 
Opaque  objects  under  view  will  differ  according  to  their  ^mounting,'  and 
to  the  manner  in  which  it  is  desired  to  illuminate  them.  If  the  object 
be  mounted  in  a  ^  slide '  of  glass  or  wood,  upon  a  large  Opaque  surface,  the 
slide  must  be  laid  on  the  stage  in  the  usual  manner,  and  the  object  brought 
as  nearly  as  possible  into  position  by  the  eye  alone  (§  141).  If  it  be  not 
so  mounted  it  may  be  simply  laid  upon  the  glass  Stage-plate,  resting 
against  its  ledge;  and  the  Diaphragm-plate  must  then  be  so  turned  as  to 
afford  it  a  black  background,  light  being  thrown  upon  it  by  a  Condensing 
Lens  or  Bull's-eye  placed  as  in  Fig.  114,  or  (still  better)  by  Beck's  Para- 
bolic Speculum,  which  gives  a  far  better  illumination  by  diffused  daylight 


^j^j^j^  ^yji^      determined  by 

Arrangement  of  Microscope  for  Opaque  Objects.  the  size  of  the  SUrface  to  be  il- 
luminated and  by  the  kind  of 
light  required.  If  the  magnifying  power  employed  be  high,  and  the 
field  of  view  be  consequently  limited,  it  will  be  desirable  so  to  adjust  the 
lens  as  to  bring  the  cone  of  rays  to  a  point  upon  the  part  of  the  object 
under  examination;  and  this  adjustment  can  only  be  rightly  made  whilst 
the  object  is  kept  in  view  under  the  Microscope,  the  condenser  being 
moved  in  various  modes  until  that  position  has  been  found  for  it  in 
which  it  gives  the  best  light.  If,  on  the  other  hand,  the  power  be  low, 
and  it  be  desired  to  spread  the  light  equably  over  a  large  field,  the  Con- 
denser should  be  placed  either  within  or  beyond  its  focal  distance;  and 

^  The  makers  of  Educational  Microscopes  supply  at  a  small  cost,  single  (triplet) 
combinations  of  3  inches,  2  inches,  or  1^  inch  focus,  or  dividing  combinations  of 
half  inch  and  1  inch,  1  inch  and  2  inchs,  or  inch  and  3  inches,  which  are  quite 
adequate  for  ordinary  requirements. 


distance  from  the  object 


than  can  be  obtained  by  any 
other  means  yet  devised,  and 
which  is  equally  well  adapted 
to  lamp-light,  when  used  in 
combination  with  the  Bull's- 
eye  (§  114).  Direct  sunlight 
cannot  be  employed  without 
the  production  of  an  injuri- 
ous glare,  and  the  risk  of 
burning  the  object;  but  the 
sunlight  reflected  from  a 
bright  cloud  is  the  best  light 
possible.  When  a  Condens- 
ing Lens  is  used,  it  should 
always  be  placed  at  right 
angles  to  the  direction  of  the 
illuminating  rays,  and  at  a 


MANAGEMENT  OF  THE  MICROSCOPE. 


149 


here,  too,  the  best  position  will  be  ascertained  by  trial.  It  will  often  be 
desirable  also  to  vary  both  the  obliquity  of  the  light  and  the  direction 
in  which  it  falls  upon  the  object;  the  aspect  of  which  is  greatly  affected 
by  the  manner  in  which  the  shadows  are  projected  upon  its  surface,  and 
in  which  the  lights  are  reflected  from  the  various  points  of  it.  Many 
objects,  indeed,  which  are  distinguished  by  their  striking  appearance 
when  the  light  falls  upon  them  on  one  side,  are  entirely  destitute  both 
of  brilliancy  of  color  and  of  sharpness  of  outline  when  illuminated  from 
the  opposite  side.  Hence  it  is  always  desirable  to  try  the  effect  of  chang- 
ing the  position  of  the  object;  which,  if  it  be  ^mounted,'  may  be  first 
shifted  by  merely  reversing  the  place  of  the  two  ends  of  the  slide,  and  then, 
if  this  be  not  satisfactory,  may  be  more  completely  as  well  as  more  gradu- 
ally altered  by  making  the  object-platform  itself  to  rotate.  With  regard 
to  the  obliquity  of  the  illuminating  rays,  it  is  well  to  remark,  that  if  the 
object  be  ^  mounted '  under  a  glass  cover,  and  the  incident  rays  fall  at 
too  great  an  angle  with  the  perpendicular,  a  large  proportion  of  them 
will  be  reflected,  and  the  brilliancy  of  the  object  will  be  greatly  impaired; 
and  hence  when  Opaque  objects  are  being  examined  under  high  powers 
with  a  very  oblique  illuminating  pencil,  they  should  always  be  uncovered. 

149.  The  same  general  arrangement  must  be  made  when  Artificial 
light  is  used  for  the  illumination  of  Opaque  objects;  the  Lamp  being 
placed  in  such  a  position  in  regard  to  the  Stage  that  its  rays  may  fall  in 
the  direction  indicated  in  Fig.  114,  and  these  rays  being  collected  and 
concentrated  by  the  Condenser,  as  already  directed.  Since  the  rays  pro- 
ceeding from  a  lamp  within  a  short  distance  are  already  diverging,  they 
will  not  be  brought  by  the  Condenser  to  such  speedy  convergence  as  are 
the  parallel  rays  of  daylight;  and  it  must,  therefore,  be  farther  removed 
from  the  object  to  produce  the  same  effect.  By  modifying  the  distance 
of  the  Condenser  from  the  lamp  and  from  the  object  respectively,  the 
cone  of  rays  may  be  brought  nearly  to  a  focus,  or  it  may  be  spread  almost 
equably  over  a  large  surface,  as  may  be  desired.  And  the  same  effect 
may  be  produced  by  shifting  the  position  of  the  Condenser,  when  the 
Parabolic  Speculum  is  employed  in  combination  with  it.  No  more 
effective  illumination  can  be  desired  for  objects  viewed  under  the  low 
powers  to  which  the  Parabolic  Speculum  is  adapted,  than  that  which  is 
afforded  by  this  combination;  the  Beckett  Lamp  (Fig.  108)  supplying  a 
most  convenient  means  of  using  it,  as  the  Author  can  testify  from  a  very 
large  experience.  In  the  illumination  of  Opaque  objects.  Artificial  light 
has  the  advantage  over  ordinary  daylight,  of  being  more  easily  concen- 
trated to  the  precise  degree,  and  of  being  more  readily  made  to  fall  in 
the  precise  direction,  that  may  be  found  most  advantageous.  Moreover, 
the  contrast  of  light  and  shadow  will^be  more  strongly  marked  when  no 
light  falls  upon  the  object  except  that  proceeding  from  the  Lamp  used 
for  its  illumination,  than  it  can  be  when  the  shadows  are  partially  light- 
ened by  the  rays  which  fall  upon  the  object  from  every  quarter,  as  must 
be  the  case  if  it  be  viewed  by  daylight. — If  a  more  concentrated  light  be 
required,  the  flame  of  the  lamp  may  be  turned  edgewise  to  the  object, 
and  the  small  Condensing-lens  may  be  used  in  combination  with  the 
Bull's  eye  ;  being  so  placed  as  to  receive  the  cone  projected  by  it,  and  to' 
bring  its  rays  to  a  more  exact  convergence.  It  was  in  this  way  that  Mr.l 
Beck  obtained  the  views  of  the  Fodura'scsle  given  in  plate  II.,  Figs.  4, 
5.  In  this  manner  very  minute  bodies  may  be  viewed  as  Opaque  objects 
under  high  magnifying  powers,  provided  that  the  brasswork  of  the  ex- 
tremities of  the  Objectives  be  so  bevelled-off  as  to  allow  the  illuminating 


150 


THE  MICROSCOPE  AND  ITS  REVELATIONS. 


cone  to  have  access  to  the  object.  As  none  but  a  yery  oblique  illumina- 
tion, however,  can  be  thus  obtained,  the  view  of  the  object  will  be  by  no 
means  complete,  unless  it  be  supplemented  by  that  which  maybe  obtained 
by  means  of  the  Vertical  Illummator  (§  116),  which  supplies  for  high 
powers  the  kind  of  illumination  that  is  given  by  the  Lieberkuhn  for  the 
lower. 

150.  There  are  many  Opaque  objects,  such  as  Foraminifera,  wh  h 
it  is  desirable  to  view  from  all  sides,  in  order  that  their  features  may  be 
completely  made  out.  This  may  be  readily  done  with  objects  mounted 
in  slides,  when  the  Microscope  is  provided  with  the  Zentmayer  stage,  by 
inclining  the  stage  to  one  side  or  the  other  (first  taking  care  that  the  ob- 
ject is  well  secured  upon  it),  and  then  giving  rotation  to  the  object-plat- 
form. For  such  objects  as  can  be  conveniently  attached  to  small  disks. 
Beck's  Disk-holder  (Fig.  94),  affords  by  far  the  most  convenient  and  effect- 
ive mode  of  presenting  them  in  every  variety  of  aspect;  but  the  disks  may 
also  be  held  by  attached  pins,  either  in  the  Stage-forceps,  or  by  the  inser- 
tion of  the  pins  into  the  cork-box  at  its  other  end  (§  118),  a  variety  of 
movements  being  given  in  either  case  by  turning  the  forceps  in  its  tube. 
So,  again,  many  small  objects,  such  as  parts  of  Insects,  may  be  grasped 
in  the  Stage-forceps  itself,  and  by  a  little  care  in  manipulation,  each 
aspect  may  be  brought  into  view  successively.  In  either  of  these  cases, 
the  Lieberkuhn  may  be  employed  for  their  illumination;  and  light  of  con- 
siderable obliquity  may  be  obtained  by  its  means,  either  by  turning  the 
Mirror  out  of  the  axis,  or  by  covering  part  of  the  reflecting  surface  of  the 
Lieberkiihn  by  a  cap,  or  by  a  combination  of  both  methods.  Whenever 
the  Lieberkuhn  is  employed,  care  must  be  taken  that  the  direct  light  from 
the  Mirror  is  entirely  stopped-out  by  the  interposition  of  a  ^  dark  well  ^ 
or  of  a  black  disk,  of  such  a  size  as  to  fill  the  field  given  by  the  partic- 
ular Objective  employed,  but  not  to  pass  much  beyond  it.  Opaque  objects 
that  are  permanently  mounted  either  upon  cardboard  disks,  or  in  the 
slides  specially  provided  for  them,  may  be  presented  to  the  Microscope 
in  a  considerable  variety  of  directions  by  means  of  Morris's  Object-holder 
(Fig.  95);  which,  however,  can  only  be  employed  with  side-illumination. 
If  it  be  desired  to  make  the  most  advantageous  use  of  this  appliance,  ob- 
jects mounted  in  slides  should  be  so  placed  that  the  parts  to  be  brought 
into  view  by  its  tilting  movement  may  look  towards  the  long  edges  of  the 
slide ;  since  it  is  obvious  that  a  much  greater  inclination  may  be  given  to 
it  in  either  of  these  directions,  than  in  the  direction  of  either  of  its  ex- 
tremities. 

151.  Errors  of  Interpretation. — The  correctness  of  the  conclusions 
which  the  Microscopist  will  draw  regarding  the  nature  of  any  object, 
from  the  visual  appearances  which  it  presents  to  him  when  examined  in 
the  various  modes  now  specified,  will  necessarily  depend  in  a  great  degree 
upon  his  previous  experience  in  Microscopic  observation,  and  upon  his 
knowledge  of  the  class  of  bodies  to  which  the  particular  specimen  may 
belong.  Not  only  are  observations  of  any  kind  liable  to  certain  fallacies, 
arising  out  of  the  previous  notions  which  the  observer  may  entertain  in 
regard  to  the  constitution  of  the  objects  or  the  nature  of  the  actions  to 
which  his  attention  is  directed,  but  even  the  most  practised  observer  is 
apt  to  take  no  note  of  such  phenomena  as  his  mind  is  not  prepared  to 
appreciate.  Errors  and  imperfections  of  this  kind  can  only  be  corrected, 
it  is  obvious,  by  general  advance  in  scientific  knowledge;  but  the  history 
of  them  affords  a  useful  warning  against  hasty  conclusions  drawn  from  a 
too  cursory  examination.    If  the  history  of  almost  any  scientific  investi' 


MANAGEMENT  OF  THE  MICROSCOPE. 


151 


gation  were  fully  made  known,  it  would  generally  appear  that  the  sta- 
bility and  completeness  of  the  conclusions  finally  arrived-at  had  only  been 
attained  after  many  modifications,  or  even  entire  alterations,  of  doctrine. 
And  it  is,  therefore,  of  such  great  importance  as  to  be  almost  essential  to 
the  correctness  of  our  conclusions,  that  they  should  not  be  finally  formed 
and  announced  until  they  have  been  tested  in  every  conceivable  mode. 
I  It  is  due  to  Science  that  it  should  be  burdened  with  as  few  false  facts  and 
false  doctrines  as  possible.  It  is  due  to  other  truth-seekers  that  they 
should  not  be  misled,  to  the  great  waste  of  their  time  and  pains,  by  our 
errors.  And  it  is  due  to  ourselves  that  we  should  not  commit  our  repu- 
tation to  the  chance  of  impairment,  by  the  premature  formation  and 
publication  of  conclusions,  which  may  be  at  once  reversed  by  other  ob- 
servers better  informed  than  ourselves,  or  may  be  proved  to  be  fallacious 
at  some  future  time,  perhaps  even  by  our  own  more  extended  and  care- 
ful researches.  The  suspension  of  the  judgment,  whenever  there  seems 
room  for  doubt y  is  a  lesson  inculcated  by  all  those  Philosophers  who  have 
gained  the  highest  repute  for  practical  wisdom;  and  it  is  one  which  the 
Microscopist  cannot  too  soon  learn,  or  too  constantly  practise. — Besides 
these  general  warnings,  however,  certain  special  cautions  should  be  given 
to  the  young  Microscopist,  with  regard  to  errors  into  which  he  is  liable 
to  be  led,  even  when  the  very  best  instruments  are  employed. 

152.  Errors  of  interpretation  arising  from  the  imperfection  of  the 
focal  adjustment  are  not  at  all  uncommon  amongst  young  Microscopists. 
With  lenses  of  high  power,  and  especially  with  those  of  large  angular 
aperture,  it  very  seldom  happens  that  all  the  parts  of  an  object,  however 
minute  and  flat  it  may  be,  can  be  in  focus  together;  and  hence,  when  the 
focal  adjustment  is  exactly  made  for  one  part,  everything  that  is  not  in 
exact  focus  is  not  only  more  or  less  indistinct,  but  is  often  wrongly  repre- 
sented. The  indistinctness  of  outline  will  sometimes  present  the  appear- 
ance of  a  pellucid  border,  which,  like  the  diffraction-band,  may  be  mis- 
taken for  actual  substance.  But  the  most  common  error  is  that  which 
is  produced  by  the  reversal  of  the  lights  and  shadows  resulting  from  the 
refractive  powers  of  the  object  itself;  thus,  the  bi-concavity  of  the  blood- 
disks  of  Human  (and  other  Mammalian)  Blood  occasions  their  centres  to 
appear  dark  when  in  the  focus  of  the  Microscope,  through  the  divergence 
of  the  rays  which  it  occasions;  but  when  they  are  brought  a  little  within 
the  focus  by  a  slight  approximation  of  the  object-glass,  the  centres  appear 
brighter  than  the  peripheral  parts  of  the  disks.  An  opposite  reversal 
presents  itself  in  the  markings  of  certain  Diatoms^  which,  like  Fleuro- 
sigma  angulatum,  present,  when  ex- 
actly focussed,  the  aspect  of  rows  of 
hemispherical  beads  (Fig.  166,  a). 
When  the  surface  is  viewed  a  little 
inside  the  focus,  its  aspect  is  that 
shown  at  A,  Fig.  115;  whilst,  when 
the  surface  is  slightly  beyond  the 
focus  (b),  the  hexagonal  areolae  are 
dark,  and  the  intervening  partitions 
light.  —  The  experienced  Microscopist 

False  hexagonal  areolation  of  Pleuro- 

will  find  in  the  optical  eilects  produced  Z^mllo  itoooX^^eterf"'"- 

by  variations  of  focal  adjustment  the 

most  certain  indications  in  regard  to  the  nature  of  such  inequalities  of 
surface  as  are  too  minute  to  be  made  apparent  by  the  use  of  the  Stereo- 
scopic Binocular.    For  superficial  elevations  must  necessarily  appear 


152 


THE  MICROSCOPE  AND  ITS  REVELATIONS. 


brightest  when  the  distance  between  the  Objective  and  the  object  is  in- 
creased,  whilst  depressions  must  apper.r  brightest  when  that  distance  is 
diminished. — The  Student  should  be  warned  against  supposing  that  in 
all  cases,  the  most  positive  and  strihing  appearance  is  the  truest;  for 
this  is  often  not  the  case.  Mr.  Slack's  optical  illusion,  or  silica-crack 
slide,^  illustrates  an  error  of  this  description.  A  drop  of  water  holding 
colloid  silica  in  solution  is  allowed  to  evaporate  on  a  glass  slide,  and, 
when  quite  dry,  is  covered  with  thin  glass  to  keep  it  clean.  The  silica 
deposited  in  this  way  is  curiously  cracked;  and  the  finest  of  these  cracks 
can  be  made  to  present  a  very  positive  and  de/eptive  appearance  of  being 
raised  bodies  like  glass  threads.  It  is  also  easy  to  obtain  diflraction-lines 
at  their  edges,  giving  an  appearance  of  duplicity  to  that  which  is  really 
single. 

153.  A  very  important  and  very  frequent  source  of  error,  which  some- 
times operates  even  on  experienced  Microscopists,  lies  in  the  refractive 
influence  exerted  by  certain  peculiarities  in  the  internal  structure  of  ob- 
jects upon  the  rays  of  light  transmitted  through  them;  this  influence 
being  of  a  nature  to  give  rise  to  appearances  in  the  image,  which  suggest 
to  the  observer  an  idea  of  their  cause  that  may  be  altogether  different 
from  the  reality.  Of  this  fallacy  we  have  a  ^  pregnant  instance  ^  in  the 
misinterpretation  of  the  nature  of  the  lacicnce  and  canaliculi  of  Bone, 
which  were  long  supposed  to  be  solid  corpuscles  with  radiating  filaments 
of  peculiar  opacity,  instead  of  being,  as  is  now  universally  admitted, 
minute  chambers  with  diverging  passages  excavated  in  the  solid  osseous 
substance.  For,  just  as  the  convexity  of  its  surface  will  cause  a  transpa- 
rent cylinder  to  show  a  bright  axial  band,^  so  will  the  concavity  of  the 
internal  surfaces  of  the  cavities  or  tubes  hollowed-out  in  the  midst  of 
highly-refracting  substances,  occasion  a  divergence  of  the  rays  passing 
through  them,  and  consequently  render  them  so  dark  that  they  are 
easily  mistaken  for  opaque  solids.  That  such  is  the  case  with  the  so-called 
^  bone  corpuscles,'  is  shown  by  the  effect  of  the  infiltration  of  Canada 
balsam  through  the  osseous  substance;  for  when  this  fills  np  the  excava- 
tions, being  nearly  of  the  same  refractive  power  with  the  bone  itself,  it 
obliterates  them  altogether. — So,  again,  if  a  person  who  is  unaccustomed 
to  the  use  of  the  Microscope  should  have  his  attention  directed  to  a 
preparation  mounted  in  liquid  or  in  balsam  that  might  chance  to  contain 
air-buiUes,  he  will  be  almost  certain  to  be  so  much  more  strongly  im- 
pressed by  the  appearances  of  these  than  by  that  of  the  object,  that  his 
first  remark  will  be  upon  the  number  of  strange-looking  black  rings 
which  he  sees,  and  his  first  inquiry  will  be  in  regard  to  their  meaning. 

154.  Although  no  experienced  Microscopist  could  now  be  led  astray 
by  such  obvious  fallacies  as  those  alluded  to,  it  is  necessary  to  notice  them 
as  warnings  to  those  who  have  still  to  go  through  the  same  education. 
The  best  method  of  learning  to  appreciate  the  class  of  appearances  in 
question,  is  the  comparison  of  the  aspect  of  globules  of  Oil  in  water,  with 
that  of  globules  of  Water  in  oil,  or  of  bubbles  of  Air  in  water  or  Canada 
balsam.  This  comparison  may  be  very  readily  made  by  shaking  up  some 
oil  with  water  to  which  a  little  gum  has  been  added,  so  as  to  form  an 
emulsion;  or  by  simply  placing  a  drop  of  oil  of  turpentine  (colored  by 
magenta  or  carmine)  and  a  drop  of  water  together  on  a  slip  of  glass,  lay- 

*    Monthly  Microscopical  Journal,"  Vol.  v.  (1872),  p.  14. 

^  This  was  the  appearance  which  gave  rise  to  the  erroneous  notion  that  long 
prevailed  amongst  Microscopic  observers,  and  still  lingers  in  the  Public  mind,  of 
the  tubular  structure  of  the  Human  Hair, 


MANAGEMENT   OF  THE  MICROSCOPE. 


158 


ing  a  thin-glass  cover  upon  them,  and  then  moving  the  cover  several 
times  backwards  and  forwards  upon  the  slide.  Now  when  such  a  mix- 
ture is  examined  with  a  sufficiently  high  magnifying  power,  all  the  glo- 
bules present  nearly  the  same  appearance,  namely,  dark  margins  with 
bright  centres;  but  when  the  test  of  alteration  of  the  focus  is  applied  to 
them,  the  difference  is  at  once  revealed;  for  whilst  tlie  globules  of  Oil 
surrounded  by  water  become  darher  as  the  object-glass  is  depressed,  and 
lighter  as  it  is  raised,  those  of  Water  surrounded  by  oil  become  more 
luminous  fis  the  object-glass  is  depressed,  and  darker  as  it  is  raised.  The 
reason  of  this  lies  in  the  fact  that  the  high  refracting  power  of  the  Oil 
causes  each  of  its  globules  to  act  like  a  ^ow^X^-convex  lens  of  very  short 
focus;  and  as  this  will  bring  the  rays  which  pass  through  it  into  conver- 
gence above  the  globule  (i.  e.,  between  the  globule  and  the  Objective),  its 
brightest  image  is  given  when  the  object-glass  is  removed  somewhat 
farther  from  it  than  the  exact  focal  distance  of  the  object.  On  the  other 
hand,  the  globule  of  Water  in  oil,  or  the  minute  bubble  of  air  in  water 
or  balsam,  acts,  in  virtue  of  its  inferior  refractive  power,  like  a  double- 
concave  lens;  and  as  the  rays  of  this  diverge  from  a  virtual  focus  heloio 
the  globule  {i,  e.,  between  the  globule  and  the  mirror),  the  spot  of  great- 
est luminosity  will  be  found  by  causing  the  object-glass  to  approach  tvitliin 
the  proper  focus.  A  thorough  mastery  of  these  appearances  is  very  im- 
portant in  the  study  of  the  ^protoplasm'  of  Plants — the  ^sarcode'  of 
Animals, — which  includes  oil-particles,  together  with  spaces  occupied  by 
a  watery  fluid,  which  (having  been  at  one  time  supposed  to  be  void)  are 
known  as  Vacuoles.' 

155.  Among  the  sources  of  fallacy  by  which  the  young  Microscopist  is 
liable  to  be  misled,  one  of  the  most  curious  is  the  movement  exhibited  by 
very  minute  particles  of  nearly  all  bodies  that  are  sufficiently  finely  di- 
vided, when  suspended  in  water  or  other  fluids.  This  movement  was  first 
observed  in  the  fine  granular  particles  which  exist  in  great  abundance 
in  the  contents  of  the  Pollengrains  of  plants  (sometimes  termed  the 
fovilla),  and  which  are  set  free  by  crushing  them;  and  it  was  imagined 
that  they  indicated  the  possession  of  some  special  vital  endowment  by 
these  particles,  analogous  to  that  of  the  Spermatozoa  of  animals.  In  the 
year  1827,  however,  it  was  announced  by  Dr.  Eobert  Brown  that  numer- 
ous other  substances.  Organic  and  Inorganic,  when  reduced  to  a  state  of 
equally  minute  division,  exhibit  a  like  movement,  so  that  it  cannot  be 
regarded  as  indicative  of  any  endowment  peculiar  to  the  fovilla  granules; 
and  subsequent  researches  have  shown  that  there  is  no  known  exception 
to  the  rule  that  such  motion  takes  place  in  the  particles  of  all  substances, 
though  some  require  to  be  more  finely  divided  than  others  before  they 
will  exhibit  it.  The  closer  the  conformity  between  the  specific  gravity 
of  the  solid  particles  and  that  of  the  liquid,  the  less  minute  need  be  that 
reduction  in  their  size  which  is  a  necessary  condition  of  their  movement: 
and  thus  Carmine,  Indigo,  or  Gamboge  rubbed  up  with  water,  show  it 
extremely  well;  whilst  the  particles  of  Metals,  which  are  from  seven  to 
twenty  times  as  heavy  as  water,  require  to  be  reduced  to  a  minuteness 
many  times  greater,  before  they  will  exhibit  it.  The  movement  is  chiefly 
of  an  oscillatory  kind ;  but  the  particles  also  rotate  backwards  and  for- 
wards upon  their  axes,  and  gradually  change  their  places  in  the  field  of 
view.  The  movement  of  the  smallest  particles  is  the  most  energetic,  and 
the  largest  (exceeding  l-5000th  of  an  inch)  are  quite  motionless,  whilst 
those  of  intermediate  size  move  with  comparative  inertness.  A  drop  of 
common  ink  which  has  been  exposed  to  the  air  for  some  weeks,  or  a  drop 


154 


THE  MICROSCOPE  AND  ITS  REVELATIONS. 


of  fine  clay  (such  as  the  prepared  haolin  used  by  Photographers)  shaken- 
up  with  water,  is  recommended  by  Prof.  Jevons/  who  has  recently 
studied  this  subject,  as  showing  the  movement  (which  he  designates 
pedesis)  extremely  well.  But  none  of  the  particles  he  has  examined  are 
so  active  as  those  of  pumice-stone  that  has  been  ground-up  in  an  agate 
mortar;  for  these  are  seen  under  the  microscope  to  leap  and  swarm  with 
an  incessant  quivering  movement,  so  rapid  that  it  is  impossible  to  follow 
the  course  of  a  particle  which  probably  changes  its  direction  of  motion 
15  or  20  times  in  a  second.  The  distance  through  which  a  particle  moves 
at  any  one  bound  is  usually  less  than  l-5000th  of  an  inch.  This 
'  Brownian  movement '  (as  it  is  commonly  termed)  is  not  due  to  evapora- 
tion of  the  liquid :  for  it  continues,  without  the  least  abatement  of  energy, 
in  a  drop  of  aqueous  fluid  that  is  completely  surrounded  by  oil,  and  is 
therefore  cut  off  from  all  possibility  of  evaporation;  and  it  has  been  known 
to  continue  for  many  years  in  a  small  quantity  of  fluid  inclosed  between 
two  glasses  in  an  air-tight  case.  And,  for  the  same  reason,  it  can  scarcely 
be  connected  with  chemical  change.  But  the  observations  of  Prof. 
Jevons  (loc.  cit.)  show  that  it  is  greatly  affected  by  the  admixture  of 
various  substances  with  water;  being,  for  example,  increased  by  a  small 
admixture  of  gum,  while  it  is  checked  by  an  extremely  minute  admixture 
of  sulphuric  acid  or  of  various  saline  compounds,  these  (as  Prof.  J.  points 
out)  being  all  such  as  increase  the  conducting  power  of  water  for  Elec- 
tricity. The  rate  of  subsidence  of  finely-divided  clays  or  other  particles 
suspended  in  water,  thus  greatly  depends  upon  the  activity  of  their 
^Brownian  movement;'  for,  when  this  is  brought  to  a  stand,  the  particles 
aggregate  and  sink,  so  that  the  liquid  clears  itself. — In  any  case  in  which 
the  motions  of  very  minute  particles,  of  whatever  kind,  are  in  question, 
it  is  necessary  to  make  allowance  for  this  ^  molecular^  movement;  and  the 
young  Microscopist  will  therefore  do  well  to  familiarize  himself  with  its 
ordinary  characters,  by  the  careful  observation  of  it  in  such  cases  as  those 
just  named,  and  in  any  others  in  which  he  may  meet  with  it.^ 

156.  Diffraction. — The  course  of  Light-rays  is  altered  not  only  by 
refraction  when  they  pass  from  one  transparent  medium  into  another, 
and  by  reflection  when  they  fall  on  polished  surfaces  which  they  do  not 
enter,  but  also  by  inflection  at  the  edges  of  objects  by  which  they  pass; 
and  as  the  differently  colored  rays  which  altogether  make  up  white  light 
are  affected  by  such  inflection  in  different  degrees,  they  are  separated  by 
it  (as  by  refractive  ^dispersion  ')  into  colored  bands;  the  phenomenon  be- 
ing altogether  known  as  diffraction.  This  may  be  made  evident  by  caus- 
ing abeam  of  sunlight  to  enter  a  darkened  room  through  a  very  narrow 
slit,  and  to  fall  on  a  white  screen;  for  the  narrow  line  of  white  light  will 
show  a  border  of  colored  fringes,  which  become  wider  as  the  slit  is  nar- 
rowed; and  if  these  fringes  be  viewed  through  a  piece  of  colored  glass, 
Avhich  allows  only  rays  of  its  own  color  to  pass,  they  will  appear  as  a  suc- 
cession of  bands  alternately  bright  and  dark.  This  alternation  is  pro- 
duced by  the  interference  of  the  Light- waves;  just  as  the  alternations  of 
sound  and  comparative  silence  termed  ^  beats,'  which  are  heard  when  two 
slightly  different  tones  are  being  sounded  together,  are  due  to  the  inter- 


1    Quarterly  Journal  of  Science,"  N.  S.,  Vol.  viii.  (1878),  p.  172. 

'-^  See  also  the  Rev.  J.  Delsaulx  **On  the  Thermo-Dynamic  Origin  of  the 
Brownian  Motions"  in  Monthly  Journ.  of  Microsc.  Sci  ,"  Vol.  xviii.  (1877),  p.  1; 
and  Dr.  W.  M.  Ord  '*On  some  Causes  of  Brownian  Movements"  in  Journ.  of 
Roy.  Microsc.  Soc,"  Vol.  ii.  (1879),  p.  656. 


MANAGEMENT  OF  THE  MICROSCOPE. 


155 


ference  of  the  Sound-waves.^  The  colored  fringes  are  produced  by  the 
superposition  of  all  these  bands. — When,  again,  a  small  opaque  plate  of 
any  substance  is  interposed  in  the  course  of  the  pencil  of  solar  light  ad- 
mitted into  a  darkened  room  through  a  very  small  hole  in  a  card,  or  di- 
verging from  the  point  at  which  it  has  been  collected  by  a  convex  lens  of 
short  focus,  the  shadow  thrown  by  it  on  the  screen  will  be  surrounded  by 
a  series  of  colored  fringes,  and  the  shadow  itself  will  be  larger  than  the 
geometrical  shadow.— But,  further,  if  a  piece  of  glass  be  ruled  by  a  dia- 
mond with  parallel  lines,  some  hundreds  or  thousands  to  an  inch,  so  as 
to  form  what  is  called  a  ^grating,'  and  the  narrow  beam  proceedmg  from 
the  slit  be  looked-at  through  this  grating  (so  held  that  the  direction  of 
its  lines  is  parallel  to  that  of  the  slit),  a  number  of  spectra  come  into 
view,  ranged  at  nearly  equal  distances  on  both  sides  of  the  slit.^  Now,  it 
is  manifest  that  when  a  beam  of  light  is  made  to  pass  through  an  object 
that  is  being  examined  Microscopically,  the  light  and  shade  in  the  picture 
seen  by  the  eye  must  be  occasioned  by  the  greater  or  less  transparence  of 
the  different  parts  of  that  object;  and  that,  wherever  there  are  definite 
lines  or  margins  sufficiently  opaque  to  throw  a  definite  shadow,  such 


Fia.  116. 


Scale  of  Gnat  showing  beaded  markings;  photographed  by  Dr.  Woodward. 


shadow  must  be  bordered  more  or  less  obviously  by  ^interference ^  or 
'  diffraction '  spectra,  especially  in  the  case  of  objects  having  strongly- 
marked  lines  with  very  transparent  intermediate  spaces.  There  are  many 
objects  of  great  delicacy,  in  which  ^diffraction-bands^  are  liable  to  be 
mistaken  for  indications  of  actual  substance;  whilst,  on  the  other  hand, 
the  presence  of  an  actual  substance  of  extreme  transparence  may  some- 
times be  doubted  or  denied,  through  its  image  being  attributed  to  diffrac- 
tion. No  rules  can  be  given  for  the  avoidance  of  such  errors;  since  they 
can  only  be  escaped  by  the  discriminating  power  which  education  and 
habit  confer  on  the  experienced  Microscopist.— A  good  example  of  this 
kind  is  afforded  by  the  minute  beading  presented  in  the  scales  of  the  Gnat 
and  Mosquito  (Fig.  116).  These  scales  are  composed  of  a  very  delicate 
double  membrane,  strengthened  by  longitudinal  ribs  on  either  side,  those 

^  The  colors  of  thin  plates, — as  seen  wlien  the  sun  shines  on  a  soap-bubble  or 
on  a  film  of  oil  spread  out  over  a  surface  of  water — or  when  we  look  at  a  window 
through  two  glasses  separated  by  an  attenuated  film  of  air, — are  familiar  exam- 
ples of  *  interference-fringes,' which,  when  displayed  annularly,  are  known  as 
'  Newton's  rings.' 

Such  '  gratings'  are  now  much  used  in  Spectroscopic  observation;  and  afford 
the  best  means  of  determining  the  wave-lengths  of  the  rays  of  the  several  parts 
of  the  spectrum. 


156  THE  MICROSCOPE  AND  ITS  KEVELATION8. 

of  the  opposite  sides  uniting  at  the  broad  end  of  the  scale,  where  they 
generally  project  as  bristle-like  appendages  beyond  the  intermediate  mem- 
brane; and  they  are  crossed  transversely  by  fine  markings,  which  are 
probably  ridge-like  corrugations  of  their  membrane,  these  also  existing 
on  both  surfaces  of  the  scale.  The  attention  of  Dr.  Woodward  having 
been  drawn  by  Dr.  Anthony  to  the  presence  in  these  scales  of  three  ujii- 
form  parallel  rows  of  beads  in  every  interspace  between  two  adjoinijig  ribs, 
he  was  at  first  inclined  to  believe  that  the  markings  are  real,  represent- 
ing an  actual  structure  in  the  scale;  but  having  obtained  an  excellent 
Photograph  of  it  by  monochromatic  sun-light,  under  a  power  of  1,350 
diameters,  he  was  led  to  alter  his  opinion,  and  to  regard  them  as  pro- 
duced by  the  crossing  of  the  transverse  markings  by  longitudmal  diffrac^ 
tioii'lines,  conditioned  by  the  longitudinal  7'ibs  and  parallel  to  them.^ 
His  chief  reasons  for  so  regarding  them  were  (1),  that  ^^the  longitudinal 
diffraction-lines  are  clearly  seen  alike  in  the  Microscopic  image  and  in 
the  Photographs,  to  extend  into  empty  space  beyond  the  contour  of  the 
scales,  almost  as  far  as  the  ends  of  the  bristles  in  which  the  parallel 
ribs  terminate;''  and  (2),  ^^that  they  vary  in  number  with  varying  ob^ 
liquity  of  illumination,  so  that  in  the  same  scale  two,  three,  four,  and 
five  rows  of  beads  can  be  seen,  and  photographed  at  pleasure,  in  every 
intercostal  space.''  The  true  appearance.  Dr.  Woodward  considers,  is 
given  when  the  Achromatic  Condenser  is  so  adjusted  that  its  light  is 
either  central  or  slightly  oblique  in  the  longitudinal  direction  of  the 
scale. 

157.  The  recent  researches  of  Prof.  Abbe  of  Jena  appear  to  have  con- 
clusively proved  that  Diffraction  has  a  most  important  share,  previously 
altogether  unsuspected,  in  the  formation  of  the  Microscopic  images  of 
very  closely  approximated  lines  or  other  markings,  in  objects  viewed 
under  high  magnifying  powers  of  large  Angular  aperture. — All  that  has 
been  hitherto  said  of  the  formation  of  Microscopic  images,  relates  to 
such  as  are  produced,  in  accordance  with  the  laws  of  refraction,  by  the 
alteration  in  their  course  which  the  Light-rays  undergo  in  their  passage 
through  the  lenses  interposed  between  the  object  and  the  eye.  These 
dioptric  images,  when  formed  by  lenses  free  from  Spherical  and  Chro- 
matic aberration,  are  geometrically  correct  pictures,  truly  representing  the 
appearances  which  the  objects  themselves  would  present,  were  they  en- 
larged to  the  same  scale,  and  viewed  under  similar  illumination.  And 
we  are  fully  justified,  therefore,  in  drawing  from  such  Microscopic  images 
(provided  that  they  are  free  from  diffraction-spectra)  the  same  conclu- 
sions in  regard  to  the  structure  of  the  objects  they  picture,  as  we  should 
draw  from  the  direct  vision  of  actual  objects  having  the  same  dimensions. 
There  is,  however,  an  optical  limit  as  to  the  completeness  of  such  images 
in  regard  to  minute  detail;  as  it  appears  from  the  theoretical  researches 
of  Prof rs.  Helmholtz  and  Abbe,  that  no  amount  of  magnifying  power 
can  separate  dioptrically  two  lines,  apertures,  or  markings  of  any  kind, 
not  more  than  l-2500th  of  an  inch  apart.  The  visual  separation  or  ^  reso- 
lution '  of  more  closely  approximated  lines  or  other  markings  is  entirely 
the  result  of  diffraction;  the  Objective  receiving  and  transmitting,  not 
only  the  ordinary  dioptric  rays,  but  the  '  inflected  '  rays  whose  course  has 
been  altered  in  their  course  through  the  object  by  some  peculiarity  in  the 
disposition  of  its  particles.  These  rays,  when  acted-on  by  the  Objective, 
produce  '  diffraction-spectra;'  the  number  and  relative  position  of  which 


*    Monthly  Microsc.  Joum.,"  Vol.  xv.  (1876),  p.  253. 


MAl^AGEMENT  OF  THE  MICROSCOPE. 


157 


bear  a  relation  to  the  structural  arrangement  on  which  their  production 
depends.^  If  the  Objective  be  perfectly  corrected,  and  all  the  diffraction- 
spectra  lie  within  its  field,  they  will  be  re-united  by  the  Eye-piece  to  form 
u  secondary  or  '  dilfraction '  image,  lying  in  the  same  focal  plane  with  the 
dioptric  image,  and  coinciding  with  it,  while  filling  up  its  outlines  by 
supplying  intermediate  details.  But  where  the  markings  (of  whatever 
nature)  are  so  closely  approximated  as  to  produce  a  wide  dispersion  of 
the  interference-spectra,  only  a  part  of  them  may  fall  within  the  range  of 
the  Objective;  and  the  re-combination  of  these  may  produce  a  diffraction- 
image  differing  more  or  less  completely  (perhaps  even  totally)  from  the 
real  structure;  whilst,  if  they  should  lie  entirely  outside  the  field  of  the 
Objective,  no  secondary  or  diffraction  image  will  be  produced.  Thus, 
whilst  the  dioptric  image  represents  the  actual  object,  a  diffraction- 
image  formed  by  the  reunion  of  some  of  the  interference-spectra  is  only 
an  optical  expression  of  the  result  of  their  partial  re-combination,  which 
may  represent  something  entirely  different  from  the  real  structure; — the 
sa)ne  arrangement  of  lines  (for  example)  being  presented  to  the  eye  by  dif- 
/<?r6?^^/^/-lined  surfaces,  and  different  arrangements  hj  similarly  lined  sur- 
faces, according  to  the  numbers  and  positions  of  the  re-united  spectra.  ^ — 
This  doctrine,  originally  based  on  elaborate  theoretical  investigations  in 
connection  with  the  '  Undulatory  Theory  of  Light,'  has  been  so  fully 
borne  out  by  experimental  inquiries  instituted  to  test  it,  and  is  in  such 
complete  harmony  with  the  most  certain  experiences  of  Microscopists,  thiit 
its  truth  scarcely  admits  of  doubt.  Although  any  attempt  to  explain  its 
theory  in  a  Treatise  like  the  present  must  necessarily  be  altogether  futile, 
yet  a  selection  from  the  experiments  by  which  Prof.  Abbe  has  verified  it, 
will  not  only  assist  in  the  comprehension  of  the  doctrine,  but  will  enable 
every  Microscopist  to  satisfy  himself  of  their  correctness. 

A  *  grating'  should  be  provided,  ruled  alternately  with  long  and  with  short 
lines,  as  in  Fig.  117,  a;  the  lines  being  traced  v«^ith  a  diamond  point  on  a  film  of 
silver  of  extreme  tenuity  deposited  on  a  thin  glass-cover;  and  the  ruled  surface 
being  cemented  to  an  ordinary  glass  slide  v^rith  Canada  balsam.'^  The  *  adapter ' 
ordinarily  used  for  rotating  the  analyzing  prism  of  the  Polariscope  between  the 
Objective  and  the  Microscope-body,  should  be  fitted  with  a  small  tube  for  the 
introduction  of  diaphragms  with  varied  slits,  so  that  these  may  be  rotated  imme- 
diately behind  the  back  combinatij^n  of  the  Objective. — The  'grating'  being 
placed  on  the  Stage  of  the  Microscope,  illuminated  from  the  mirror,  and  focussed 
under  a  1-inch  Objective,  so  as  to  show  the  ordinary  microscopic  image  of  its 
ruled  surface  as  at  A,  the  eye -piece  is  removed,  and  the  observer,  looking  into  the 
body  of  the  instrument,  and  changing  the  place  of  his  eyes,  sees  two  rows  of 
spectra,  each  having  a  central  circle,  with  ovals  on  either  side  of  it  (c).  The 
central  circle  is  bright  and  colorless;  while  each  of  the  ovals  shows  the  colors  of 
the  solar  spectrum,  with  the  blue  always  towards  the  centre.  These  ovals  are 
*  diffraction -spectra;'  of  which  the  four  closely  approximated  pairs  in  the  upper 
row  are  formed  by  the  wider  lines  of  the  single  ruling,  and  the  two  pairs  in  the 
lower  row  (which  are  at  double  the  distance  of  the  preceding)  by  the  closer  lines 
of  the  double  ruling. 

The  following  experiments  show  (1)  that  the  dioptric  image,  when  viewed  by 
the  eye-piece  separately  from  all  diffraction-spectra,  gives  no  Microscopic  repre- 


^  The  reader  may,  perhaps,  be  aided  in  comprehending  Prof.  Abbe's  doctrine 
by  the  following  analogy: — When  a  solar  spectrum  is  projected  by  a  prism  on  a 
white  surface,  its  entire  re-combination  by  a  convex  lens  will  reproduce  a  beam 
of  white  light.  But,  if  only  certain  parts  of  the  spectrum  be  thus  recombined, 
the  beam  will  have  a  color  dependent  upon  the  selection. 

2  In  the  grating  used  by  Mr.  Stephenson  ("  Monthly  Microsc,  Joum.,"  Vol. 
xvii.,  p.  83),  the  lines  in  the  upper  half  were  about  1,790  to  the  inch,  and  in  the 
lower  about  3,580. 


158 


THE  MICROSCOPE  AND  ITS  REVELATIONS. 


sentation  at  all  of  the  lined  surface;  whilst  (2)  by  varying  the  combinations  of  the 
diffraction-spectra,  the  lineation  shown  in  the  Microscopic  image  of  the  ruled 
surface  may  be  partially  or  completely  changed. 

Experiment  1. — If,  in  the  first  place,  a  diaphragm  with  a  single  diametric  slit 
(b)  be  so  placed  immediately  behind  the  Objective,  that  the  slit  is  parallel  to  th^ 


direction  of  the  ruled  lines — thus  giving  passage  to  the  direct  rays  forming  the 
dioptric  image,  but  excluding  all  diffraction-spectra — the  field  seen  through  the 
replaced  eye-piece  shows  no  lineation  whatever^  the  *  grating '  being  replaced  by  a 
plain  silver  band.  Yet,  if  the  diaphragm  be  turned  a  quarter  round,  so  that  the 
slit  lies  transversely  to  the  lines,  and  admits  the  pairs  of  diffraction-spectra  in 


MANAGEMENT  OF  THE  MICROSCOPE. 


159 


each  row  that  lie  nearest  the  centre,  the  delineation  reappears^  as  it  actually  is 
in  the  grating. 

Experiment  2. — If,  now,  a  diaphragm  be  used  having  three  sUts  (e),  of  which 
the  central  admits  the  direct  rays  only,  while  the  two  lateral  receive  the  first  pair 
of  the  wider  spectra  and  the  second  pair  of  the  closer,  it  will  be  found,  on  replac- 
ing the  eye-piece,  that  the  whole  field  is  covered  with  the  closer  lines,  as  at  F. 
For  the  stopping-out  of  the  alternate  spectra  of  the  upper  series  brings  inio  com- 
bination only  that  pair  which  corresponds  with  the  lower,  and  therefore  makes 
the  apparent  lineation  of  the  upper  half  correspond  with  the  real  lineation  of  the 
lower,  by  the  introduction  of  an  intermediate  set  of  spectral  lines,  scarcely  dis- 
tinguishable from  those  of  which  they  seem  to  be  prolongations. 

Experiment  3. — Further,  by  carrying  the  two  lateral  slits  (as  at  h)  to  the  dis- 
tance of  the  extreme  spectra  of  both  rows — which  distance  represents  that  of  the 
spectra  that  would  be  produced  by  a  lineation  twice  as  close  as  that  of  the  lower 
half  of  A,  and  four  times  as  close  as  that  of  the  upper  half — the  entire  field,  when 
the  eye-piece  is  replaced,  is  seen  to  be  covered  with  the  doubly-close  lines  corre- 
sponding to  that  distance,  as  shown  at  I. 

Experiment  4. — On  the  other  hand,  by  using  a  single  aperture  shaped  as  in  D, 
which  is  broad  enough  to  admit  the  innermost  pair  of  spectra  in  the  upper  row, 
but  not  to  admit  any  of  the  spectra  of  the  lower  row,  the  field,  when  the  eye- 
piece is  replaced,  shows  the  wide  lines  (G)  of  the  upper  half  of  the  grating,  whilst 
its  lower  half  is  perfectly  blank. 

It  has  thus  been  experimentally  demonstrated  that  the  formation  of 
the  true  image  of  the  grating  is  dependent  upon  the  normal  re-combi- 
nation of  its  diffraction-spectra,  while  the  entire  exclusion  of  these 
altogether  obliterates  the  lineation.  And  we  thus  have  now  for  the  first 
time  the  scientific  rationale  of  the  fact  which  has  long  been  practically 
known — the  relation  of  the  ^resolving  power' of  Objectives  to  their 
Angle  of  aperture.  For  it  is  obvious  that  since  the  ^inflected'  rays 
which  form  the  ^diffraction-spectra' diverge  more  and  more  widely  in 
proportion  to  the  approximation  of  the  lines  that  separate  them — so  that 
those  spectra  (as  already  shown)  are  carried  apart  to  greater  and  yet 
greater  distances — the  separation  of  those  of  a  very  close  lineation  may 
be  such  as  to  carry  them  completely  beyond  the  aperture  of  an  Objective 
which  may  take  in  the  spectra  of  a  more  open  lineation  (Exper.  4).  And 
thus  an  Objective  may  be  able  clearly  to  separate  lines  of  50,000  to  an 
inch,  from  which  no  amount  of  ^coaxing'  by  oblique  or  any  other  kind 
of  illumination  can  obtain  a  resolution  of  lines  of  80,000  to  an  inch. — 
But  further,  it  has  been  made  clear  that  most  distinct  ^spectral Mines 
can  be  produced  in  the  Microscopic  image,  by  the  re-combination  of 
selected  pairs  of  diffraction-spectra,  without  any  real  lines  answering  to 
them;  and  hence,  that  the  images  thus  formed  cannot  be  regarded  as 
indicative  of  the  actual  structure  of  the  objects  they  represent;  the 
grating,  for  example,  whose  real  lineation  is  shown  at  a,  being  made  to 
appear  (according  to  the  manner  in  which  it  is  viewed)  either  entirely 
blank,  as  half -blank  (g),  as  having  the  intermediate  lines  of  its  lower  half 
extended  over  its  upper  (f),  or  as  having  its  whole  field  covered  with 
lines  at  only  half  the  distance  of  those  of  its  closest  part  (i).  The  same 
effects  of  obliteration  or  duplication  of  lines  may  be  produced  on  such 
objects  as  the  scale  of  Lepisma  saccharina  (Fig.  417),  by  using  higher 
powers  with  suitable  diaphragms. — It  will  now  be  shown  that  the  varia- 
tions producible  by  similar  treatment  in  the  appearances  of  cross-lined 
objects,  are  yet  more  remarkable. 

A  grating  with  lines  crossed  at  an  angle  may  be  prepared  by  cementing  a 
cover-glass,  with  one  set  of  lines  ruled  through  a  silver-film  on  its  under  side, 
upon  a  glass  slide  having  another  set  ruled  on  a  silver-film  on  its  upper  surface. 
If  the  two  sets  of  lines  are  placed  at  right  angles  to  each  other,  a  rectangular 
grating  is  the  result  (n);  if  at  any  oblique  angle,  the  grating  is  rhombic  (k). 


160 


THE  MICBOSCOPE  AND  ITS  REVELATIONS, 


If  the  square  grating  be  focusse^,  and  its  image  examined  by  looking  into  the 
tube  of  the  Microscope  without  the  eye-piece,  the  diffraction-spectra  will  exhibit 
the  regular  arrangement  shown  at  R;  the  round  image  being  in  the  centre  of  the 
field,  and  the  ovals  being  disposed  in  five  rows  at  equal  distances. 

Experiment  5. — A  diaphragm  being  interposed  (l)  with  an  oblique  slit  just 
large  enough  to  admit  the  central  circle  and  one  of  the  diffraction  spectra,  and 
the  eye-piece  being  replaced,  the  real  rectangular  lines  will  not  he  seen  at  all,  but 
the  field  will  be  traversed  by  oblique  spectral  lines  (o),  whose  direction  is  at  right 
angles  to  that  of  the  slit. 

If  the  image  of  the  rhombic  grating  be  examined  in  the  same  manner  as  that 
of  the  square,  it  will  exhibit  the  arrangement  shown  at  P  (the  dotted  inner  circle 
being  here  disregarded). 

Experiment  6. — By  using  a  diaphragm  with  a  single  slit  in  the  direction  of  one 
of  the  diagonals  of  the  rhomb,  a  Microscopic  image  will  be  presented  from  which 
both  sets  of  real  lines  are  entirely  absent,  whilst  a  single  set  of  spectral  lines  is 
seen,  whose  direction  is  at  right  angles  to  the  slit,  that  is,  in  the  direction  of  the 
other  diagonal. 

Experiment  7. — Again,  by  using  a  diaphragm  with  two  slits  at  right  angles  to 
each  other  (Q),  the  Microscopic  image  will  show  two  sets  of  spurious  lines  cross- 
ing one  another  at  right  angles  (m),  in  the  directions  of  the  two  diameters  of  the 
rhomb,  the  real  lines  being  altogether  invisible. 

Experiment  8. — A  very  singular  effect  is  produced  by  the  use  of  a  single  circu- 
lar diaphragm,  whose  aperture  is  reduced  so  as  only  to  include  the  six  spectral 
ovals  lying  within  the  dotted  circle  at  p;  for  on  then  replacing  the  eye-piece,  the 
entire  field  is  seen  to  be  marked  out  in  hexagons. 

Now  if  a  valve  of  Pleurosigma  angulatum  be  focussed  with  central  illumina- 
tion under  an  Objective  of  sufficiently  high  power  and  large  aperture,  and  the 
eye-piece  be  removed,  there  will  be  seen  on  looking  down  the  body  a  bright  cen- 
tral beam,  with  six  colored  spectra  arranged  round  it— just  as  in  the  interior 
spectrum  (P)  of  the  rhomboidal  grating;  the  reason  that  no  other  spectra  are  seen, 
being  that  the  approximation  of  the  markings  carries  these  six  spectra  to  the 
extreme  border  of  the  field  of  even  the  largest-angled  Objective.  An  Objective 
of  smaller  angle  will  not  show  them  at  all  with  central  ligifit;  but  if  oblique  light 
be  used,  the  circular  beam  is  carried  to  one  margin,  and  a  single  spectral  oval  is 
seen  at  the  other;  and  the  recombination  of  these  suffices  to  make  one  set  of  lines 
visible.  Again,  by  recombining,  by  means  of  appropriate  diaphragms,  any  three 
of  the  spectral  ovals,  or  any  two  of  these  with  the  central  beam,  the  very  same 
part  of  the  valve  may  be  made  to  show  a  great  variety  of  appearances — such  as 
are  actually  seen  in  different  parts  of  the  same  valve  under  the  same  illumination 
(Fig.  166).  1 

The  foregoing  experiments,  then,  entirely  confirm  the  general  con- 
clusions drawn  from  those  of  the  previous  series,  (1)  as  to  the  entire 
distinctness  in  character  between  the  images  Dioptrically  formed  of  the 
general  outlines  and  larger  details  of  Microscopic  objects,  and  the 
representations  of  their  finer  details  which  result  from  the  reunion  of 
their  Interference-spectra;  and  (2)  as  to  the  very  limited  trustworthiness 
of  the  latter,  when  the  minuteness  of  the  structure  occasions  such  a  wide 
separation  of  the  '  diffraction-spectra,^  as  limits  the  number  thus  com- 
bined.— Thus  it  becomes  clear  (1)  that  the  ^resolving  power'  by  which 
closely-approximated  lines  or  other  markings  are  separated,  increases 
(the  completeness  of  the  corrections  for  Spherical  and  Chromatic  aberra- 
tion being  presupposed)  with  the  Angular  Aperture^  of  the  Objective; 

'  See  the  original  Memoir  by  Prof.  Abbe,  *  Beitrage  zur  Theorie  des  Micro- 
scopes,' in  Archiv  fiir  Microscop.  Anatomie,"  Bd.  ix.  (1874),  p.  418;  Dr.  H.  E. 
Fripp's  translation  of  it  in  the  ''Proceedings  of  the  Bristol  Naturalists'  Society," 
N.S.,  Vol.  i.,  part  2(1875  ;  extracts  from  Dr.F.'s  translation  in  *'  Monthly  Microsc. 
Journ.,"  Vol.  xiv.  (1875),  pp.  191,  245;  also  Mr.  Stephenson's  'Observations' 
thereon — to  which  the  Author  has  been  specially  indebted — op.  cit..  Vol.  xvii. 
(1878),  p.  82;  and  Mr.  F.  Crisp  "  On  the  Influence  of  Diffraction  in  Microscopic 
Vision,"  in  ''Journ.  of  Quekett  Club,"  Vol.  v.,  p.  79. 

2  The  term  Angular  Aperture  is  to  be  understood  as  differentiated  from  "  Angle 
s>i  Aperture"  (§  10),  by  the  allowance  made  for  the  modification  in  the  course  of 


MANAGEMENT  OF  THE  MlCROSCori: 


and  (2)  that,  as  there  is  a  like  increase  in  the  number  of  separate 
diffraction  spectra  which  can  be  combined  with  the  dioptric  image,  the 
representations  of  minute  structure  given  by  Objectives  of  widest  Angular 
aperture  are  more  trustworthy  than  those  given  by  those  of  narrower. 

158.  Relative  Qualities  of  Objectives. — In  estimating  the  comparative 
values  of  different  Objectives,  regard  must  always  be  had  to  the  purpose 
for  which  each  is  designed;  since  it  is  impossible  to  construct  a  combina- 
tion which  shall  be  equally  serviceable  for  every  requirement.  It  is 
commonly  assumed  than  an  Objective  which  will  show  certain  Test- 
objects,  musb  be  very  superior  for  everything  else  to  a  glass  which  will 
not  resolve  these;  but  this  is  known  to  every  practised  Microscopist  to  be 
a  complete  mistake,  the  qualities  which  enable  it  to  resolve  some  of  the 
more  difficult  ^  tests,^  not  being  by  any  means  identical  with  those  which 
make  it  most  useful  in  all  the  ordinary  purposes  of  Scientific  investiga- 
tion. Five  distinct  attributes  have  to  be  specially  considered  in  judging 
of  the  character  of  an  Objecfc-glass,  viz. — (1)  its  working-distance,  or 
actual  interval  between  its  front  lens  and  the  object  on  which  it  is 
focussed;  (2)  its  defining  power,  or  power  of  giving  a  clear  and  distinct 
image  of  all  well-marked  features  of  an  object,  especially  of  its  bounda- 
ries; (3)  it  penetrating  power,  or  focal  depth,  by  which  the  observer  is 
enabled  to  looh  into  the  structure  of  objects;  (4)  its  resolving  poimr,  by 
which  it  enables  closely-approximated  markings  to  be  distinguished;  and 
(5)  the  flatness  of  the  field  which  it  gives. 

I.  The  ^  Working  distance  ^  of  an  Objective  has  no  fixed  relation  to 
its  ^ focal  length;'  the  latter  being  estimated  by  its  equality  in  magnify- 
ing power  with  a  single  lens  of  given  curvature;*  while  the  former  varies 
with  the  mode  in  which  the  combination  is  constructed,  and  with  the 
angular  aperture  given  to  it.  Of  two  Objectives  of  1-inch  focus  and  the 
same  angle  of  aperture  (say  25^),  one  may  have,  in  virtue  of  its  construc- 
tion, a  much  longer  ^  working  distance  '  than  the  other;  and  this  is  not 
only  an  advantage  in  facilitating  the  side-illumination  of  opaque  objects, 
but  also  in  admitting  (as  will  presently  appear)  of  greater  '  focal  depth  ' 
or  ^penetration.'  But  it  is  especially  in  the  case  of  high  powers  that 
^  working  distance '  comes  to  be  of  essential  importance.  The  widening 
of  angular  aperture  which  is  required  to  give  them  their  highest  degree 
of  ^resolving'  power  (iv.)  necessitates  a  very  close  approximation  of  the 
front  lens  to  the. object;  and  whilst  it  is  an  absolute  necessity  that  the 
interval  should  be  sufficient  for  the  interposition  of  a  cover  of  the  thin- 
nest glass,  or  (if  this  be  inadmissible)  of  a  film  of  mica,  every  addition  to 
this  interval  is  a  clear  gain,  not  only  in  convenience  of  working,  but  also 
in  regard  to  the  'penetrating'  power  (iii.)  of  the  Objective. — The 
increase  of  ^  working  distance '  obtainable  by  the  use  of  the  Immersion 
system  is  by  no  means  the  least  of  its  advantages. 

II.  The  '  Defining  power '  of  an  Objective  depends  upon  the  com- 
pleteuess  of  its  correctio?is  for  Spherical  and  Chromatic  aberration  (§§  9- 

the  rays,  by  the  medium— whether  Air,  Water,  Glycerine,  Balsam,  or  Oil- 
through  which  they  pass  in  their  course  from  the  object  into  the  Objective.  (See 
Appendix.) 

^  Owing  to  the  want  of  some  common  standard,  Objectives  constructed  by 
different  Makers  of  the  same  nominal  focal  length,  often  differ  considerably  from 
each  other  in  magnifying  power;  and  the  proportional  amplification  given  by  the 
different  Objectives  of  any  one  Maker's  series  is  often  very  different  from  that 
indicated  by  their  nomenclature.  It  is  therefore  greatly  to  be  wished  that  some 
uniform  standard  could  be  agreed  on;  such  as  that  of  Magnifying  power  under 
an  Eye-piece  of  definite  focal  length,  at  a  fixed  distance  from  the  Objective. 
11 


162 


THE  MICROSCOPE  AND  ITS  KEVELATIONS. 


15),  especially  the  former;  and  it  is  an  attribute  essential  to  the  satisfac- 
tory  performance  of  any  Objective,  whatever  be  its  other  qualities. 
Good  definition  may  be  more  easily  obtained  with  lenses  of  small  or 
moderate,  than  with  lenses  of  large  angular  aperture;  and  as  it  is  impos- 
sible to  construct  ^dry^  Objectives  of  very  wide  angle,  without  some 
sacrifice  of  perfect  correction  (Abbe),  there  is  a  limit  which,  where 
^definitions  is  of  primary  importance,  cannot  be  advantageously  passed. 
On  the  immersion  system,  however,  and  especially  on  the  '  homogeneous 
immersion'  system  (§§  19,  20),  Objectives  can  be  constructed  of  very 
much  wider  angle,  without  any  injurious  sacrifice  of  definition  arising 
from  inadequate  correction.  But  here  there  comes  in  another  source  of 
impairment — the  difference  in  the  perspective  views  of  every  object  not  a  7nere 
mathematical  point  or  line,  which  are  received  through  the  different  parts 
of  the  area  of  the  Objective.  The  picture  given  by  the  entire  area  is — so 
to  speak — the  ^general  resultant'  of  the  dissimilar  pictures  recived 
through  these  several  parts;^  and  as  this  dissimilarity  obviously  increases 
with  the  angle  of  operture  of  the  Objective,  its  defining  power  mnst  be 
proportionately  impaired.  This  theoretical  conclusion  has  been  experi- 
mentally verified  by  Dr.  Koyston  Pigott;  who  has  found  that  by  com- 
paring Objectives  of  large  with  those  of  moderate  apertures,  on  such 
objects  as  the  cracks  in  Mr.  Slack's  silica-films  (§  152),  or  the  aerial  image 
formed  by  the  Achromatic  Condenser  of  a  hair  stretched  before  the  light 
at  some  distance,  the  advantage  was  decidedly  on  the  side  of  the  latter. 
He  has  shown^  ^^that  the  black  margins  or  black  marginal  annuli  of 
refracting  spherules  constantly  displayed  by  small  aperture  Objectives, 
are  attenuated  gradually  to  invisibility  as  the  apertures  are  widened  to  the 
utmost;  that  the  black  margins  of  cylinders,  tubules,  or  semi-tubules, 
also  suffer  similar  obliterations;  and  that,  in  consequence,  minute  detcdls 
are  concealed  or  destroyed  till  the  aperture  is  sufficiently  reduced.^^ — It  is 
also  the  experience  of  Messrs.  Dallinger  and  Drysdale,  that  for  the 
definition  of  the  immeasurably-minute  reproductive  granules  of  the 
Monadine  forms  whose  life-history  they  have  studied  (§  418),  or  of  the 
flagella  of  Bacterium  termo  (§  305),  which  may  be  characterized  as  the 
highest  feats  of  Biological  Microscopy  yet  performed,  moderate  angles  of 
aperture  are  unquestionably  uo  be  preferred  (vi.) — An  experienced  Micro- 
scopist  will  judge  of  the  defining  power  of  an  Objective  by  the  quality  of 
the  image  it  gives  of  any  fitting  object  with  which  he  is  familiar;  no  test 
being,  in  the  Author's  judgment,  more  suitable  than  the  Po^Z^^ra-scale 
(§  162).  Any  imperfection  in  Defining  power  is  exaggerated,  as  already 
pointed  out  (§§  26,  136),  by  Meep  Eye-piecing;'  so  that,  in  determin- 
ing the  value  of  an  Objective,  it  is  by  no  means  sufficient  to  estimate  its 
performance  under  a  low  Eye-piece — an  image  which  appears  tolerably 


^  This  point  has  been  long  kept  before  the  mind  of  the  Author,  by  his  studies 
in  Stereoscopic  Microscopy ;  the  condition  of  the  effect  of  relief  in  the  Binocular 
image  being  the  dissimilarity  of  the  pictures  of  any  object  not  absolutely  flat, 
that  are  formed  by  the  right  and  the  left  halves  of  the  Objective  respectively 
(§  39).  And  he  is  glad  to  tind  his  view  of  its  importance  confirmed  by  so  able  a 
practical  Optician  as  Mr.  Zentmayer ;  vrho.  in  a  Lecture  on  the  Elementary  Prop- 
erties of  Lenses,  published  in  the  Journal  of  the  Franklin  Institute  "  for  May  and 
June,  1876,  and  cited  in  the  Monthly  Microscop.  Journ.,"  Vol.  xvi.  (1876),  p.  317, 
called  attention  prominently  to  the  confusion  of  images  necessarily  attendant 
upon  large  apertures,  except  when  viewing  absolutely  flat  objects,  from  the  fact 
that  the  image  formed  by  pencils  transmitted  by  one  side  of  the  lens  are  unavoid- 
ably different  from  corresponding  images  formed  by  the  opposite  side  of  the  lens. 

"2    Proceeding  of  Royal  Society,"  June  19th,  1879, 


MANAGEMENT  OF  THE  MICROSCOPE. 


163 


dear  when  moderately  magnified,  being  often  found  exceedingly  deficient 
in  sharpness  when  more  highly  amplified.  The  use  of  the  Draw-tube 
(§83)  affords  an  additional  means  of  testing  the  Defining  power;  but 
recourse  cannot  be  fairly  had  to  this,  unless  an  alteration  be  made  in  the 
adjustment  for  the  thickness  of  the  glass  that  covers  the  object  (§  139), 
in  proportion  to  the  nearer  approximation  of  the  object  to  the  Objective 
whicli  the  lengthening  of  the  body  involves. 

III.  The  Penetrating  power  or  '  focal  depth '  of  an  Object-glass  may 
be  defined  as  consisting  in  the  vertical  range  through  whicli  the  parts  of 
an  object  not  precisely  in  the  focal  plane  may  be  seen  with  sufiicient 
distinctness  to  enable  their  relations  with  what  does  lie  precisely  in  that 
plane  to  be  clearly  traced  out;  just  as  we  could  do  by  ordinary  vision,  if 
the  object  were  itself  enlarged  to  the  size  of  its  Microscopic  image. — Now 
this  is  a  quality  which  is  very  differently  valued  by  different  observers, 
according  to  the  nature  of  the  work  on  which  they  may  be  severally 
engaged.  The  Histologist  who  is  scrutinizing  the  elementary  compo- 
nents of  a  tissue  that  is  spread  out  in  the  thinnest  possible  film  between 
two  plane  surfaces  of  glass,  considers  ^penetration'  rather  an  evidence  of 
imperfection  in  his  Objective,  which  (he  affirms)  cannot  show  him 
anything  save  what  is  exactly  in  the  focal  plane,  without  a  sacrifice  of  its 
highest  attainable  capacity  for  doing  the  latter.  On  the  other  hand,  the 
Anatomist  who  is  studying  the  general  organization  of  some  minute 
Plant  or  Animal,  or  the  structure  of  individual  organs  in  a  larger  one, 
finds  a  certain  amount  of  '  penetration '  essential  to  his  recognition  of 
the  relations  between  the  several  parts  of  the  object  whicli  are  suc- 
cessively brought  into  distinct  views  by  alterations  of  the  focal  adjust- 
ment. And  the  Physiologist  who  is  watching  the  actions  that  are  going 
on  in  a  living  Organism  or  in  some  component  part  of  it  (as,  for  exam- 
ple, the  internal  movements  of  an  Amoeba,  or  the  cyclosis  in  a  leaf-cell  of 
Vallisneria)  could  form  no  satisfactory  conception  of  such  phenomena, 
if,  instead  of  passing  gradationally  (as  an  Objective  of  good  '  penetration ' 
allows  him  to  do)  from  one  focal  plane  to  another,  he  can  only  get  a 
series  of  '  dissolving  views '  with  an  interval  of  '  chaos '  between  each,  as  he 
does  when  working  with  an  Objective  whose  '  penetration'  has  been  sacri- 
ficed to  Angular  aperture. — For  the  study  of  opaque  objects  which 
present  such  inequalities  of  surface  as  to  render  it  impossible  to  appre- 
hend their  true  forms  unless  much  more  can  be  seen  than  is  precisely  in 
focus  at  once,  good  '  penetrating'  power  is  obviously  essential;  and  this 
is  indispensable  to  the  advantageous  use  of  the  Stereoscopic  Binocular, 
which  grossly  exaggerates  the  effect  of  projection,  when  objects  are 
viewed  under  Objectives  of  too  wide  an  angle  (§  39). — No  definite  rule 
can  be  laid  down  as  to  the  relation  which  the  '  focal  depth '  of  an 
Objective  bears  to  its  Svorking  distance'  and  its  ^angular  aperture;' 
because  much  depends  upon  the  mode  of  their  construction.  But  it  may 
be  stated  generally  that  Objectives  of  longest  working  distance  have  the 
greatest  ^penetration;'  whilst  the  widening  of  the  Angular  aperture 
diminishes  penetration  at  a  rapidly  increasing  rate.  * 


^  The  Author  is  informed  by  Prof.  Abbe,  that,  theoretically— the  plan  of  con- 
struction remaining  the  same — the  '  penetration '  of  an  Objective  decreases,  as  the 
square  of  the  Angular  aperture  increases.— It  is  perfectly  well-known  to  Photo- 
graphers, that  a  good  picture  of  the  interior  of  a  long  Sculpture-gallery,  showing 
both  the  near  and  the  distant  parts  with  tolerable  distinctness,  can  only  be 
obtained  by  a  lens  of  very  narrow  angle. — The  singular  assertion  lately  made  by 
Dr.  Blackham      On  Angular  Aperture  of  Objectives,"  New  York,  1880),  that 


164 


THE  MICROSCOPE  AND  ITS  REVELATIONS. 


IV.  The  ^Resolving  power '  by  which  very  minute  and  closely  approx- 
imated markings — whether  lines,  striae,  dots,  or  apertures — are  separ- 
ately discerned,  has  now  been  clearly  shown  to  depend  upon  Angular 
aperture  (§  157);  and  this,  not  so  much — as  formerly  supposed — on  account 
of  the  greater  obliquity  of  the  rays  Avhich  large-angled  Objectives  will 
admit,  as  because  of  their  capacity  to  receive  and  recombine  the  '  dif- 
fraction-spectra Hhat  lie  Avithout  the  range  of  Objectives  of  more  lim- 
ited angle.  In  comparing  the  ^ resolving^  powers  of  different  Objec- 
tives, it  must  be  borne  in  mind  that  the  advantages  of  wide  aperture 
will  be  lost,  if  the  obliquity  of  the  illumination  does  not  correspond  with 
that  of  the  most  divergent  rays  which  enter  the  Objective  to  take  part  in 
the  formation  of  the  image.  But  when  the  question  is  not  of  the  reso- 
lution of  surface-markings  (such  as  those  of  Diatom-valves),  but  of  the 
determination  of  internal  structure  (as,  for  example,  in  the  study  of  the 
process  of  division  in  cell-nuclei),  axial  illumination  is  decidedly  to  be 
preferred,  as  being  attended  with  less  liability  than  oblique  to  produce 
deceptive  api3earances. — It  appears  from  the  theoretical  researches  of 
Prof.  Abbe,  that  the  maximum  attainable  resolving  power  with  an  An- 
gular aperture  of  180°  should  separate  118,000  lines  to  the  inch;  and 
this  agrees  well  with  what  has  been  actually  accomplished  (§  160).  But 
the  loss  of  ^ resolving^  power  consequent  upon  the  contraction  of  the 
aperture  from  180°  to  128^°  is  only  10  per  cent.;  while  a  further  reduc- 
tion to  106 J°  only  lowers  the  number  of  separable  lines  to  94,400  per 
inch. 

V.  The  ^  Flatness  of  the  field  ^  afforded  by  the  Object-glass  is  a  con- 
dition of  great  importance  to  the  advantageous  use  of  the  Microscope, 
since  the  real  extent  of  the  field  of  view  practically  depends  upon  it. 
Many  Objectives  are  so  constructed,  that,  even  with  a  perfect  flat  ob- 
ject, the  foci  of  the  central  and  of  the  peripheral  parts  of  the  field  are  so 
different,  that  when  the  adjustment  is  made  for  one,  the  other  is  ex- 
tremely indistinct.  Hence,  when  the  central  portion  is  being  looked  at, 
no  more  information  is  gained  respecting  the  peripheral,  than  if  it  had 
been  altogether  stopped  out.  With  a  really  good  Object-glass,  not  only 
should  the  image  be  distinct  even  to  the  margin  of  the  field,  but  the 
marginal  portion  should  be  as  free  from  color  as  the  central.  In  many 
microscopes  of  inferior  construction,  the  imperfection  of  the  Objectives 
in  this  respect  is  masked  by  the  contraction  of  the  aperture  of  the  dia- 
phragm in  the  Eye-piece  (§  27),  which  limits  the  dimensions  of  the 
field;  and  the  performance  of  one  Objective  within  this  limit  may  scarcely 
be  distinguishable  from  that  of  another,  although,  if  the  two  were  com- 
pared under  an  Eye-piece  of  larger  aperture,  their  difference  of  excellence 
would  be  at  once  made  apparent  by  the  perfect  correctness  of  one  to  the 
margin  of  the  field,  and  by  the  entire  failure  of  the  other  in  every  part 
save  its  centre.  In  estimating  the  relative  merits  of  two  lenses,  there- 
fore, as  regards  this  condition,  the  comparison  should  be  made  under 
an  Eye-piece  giving  a  large  field. 

VI.  The  most  perfect  objective  for  general  purposes,  is  obviously  that 
which  combines  all  the  preceding  attributes  in  the  degree  in  which  they 
are  mutually  compatible.  But  it  seems  to  be  now  clear  that  the  highest 
perfection  of  the  two  primary  qualities,  ^defining'  power  and  ^resolving 


*  depth  of  focus '  has  no  relation  to  Aperture,  but  depends  on  residual  "  {i.  6., 
uncorrected)  Spherical  Aberration,  and  that  "  the  less  the  lens  has  of  it,  the  bet- 
ter the  lens,"  does  not  require  serious  refutation. 


MANAGEMENT  OF  THE  MICROSCOPE. 


165 


power/  cannot  be  obtained  in  the  same  combination;  so  that  the  choice 
between  two  Objectives,  one  distingnished  by  the  former  of  these  attri- 
butes, and  the  other  by  the  latter,  will  depend  upon  the  kind  of  work  on 
which  it  is  to  be  employed.  If  the  resolutions  of  the  markings  on  Dia- 
tom-valves is  the  Microscopist's  special  pursuit/  he  will  rightly  prefer  an 
Objective  of  the  largest  attainable  angle,  with  the  best  definition  that 
is  compatible  with  it.  But  if  he  be  engaged  upon  difficult  Biological  in- 
vestigations, he  will  do  well  to  make  perfect  'definition^  his  stJie  qua 
11071,  and  to  be  content  with  the  largest  angle  that  can  be  obtained  with- 
out a  sacrifice  of  this.  It  is,  as  already  stated,  in  admitting  of  perfect 
correction  for  Spherical  Aberration,  even  to  an  aperture  of  180°,  that 
the  great  superiority  of  the  ^immersion  system  '  consists;  but  the  great- 
est perfection  in  the  construction  of  even  an  immersion  Objective, 
cannot  (in  the  nature  of  things)  prevent  that  impairment  of  defini- 
tion, which  has  been  experimentally  as  well  as  theoretically  shown 
by  Dr.  Eoyston  Pigott  to  be  consequent  upon  excessive  widening  of  the 
angle  of  aperture.  The  most  serviceable  Objectives  for  the  most  dif- 
ficult Biological  investigations,  therefore,  will  (in  the  Author's  judg- 
ment) be  such  as  possess  the  combination  of  qualities  attributed  by  Mr. 
Dallinger  to  the  l-35th  inch  constructed  specially  for  his  work  by  Messrs. 
Powell  and  Lealand;  ''the  angle  is  moderate;  its  definition  very  crisp 
and  clear;  and  its  penetration,  considering  its  magnifying  power,  very 
considerable.^^ 

159.  Test'Ohjects, — It  is  usual  to  judge  of  the  optical  perfection  of  a 
Microscope  by  its  capacity  for  exhibiting  certain  objects,  which  are  re- 
garded as  Tests  of  the  merits  of  its  Object-glasses;  these  tests  being  of 
various  degrees  of  difficulty,  and  that  being  accounted  the  best  instru- 
ment which  shows  the  most  '  difficult'  of  such  tests.  Now  it  must  be 
borne  in  mind  that  of  the  qualities  which  have  been  just  enumerated, 
the  '  tests  ^  usually  relied-on  have  reference  almost  exclusively  to  two — 
viz.,  definition  and  resolving  potoer;  and  that  the  greater  number  of 
them,  being  objects  whose  surface  is  marked  by  lines,  striae,  or  dots,  are 
tests  of  resolving  power,  and  thus  of  Angular  aperture  only.  Hence,  as 
already  shown,  an  Objective  may  resolve  some  very  difficult  test-objects, 
and  yet  may  be  very  unfit  for  ordinary  use.  Moreover,  these  'difficult' 
tests  are  only  suitable  to  Object-glasses  of  very  short  focus  and  high  mag- 
nifying power;  whereas  the  greater  part  of  the  real  tvorh  of  the  Micro- 
scope is  done  with  Objectives  of  low  and  medium  power;  and  the 
enlargement  of  the  Angular  aperture,  which  enables  one  of  these  to  re- 


^  It  is  assuredly  neither  the  only  nor  yet  the  chief  vs^ork  of  the  Microscope  (as 
some  appear  to  suppose)  to  resolve  the  markings  on  the  siliceous  valves  of  Dia- 
toms ;  in  fact,  the  interest  which  attaches  to  observations  of  this  class  is  entirely 
confined  to  the  value  of  these  objects  as  *  tests '  of  the  performance  of  Objectives 
(§  159).  If  one-tenth  of  the  attention  which  has  been  devoted  to  the  scrutiny  of 
these  objects  with  instruments  of  the  highest  class,  had  been  given  to  the  study 
of  the  Life-history  of  the  minute  Plants  which  furnish  them,  with  such  a  Stu- 
dent's microscope  as  thirty  years  ago  enabled  Mr.  Thwaites  to  discover  their 
*  conjugation,'  it  cannot  be  doubted  that  vast  benefit  would  have  accrued  to  Bio- 
logical Science. — It  has  been  urged  that  the  acquirement  of  the  power  of  dis- 
playing '  difficult '  Diatom-tests,  is  a  valuable  '  gymnastic '  for  the  training  of  Mi- 
croscopists;  but  the  experience  of  the  Author,  and  of  every  Biological  teacher  he 
knows,  is  that  a  much  better  training  for  the  Student  is  to  begin  with  the  study 
of  such  easy  objects — e,  g.,  the  Yeast-Plant,  and  Colorless  Blood-Corpuscles, — as 
afford  him  the  experience  which  it  is  absolutely  essential  that  he  should  acquire 
in  the  first  instance,  and  to  proceed  gradually  from  these  to  the  more  difficulty 
gaining  new  knowledge  at  every  stage. 


166 


THE  MICROSCOPE  AND  ITS  REVELATIONS. 


solve  (under  deep  Eye-pieces)  many  objects  which  were  formerly  consid- 
ered adequate  tests  for  higher  powers,  is  for  ordinary  purposes  rather 
injurious  than  beneficial,  detracting  from  the  value  of  the  Objective  for 
the  work  to  which  it  is  specially  adapted.  For  Microscopists  of  large 
.Biological  experience  know  perfectly  well  that  every  ^  power  ^  has  its  own 
proper  range  and  capacity;  and  that  they  work  most  satisfactorily  with 
the  ^powcr  ^  most  suitable  to  the  investigation  on  which  they  may  be  en- 
gaged. In  estimating  the  vahie  of  an  Object-glass^  it  should  always  be 
considered  for  tohat  purpose  it  is  intended;  and  its  merits  should  be 
judged-of  according  to  the  degree  in  which  it  fulfils  that  purpose.  We 
shall  therefore  consider  what  are  the  objects  proper  to  the  several  ^  powers ' 
of  Object-glasses — loio,  medium,  and  high;  and  what  are  the  objects  by 
its  mode  of  exhibiting  which,  each  may  be  fairly  judged. 

I.  By  Objectives  of  loiv  power  we  may  understand  any  whose  focal 
length  is  greater  than  Half -an  inch.  The  ^  powers^  usually  made  in  this 
country  are  known  as  4  inch,  3  inch,  2  inch,  \\  inch,  1  inch,  and  2  3ds 
inch  focus;  and  they  give  a  range  of  amplification  of  from  10  to  70  dia- 
meters with  the  A  eye-piece,  and  of  from  16  to  120  diameters  with  the 
B  eye-piece.  An  ^ adjustable^  low  power  is  made  by  Zeiss  of  Jena  (ob- 
tainable from  Messrs.  Baker),  in  which,  by  varying  the  position  of  the 
front-lens  by  means  of  a  screw-collar,  a  range  of  power  is  obtainable  from 
about  8  to  16  diameters  with  the  A  eye-piece,  and  from  12  to  24  with  the 
B  eye-piece.  This  has  been  found  by  the  Author  extremely  convenient 
for  the  display  of  large  opaque  objects,  of  which  it  is  desired  to  show  the 
whole  under  as  high  an  amplification  as  will  make  their  images  fill  the 
field.  Objectives  of  Ioid  power  are  most  used  in  the  examination  of 
opaque  objects,  and  of  Transparent  objects  of  large  size  and  of  compara- 
tively coarse  texture;  and  the  qualities  most  desirable  in  them  are  a  sufii- 
ciently  large  aperture  to  give  a  bright  image,  combined  with  such  accu- 
rate definition  as  to  give  a  clear  image,  with  ^  focal  depth  ^  suflicient  to 
prevent  any  moderate  inequalities  of  surface  from  seriously  interfering 
with  the  distinctness  of  the  entire  picture,  and  with  perfect  ^  flatness '  of 
the  image  when  the  object  itself  is  flat.  For  the  3  inch,  2  inch,  or  1^ 
inch  Objectives,^  no  ground  of  judgment  is  better  than  the  manner  in 
which  it  shows  such  an  injected  preparation  as  the  interior  of  a  Frog's 
Lung  (Fig.  485)  or  a  portion  of  the  villous  coat  of  the  Monkey's  Intes- 
tine (Fig.  479);  for  the  aperture  ought  to  be  sufficient  to  give  a  bright 
image  of  such  objects  by  ordinary  daylight,  without  the  use  of  any  illu- 
minator; the  border  of  every  vessel  should  be  clearly  defined,  without  any 
thickness  or  blackness  of  edge;  every  part  of  such  an  object  that  comes 
within  the  field  should  be  capable  of  being  made-out  when  the  focal  ad- 
justment is  adapted  for  any  other  part;  whilst,  by  making  that  adjust- 
ment a  medium  one,  the  whole  should  be  seen  without  any  marked  in- 
distinctness. If  the  Aperture  be  too  small,  the  image  will  be  dark:  but 
if  it  be  too  large,  details  are  brought  into  view  (such  as  the  separateness 
of  the  particles  of  the  vermilion  injection)  which  it  is  of  no  advantage  to 
see;  whilst,  through  the  sacrifice  of  penetration,  those  parts  of  the  object 
which  are  brought  exactly  into  focus  being  seen  with  over-minuteness, 
the  remainder  are  envelo])ed  in  a  thick  fog  through  which  even  their  gen- 
eral contour  can  scarcely  be  seen  to  loom.    If  the  corrections  be  imper- 


^  These  are  ordinarily  composed  of  two  pairs  of  lenses  only,  as  the  corrections 
can  be  ad«lq[uately  made  by  this  combination  for  an  Angular  aperture  of  33°, 
which  is  the  largest  that  is  found  practically  useful  for  the  1^  inch. 


MANAGEMENT  OF  THE  MICROSCOPE. 


167 


fectly  made,  no  line  or  edge  will  be  seen  with  perfect  sharpness.  For 
Defining  power,  the  Author  has  found  the  Pollen-grains  of  the  Holly- 
hock or  any  other  flower  of  the  Malloiu  kind  (Fig.  277,  a)  viewed  as  an 
opaque  object,  a  very  good  test;  the  minute  spines  with  which  they  arc 
beset  being  but  dimly  seen  with  any  save  a  good  Object-glass  of  these 
long  foci,  and  being  really-well  exhibited  only  by  adding  such  power  to 
the  Eye-piece,  as  will  exaggerate  any  want  of  definition  on  the  part  of  an 
inferior  lens.  For  Flatness  of  field  no  test  is  better  than  a  section  of 
Wood  (Fig.  253)  or  a  large  Echinus  spine  (Fig.  369),  under  an  Eye-piece 
that  will  give  a  field  of  the  diameter  of  from  9  to  12  inches.  The  general 
performance  of  Object-glasses  of  1-inch  and  2-3ds  inch  focus,  may  be 
partly  judged-of  by  the  manner  in  which  they  show  such  injections  as 
those  of  the  Gill  of  the  Eel  (Fig.  484),  or  of  the  Bird's  Lung  (Fig.  486), 
which  require  a  higher  magnifying  power  for  their  resolution  than  those 
previously  named;  still  better,  perhaps,  by  the  mode  in  which  they  ex- 
hibit a  portion  of  the  wing  of  some  Lepidopterous  Insect  having  well 
marked  scales.  The  same  qualities  should  here  be  looked-for,  as  in  the 
case  of  the  lowest  powers;  and  a  want  of  either  of  them  is  to  be  distin- 
guished in  a  similar  manner.  The  increaso  of  Angular  aperture  which 
these  Objectives  may  advantageously  receive  up  to  30%  should  render 
them  capable  of  resolving  all  the  easier  ^test'  scales  of  Lepidoptera,  such 
as  those  of  the  Morplio  menelaits  (Fig.  414),  in  which,  with  the  B  eye- 
piece, they  should  show  the  transverse  as  well  as  the  longitudinal  mark- 
ings. The  Proboscis  of  the  Blow-fly  (Fig.  428)'  is  one  of  the  best  trans- 
parent objects  for  enabling  a  practised  eye  to  estimate  the  general 
performance  of  Object-glasses  of  these  powers;  since  it  is  only  under  a 
really  good  lens,  that  all  the  details  of  its  structure  can  be  well  shown. 
In  particular,  all  the  outlines  and  edges  should  be  seen  clearly  and 
sharply,  without  any  haze  or  fringe;  the  tracheal  spires  and  rings  should 
be  well-defined,  without  any  color  between  them;  and  there  should  be  no 
indication  of  general  mist.  An  Objective  which  shows  this  well,  may  be 
trusted  for  any  other  object  of  its  kind.  For  Flatness  of  field,  sections  of 
small  Echinus-spines  (Plate  IT.,  fig.  1)  are  very  good  tests.  The  exact- 
ness of  the  corrections  in  lenses  of  these  foci  may  be  judged-of  by  the  ex- 
amination of  objects  which  are  almost  sure  to  exhibit  Color  if  the  correc- 
tion be  otherwise  than  perfect.  This  is  the  case,  for  example,  with  the 
so-called  glandulce  of  Coniferous  wood  (Fig.  248),  the  centres  of  which 
ought  to  be  clearly  defined  under  such  objectives,  and  to  be  quite  free 
from  color;  and  also  with  the  tracliece  of  Insects  (Fig.  432),  the  spires  of 
which  ought  to  be  distinctly  separated  from  each  other,  without  any  ap- 
pearance of  intervening  chromatic  fringes. 

II.  We  may  consider  as  Objectives  of  medium  power  the  Half-inch, 
4-lOths  inch,  l-4th  inch,  and  l-5th  inch;  the  magnifying  power  of  which 
ranges  from  about  90  to  250  diameters  under  the  A  eye-piece,  and  from 
about  150  to  400  diameters  with  the  B  eye-piece.  The  first  three,  when 
used  by  reflected  light,  can  be  advantageously  employed  in  the  examina- 
tion of  such  small  opaque  objects  as  Diatoms,  Polycystina,  portions  of 
small  Feathers,  capsules  of  the  lesser  Mosses,  Hairs,  etc. ;  they  should  be 
so  mounted  on  cones  as  to  allow  of  side  illumination;  and  the  l-4th 
should  have  suflBcient  working  distance  to  permit  its  easy  use  for  these 


^  This  object  should  be  mounted  in  Glycerine- jelly;  for  when  mounted  in  Bal- 
sam, the  parts  are  usually  flattened-out  and  squeezed  together,  so  that  their  real 
forms  and  relative  positions  cannot  be  seen. 


168 


THE  MICROSCOPE  AND  ITS  REVELATIONS. 


purposes,  with  an  aperture  not  exceeding  80°.  Larger-angled  l-4ths  and 
l-5ths  cannot  be  conveniently  used  for  opaque  objects,  unless  these  are 
shown  by  Prof.  Smith's  or  some  analogous  illumination  (§  116). — The 
great  value  of  these  powers  lies  in  the  information  they  enable  us  to 
obtain  regarding  the  details  of  organized  structures  and  of  living  actions, 
by  the  examination  of  properly-prepared  transparent  objects  by  trans- 
mitted light;  and  it  is  to  them  that  the  remarks  already  made  respecting 
Angular  aperture  (§  158,  ii.)  especially  apply;  since  it  is  here  that  the 
greatest  difference  exists  between  the  ordinary  requirements  of  the 
Scientific  investigator,  and  the  special  needs  of  those  who  devote  them- 
selves to  the  particular  classes  of  objects  for  which  the  greatest  ^resolv- 
ing '  power  is  required.  A  moderate  amount  of  such  power  is  essential 
to  the  value  of  every  Objective  within  the  above-named  range  of  foci: 
thus,  even  a  good  half-inch  should  enable  the  markings  of  the  larger 
scales  of  the  Polyommatus  argiis  (^azure-blue  Butterfly')  to  be  well  dis- 
tinguished— these  being  of  the  same  kind  with  those  of  the  Menelaus,  but 
more  delicate — and  should  clearly  separate  the  dots  of  the  small  or 
^battledoor'  scales  (Fig.  41G)  of  the  same  Insect,  which,  if  unresolved, 
are  seen  as  coarse  longitudinal  lines;  a  good  4:-10ths  inch  should  resolve 
the  larger  scales  of  the  Podura  (Plate  II.,  fig.  2)  without  difficulty;  and 
a  good  l-4th  or  l-5th-inch  should  bring  out  the  markings  on  the  smaller 
scales  of  the  Podura,  and  should  resolve  the  markings  on  the  Pleuro- 
sifjma  angulatum  into  lozenge-lines,  the  B  and  0  eye-pieces  being  used 
when  the  scales  are  very  small  and  their  markings  delicate.  Even  the 
half-inch  or  the  4-lOths  inch  may  be  made  with  angles  of  aperture  suffi- 
ciently wide  to  resolve  the  objects  named  as  difficult  tests  for  the  powers 
above  them ;^  but  for  the  reasons  already  stated,  the  Author  thinks  it 
most  undesirable  that  they  should  be  thus  forced  up  to  the  work 
altogether  unsuited  to  their  powers,  by  a  sacrifice  of  those  very  qualities 
which  constitute  their  special  value  in  the  study  of  the  objects  whereon 
they  can  be  most  appropriately  and  effectively  employed.  And  he  is 
decidedly  of  opinion  that  an  angular  aperture  of  50°  is  as  great  as  should 
be  given  to  a  IIalf-?nch,  60°  to  a  4:-10ths  inch,  and  90°  to  a  l-4th  inch, 
that  are  destined  for  the  ordinary  purposes  of  Scientific  investigation: 
whilst  his  own  experience  would  lead  him  to  prefer  an  angle  of  40°  for 
the  Half-inch  (§  39),  and  of  80°  for  the  l-4th  inch,  provided  the  correc- 
tions are  perfect.  Objectives  of  these  apertures  should  show  the  easier 
tests  first  enumerated,  with  perfect  Definition,  a  fair  amount  of  Pene- 
trating power,  and  complete  Flatness  of  field.  No  single  object  is  so 
useful  as  the  Podura-scale  for  the  purpose  of  testing  these  qualities  in  a 
l-4th  inch  or  l-5th  inch  Objective;  and  it  may  be  safely  said  that  a  lens 


^  By  Mr.  Tolles  (Boston,  N.E.)  the  Angle  of  the  half-inch  is  carried  to  80'';  and 
that  of  the  4-lOths  to  145°.  And  it  has  lately  been  seriously  maintained  that  an 
Objective  of  the  latter  focus  supplies  almost  every  need  of  the  Biologist,  since,  as 
even  diflScult  Diatom-tests  can  be  shown  by  it,  it  can  be  worked  up  by  deep  Eye- 
piecing  to  the  highest  power  that  he  requires,  except  for  special  investigation f. 
But  the  resolution  of  a  Diatom  is  one  thing,  while  the  prosecution  of  investiga- 
tion continued  through  several  hours  at  a  time  is  quite  another;  and  the  Author, 
regarding  the  advice  of  this  writer  as  most  dangerous  to  the  eyes  of  those  who 
may  follow  it,  deems  it  his  duty  to  enter  his  protest  against  it. — Many  excellent 
makers  now  make  first-class  Objectives  of  narrow  as  well  as  wide  angles;  thus, 
Messrs.  Powell  and  Lealand,  followed  by  several  others,  make  the  half-inch  of 
40°  (first  constructed  for  the  Author,  to  be  used  with  the  Stereoscopic  Binocular), 
as  well  as  a  half -inch  of  70°;  Messrs.  Beck  make  a  4-lOths  of  55°,  as  well  as  one  of 
90° ;  and  Mr.  Crouch  a  l-4th  of  60°,  another  of  105^,  and  another  of  140°. 


I 


MANAGEMENT  OF  THE  MICROSCOPE. 


169 


which  brings  out  its  markings  satisfactorily  will  suit  the  requirements  of 
the  ordinary  working  Microscopist,  although  it  may  not  resolve  difficult 
Diatoms.  In  every  case,  the  Objective  should  be  tried  with  the  B  and 
C  as  well  as  with  the  A  eye-piece;  and  the  effect  of  this  substitution  will 
be  a  fair  test  of  its  merits.  Where  markings  are  undistinguishable  under 
a  certain  Objective,  merely  because  of  their  minuteness  or  their  too  close 
approximation,  they  m.ay  be  enlarged  or  separated  by  a  deeper  Eye-piece, 
provided  that  the  Objective  be  well  corrected.  But  if,  in  such  a  case,  the 
image  be  darkened  or  blurred,  so  as  to  be  rather  deteriorated  than 
improved,  it  may  be  concluded  that  the  Objective  is  of  inferior  quality, 
having  either  an  insufficient  Angular  aperture,  or  being  imperfectly 
corrected,  or  both. 

HI.  All  Object-glasses  of  less  than  l-5th  inch  focus  may  be  classed 
as  liigh  powers;  the  focal  lengths  to  which  they  are  ordinarily  constructed 
being  l-6th,  l-8tli,  1-lOth,  l-12th,  l-16th,  l-20th,  l-25th,  l-40th,  and 
l-50th  of  an  inch  respectively;  the  l-12th,  l-16th,  ]-25th,  and  l-50th 
being  made  by  Messrs.  Powell  and  Lealand,  and  the  1-lOth,  l-20th,  and 
l-40th  by  Messrs.  Beck.  The  magnifying  powers  which  Objectives  from 
l-6th  to  l-25th  inch  focus  are  fitted  to  afford,  range  from  about  320  to 
1250  diameters  with  the  shallower  Eye-piece,  and  from  480  to  1850  dia- 
meters with  the  deeper:  but  by  the  use  of  still  deeper  Eye-pieces,  or  by 
the  Objective  of  l-50th  inch,  or  the  l-80th  recently  constructed  by 
Messrs.  Powell  and  Lealand,  a  power  of  4000  or  more  may  be  obtained. 
It  is  seldom,  howeyer,  that  anything  is  really  gained  thereby. — The  in- 
troduction of  ifmnersion-lenscs  (§  19)  has  considerably  increased  the 
utility  of  what  may  be  called  moderately  high  powers,  such  as  l-8th, 
1-lOth,  and  l-12th.  These,  if  really  good,  can  be  used  when  necessary 
with  deep  Eye-pieces;  and  yery  little  of  scientific  importance  that  is  be- 
yond their  reach  has  yet  been  seen  by  higher  Objectives,  though  the  lat- 
ter have,  no  doubt,  special  value  in  certain  circumstances  when  skilfully 
employed.  With  these  and  higher  powers  not  intended  for  exclusive 
use  upon  ^yexatious'  Diatoms,  the  Angle  of  aperture  should  be  so  pro- 
portioned to  focal  length,  as  not  to  sacrifice  the  '  definition '  and  '  penetra- 
tion ^  required  to  show  the  internal  organs  of  small  Eotifers,  large  Infu- 
soria, minute  Worms,  etc.  An  Objective  that  will  show  surfaces  only, 
may  be  broadly  stated  to  be  of  little  use  for  Biological  investigation. 
Dry-front  l-8ths  or  l-12ths  with  an  aperture  closely  approaching  170°, 
are  of  very  limited  utility,  from  want  of  penetration,  and  from  focussing 
extremely  close  to  their  objects;  while  with  30°  to  40^  less  aperture  and 
good  corrections,  they  are  much  more  serviceable,  losing  yery  little  (as 
already  shown,  §  158,  iv.)  in  ^ resolving^  power,  and  gaining  much  in 
working  distance  and  penetration. 

160.  For  Eesolving  power,  the  best  tests  are  afforded  by  the  lines  ar- 
tificially ruled  by  M.  Nobert,  and  by  the  more  ^ difficult^  Diatoms. — 
What  is  known  as  Noierfs  Test  is  a  plate  of  glass,  on  a  small  space  of 
which,  not  exceeding  one-fiftieth  of  an  inch  in  breadth,  are  ruled  from 
ten  to  nineteen  series  of  lines,  forming  as  many  separate  bands  of  equal 
breadth.  In  each  of  these  bands,  the  lines  are  ruled  at  a  certain  known 
distance;  and  the  distances  are  so  adjusted  in  the  successive  bands,  as  to 
form  a  regular  diminishing  series,  and  thus  to  present  a  succession  of 
tests  of  progressively  increasing  difficulty.  The  distances  of  the  lines 
differ  on  different  plates;  all  the  bands  in  some  series  being  resolvable 
under  a  good  Objective  of  l-4th  inch  focus,  whilst  the  closest  bands  in 
others  long  defied  the  resolving  power  of  l-12th  inch  Objectives  of  large 


170 


THE  MICROSCOPE  AND  ITS  REVELATIONS. 


Aperture.    On  the  ninefceen-band  Test-plate  the  lines  are  ruled  at  the 
following  distances,  expressed  in  parts  of  a  Paris  line,  which,  to  an  Eng- 
lish inch,  is  usually  reckoned  as  .088  to  1.000,  or  as  11  to  125: — 
Band  1.  1-lOOOth.        Band  8.  l-4500th.        Band  14.  l-7500th. 

2.  l-1500th.  "    9.  l-5000th.  "    15.  l-SOOOth. 

3.  l-2000th.  10.  l-5500th.  16.  l-8500th. 

"    4.  l-2500th.  11.  l-6000th.  "    17.  l-OOOOth.  > 

"    5.  l-3000th.  12.  l-6500th.  18.  l-9500th. 

6.  l-3500th.  "  13.  l-7000th.  19.  MOOOOth. 

7.  l-4000th. 

The  following  exact  estimates  of  the  numbers  of  the  lines  to  the  Eng- 
lish inch,  in  some  of  the  Bands,  are  given  by  Dr.  Koyston  Pigott:^ — 

Band.  Band,        of  spaces  ^^^^  No,  of  spaces 

per  inch.  per  inch.  per  inch, 

I.  11,259.51358.  IX.  56,297.56790.  XV.  90,076.10864. 

III.  22,519.02716.  XL  67,557.08148.  XVII.  101,335.62222. 

IV.  33,778.54074.  XIII.  78,816.59506.  XIX.  112,595.13580. 
VII.  45,038.05432. 

In  objects  like  Nobert's  Test-plate,  spurious  diffraction  lines  are  easily 
mistaken  for  genuine  resolution;  and  the  difficulty  of  resolving  the  higher 
bands  of  his  series  was  formerly  supposed  to  be  an  optical  impossibility. 
The  more  recent  investigations  of  Helmholtz  and  Abbe,  however,  have 
disposed  of  this  theoretical  objection;  and  the  ^resolution  ^  of  Robert's  19th 
band,  which  was  long  supposed  to  be  a  sort  of  crux  of  Microscopy,  is 
now  easily  demonstrable. 

161.  It  cannot  be  questioned  that  the  recognition  of  the  value  of  the 
markings  on  the  siliceous  valves  of  the  Diatoms  as  Test-objects  (first 
made  by  Messrs.  Harrison  and  SoUitt,  of  Hull,  in  1841)  has  largely  con- 
tributed to  the  success  of  the  endeavors  which  have  since  been  so  effectu- 
ally made,  to  perfect  high-power  Objectives,  and  to  devise  new  methods 
of  using  them  to  the  best  advantage.  But  it  has  now  been  demonstrated, 
both  theoretically  and  practically,  that  the  power  of  ^resolving'  these 
markings  essentially  depends  on  the  Angular  aperture  of  the  Objective: 
so  that,  as  a  lens  which  possesses  it  in  a  high  degree  may  be  very  defi- 
cient in  ^definition,'  and  will  probably  have  an  inconveniently  short 
^  working  distance' with  very  little  ^penetration' — qualities  essential  to 
an  Objective  to  be  employed  in  Biological  investigation, — the  resolution 
of  difficult  Diatom-tests  by  no  means  proves  the  fitness  of  an  Objective 
for  the  ordinary  work  of  the  Microscopist. — Still,  these  tests  are  of  great 
value  for  the  purpose  to  which  they  are  really  adapted;  and  it  Avill  there- 
fore be  desirable  here  to  specify  their  relative  degrees  of  ^difficulty,' 
which  is  indicated  by  the  closeness  of  their  lineation,  leaving  for  future 
discussion  (§  277)  the  nature  of  the  structure  to  which  that  lineation  is 
due.  The  greater  part  of  the  Piatoms  now  in  use  for  this  purpose,  are 
comprehended  in  the  genus  Pleurosigma  of  Prof.  W.  Smith;  which 
includes  those  Naviculm  whose  ^frustules'  are  distinguished  by  their 
sigmoid  (S-like)  curvature  (Pig.  165). 

^  '*  Monthly  Microscopical  Journal,"  Vol.  ix.  (1878),  p.  (53.— A  much  larger 
number  of  lines  to  the  inch  has  been  assigned  to  Nobert's  Test-plate  by  Mr.  J. 
Allan  Broun  ("Proceedings  of  Royal  Society,"  Vol.  xxiii.,  1875,  p.  531),  on  the 
basis  of  his  measurement  of  Photographs  taken  by  Dr.  E.  Carter  (Surgeon  U.  S. 
Army);  but  there  seems  strong  ground  to  believe  that  either  from  diffraction, 
or  from  some  mistake  in  the  magnifying  power  employed,  Mr.  Broun's  estimate 
must  be  greatly  in  excess  of  the  reality. 


MANAGEMENT  OF  THE  MICEOSCOPE. 


171 


Direction      Strice  in  1-I00f7i  of  an  inch. 


OiUl  1  JEl« 

SOLLITT. 

1.  Pleurosigma  formosum 

...  uidguiiai     • . 

—  /iy) 

o 

/Q, 

 strigile 

•  •  •     tx  dllO  V  Cl  otJ    .  . 

36 

OA 

Q 

 Balticum 

. . .  transverse  . . 

ACi 

OA 

A 

4, 

 attenua.tum 

40 

A(\ 

OK 
  OO 

Ft 

o. 

 hippocampus  . 

•  .  .     tldllO  VtJJ  OtJ    .  , 

40 

A  w 

.  .  .  40 

A  A 

—  40 

6. 

 strigosum 

...  diagonal     . . 

AA. 

QCi 

A  A 

—  40 

7. 

 quadratum 

...    LUdgUIldi        .  . 

—  OO 

8. 

 elongatum 

H 1 Q  rr/^n  q  1 
.  .  1    Llict^vJlicli        .  . 

AH 

9. 

 lacustre 

. . .  transverse 

48 

10. 

 angulatum 

. . .  diagonal 

...  52 

...51 

—  46 

11. 

 aestuarii 

. . .  diagonal 

...  54 

12. 

 fasciola 

...    64  ... 

,  .  90 

—  50 

13. 

Navicula  rhomboides 

...    85    . . , 

,  .  Ill 

—  60 

14.  Nitzschia  sigmoidea 

. . .  transverse  . . 

.,.  85 

15.  Amphipleura  pellucida. 

...130 

—120 

(Navicula  acus). 


Good  specimens  of  the  first  ten  of  the  foregoing  list  may  be  resolved, 
with  Judicious  management,  by  good  small-angled  l-4th  or  l-5th  inch 
Objectives,  and  even,  with  very  oblique  illumination,  by  Objectives  of 
one-half  and  4-10 ths  inch,  having  an  angular  aperture  of  90°;  the  re- 
mainder require  the  larger  aperture  proper  to  the  l-8th  inch  or  higher 
power,  for  the  satisfactory  exhibition  of  their  markings.  The  first  column 
of  measurements  in  the  above  table  gives  the  numbers  stated  by  Prof.  W. 
Smith  as  averages;  the  second  column  gives  the  numbers  subsequently 
assigned  as  the  extreines  by  Mr.  Sollitt,^  who  pointed  out  that  great  dif- 
ferences exist  in  the  fineness  of  the  markings  of  specimens  of  the  same 
species  obtained  from  different  localities — a  statement  now  so  abundantly 
confirmed,  as  to  be  entitled  to  rank  as  an  established  fact.  It  is  in  regard 
to  Amphipleura  pellucida^  however,  that  the  greatest  diversity  of  opinion 
has  existed;  and  the  conclusion  which  the  Author  had  expressed  in  the 
earlier  editions  of  this  Manual,  that  Mr.  Sollitt's  estimate  was  much  too 
high  (having  been  based  on  ^spurious'  lineation),  has  been  fully  con- 
firmed by  Col.  Dr.  Woodward;  who,  having  succeeded  in  obtaining  very 
perfect  Photographs  of  this  Diatom,  under  powers  of  1500  and  16*50  diam- 
eters, has  found  that  the  stria3  on  the  largest  valves  w^ere  never  more  than 
91  in  1-lOOOth  of  an  inch,  while  those  on  the  smallest  never  exceeded  100 
in  the  lOOOth  inch.^  The  ^resolution'  of  the  lines  on  this  test  may  be 
made  without  much  difficulty  by  ^immersion'  Objectives  of  l-8th  inch 
without  any  excessive  Aperture;  but  the  resolution  of  the  lines  into  dis- 
tinct dots  is  a  severe  test  for  Objectives  of  largest  Aperture. — Several 
very  difficult  tests  of  this  description  have  been  furnished  by  the  late 
Prof.  Bailey^  of  West  Point  (U.S.);  among  them  the  very  beautiful  Gram- 
matophora  subtilissimu  and  the  Hyalodiscus  suMilis,  the  latter  being  of 
discoid  form,  and  having  markings  which  radiate  in  all  directions,  very 
much  like  those  of  an  engine-turned  watch. — To  these  may  be  added  the 
Surirella  gemr)ia,  which  presents  appearances  of  a  very  deceptive  character. 
These  appearances,  as  represented  by  M.  Hartnack,  are  shown  in  Fig. 
118,  A,  b;  the  upper  part  of  the  valve  A  being  illuminated  by  oblique 
light  in  the  direction  of  its  axis,  and  the  lower  part  by  oblique  light  in  a 


*  *  On  the  Measurement  of  the  Striae  of  Diatoms,'  in  Quart.  Journ.  of  Microsc. 
Science,"  Vol.  viii.  (1860),  p.  48. 

2    Monthly  Microsc.  Journ.,"  Vol.  v.  (1871),  p.  163. 

2  See  his  interesting  Memoirs  in  Vols.  ii.  and  vii.  of  the  Smithsonian  Contri- 
butions to  Knowledge."  On  Hyalodiscus  subtilis,  see  Hendry,  in  Quart.  Journ. 
of  Microsc.  Science,"  Vol.  i.,  N.S.  (1861),  p.  179. 


172 


THE  MICROSCOPE  AKD  ITS  REVELATIONS. 


direction  transverse  to  its  axis;  while  b  shows  a  portion  more  highly 
magnified  under  the  last  illumination.  This  Diatom,  however,  has  been 
successfully  photographed  by  Dr.  Woodward  (Fig.  118,  c),  who  says  of 

it:  A  careful  examination  of  specimens  mounted  dry,  has  satisfied  me 

that  Hartnack's  interpretation  is  erroneous.  The  fine  striae  are,  I  think, 
rows  of  minute  hemispherical  beads;  the  appearance  of  hexagons  is  tho 
optical  result  of  imperfect  definition  or  of  unsuitable  illumination.  For 
photographing  this  object,  I  have  selected  a  frustule  of  somewhat  less 
than  the  medium  size.  It  measures  1  290th  of  an  inch  in  length. 
Longitudinally  the  fine  striae  count  at  the  rate  of  72,000  to  the  inch. 
These  striae  are  resolved  into  beaded  appearances,  which  count  laterally 
84,000  to  the  inch."' 

162.  As  a  test  for  those  qualities  of  Objectives  which  best  fit  them 
for  the  general  purposes  of  Biological  investigation,  the  Author  remains 
of  the  opinion  (which  he  finds  to  be  shared  by  many  able  and  experienced 


Valve  of  Surirella  gemma,  with  portion  (b)  more  highly  magnified,  showing:  two  systc-ms  ot 
markings  a  and  6,  as  represented  by  Hartnack;  while  c  is  copied  from  a  photograph  taken  oy  Dr. 
Woodward. 

Microscopists,  and  by  Makers  specially  familiar  with  their  requirements) 
that  nothing  is  better  than  the  scale  of  the  Lepidocyrtus  cervicolUs, 
commonly  known  as  the  Podura  (Fig.  419).  It  is  a  fact  perfectly 
familiar  to  such  Makers,  that  an  Objective  may  serve,  in  virtue  of  its 
wide  Angular  aperture,  to  resolve  Diatom- tests  of  considerable  difficulty, 
and  may  yet  fail  utterly  on  the  Podura-scale,  in  consequence  of  its 
inferior  defining  power;  and  such  an  Objective  can  be  of  very  little  ser- 
vice to  the  Biological  investigator.  On  the  other  hand,  although  the 
exact  structure  of  the  Podura-scale  is  still  (like  that  of  the  Diatom-valve) 
a  matter  of  discussion,  yet  all  are  agreed  as  to  the  appearances  it  presents 
under  Objectives  that  combine  in  the  fullest  degree  the  attributes  already 
specified  as  best  qualifying  them  for  Scientific  work;  so  that  any  glass 
which  shows  these  appearances  satisfactorily,  may  be  safely  accounted 
suitable  for  that  purpose.    The  surface  of  this  scale,  when  viewed  under 

1    Monthly  Microsc.  Journ.,"  Vol.  vi.  (Ib71),  p.  100.  - 


MANAGEMENT  OF  THE  MICROSCOPE. 


173 


a  sufficiently  high  amplification,  is  seen  to  be  covered  with  the  peculiar 
markings  shown  in  Plate  ii.,  Figs.  2,  3,  which  are  sometimes  designated 
^  spines,"^  but  are  more  commonly  known  as  ^  notes  of  admiration  '  or  'ex- 
clamation-markings.' These  should  be  clearly  separated  from  each  other, 
and  their  margins  well  defined.  An  Objective  of  small  angle  (such  as  a 
l-4th  inch  of  60°)  will  show  the  ^spines'  dark  throughout;  a  l-4th  inch 
of  100  will  show  a  light  streak  extending  from  the  large  end,  down  the 
centre  of  each  marking;  and  a  further  enlargement  of  the  aperture  will 
show  an  extension  of  this  streak  through  the  entire  length  of  each 
'  spine.'  The  degree  in  which  these  markings  retain  their  brightness  and 
distinctness  under  deep  Eye-piecing,  may  be  considered  a  most  valuable 
test  of  the  excellence  of  the  defining  power  of  the  Objective.  As  it  is 
impossible  that  large-angled  Objectives  used  *dry,'  should  be  perfectly 
corrected  for  spherical  aberration  (so  as  to  possess  the  greatest  possible 
defiyiing  power)  without  some  residuum  of  chroinatic  aberration,  all  the 
best  defining  glasses  will  show  the  thick  part  of  the  spines  tinged  with 
either  blue  or  red.  Perfect  Achromatism,  on  the  other  hand,  is  only 
attainable  with  'dry'  lenses  at  some  sacrifice  of  resolving  and  defining 
power;  and  many  Microscopists  prefer  to  keep  the  latter  to  their  highest 
point,  even  at  the  expense  of  complete  color-correction.  Most  Physiolo- 
gists, hower,  will  prefer  the  highest  attainable  achromatism,  at  some  sac- 
rifice of  aperture.  But  it  ^is  one  of  the  advantages  of  the  *  immersion- 
system,'  that  the  residual  aberrations  of  even  large-angled  Objectives  can 
be  much  more  perfectly  compensated  than  they  can  be  in  'dry'  Objec- 
tives; so  that  on  this  as  on  several  other  accounts,  their  use  is  to  be  re- 
commended whenever  permitted  by  the  nature  of  the  research. 

163.  Determination  of  Magnifyijig  Power. — The  last  subject  to  be 
here  adverted  to  is  the  mode  of  estimating  the  magnifying  power  of 
Microscopes,  or,  in  other  words,  the  number  of  times  that  any  object  is 
magnified.  This  will  of  course  depend  upon  a  comparison  of  the  real 
size  of  the  Object  with  the  apparent  size  of  the  Image;  but  our  estimate 
of  the  latter  will  depend  upon  the  distance  at  which  we  assume  it  to  be 
seen;  since,  if  it  be  projected  at  different  distances  from  the  Eye,  it  will 
present  very  different  dimensions.  Opticians  generally,  however,  have 
agreed  to  consider  ten  inches  as  the  standard  of  comparison;  and  when, 
therefore,  an  object  is  said  to  be  magnified  100  diameters,  it  is  meant 
that  its  visual  image  projected  at  ten  inches  from  the  Eye  (as  when 
thrown  down  by  the  Camera  Lucida,  §  94,  upon  a  surface  at  that  distance 
beneath),  has  100  times  the  actual  dimensions  of  the  object.  The  mea- 
surement of  the  magnifying  power  of  Simple  or  Compound  Microscopes 
by  this  standard  is  attended  with  no  difficulty.  All  that  is  required  is  a 
Stage-Micrometer  accurately  divided  to  a  small  fraction  of  an  inch  (the 
1-lOOth  will  answer  very  well  for  low  powers,  the  1-lOOOth  for  high),  and 
a  common  foot-rule  divided  to  tenths  of  an  inch.  The  Micrometer  being 
adjusted  to  the  focus  of  the  Objective,  the  rule  is  held  parallel  with  it  at 
the  distance  of  ten  inches  from  the  eye.  If  the  second  eye  be  then 
opened  whilst  the  other  is  looking  through  the  Microscope,  the  circle  of 
light  included  within  the  field  of  view  crossed  by  the  lines  of  the  Micro- 
meter will  be  seen  faintly  projected  upon  the  rule;  and  it  will  be  very 
Basy  to  mark  upon  the  latter  the  apparent  distances  of  the  divisions  on 
the  Micrometer,  and  thence  to  ascertain  the  magnifying  power.  Thus, 
supposing  each  of  the  divisions  of  1-lOOth  of  an  inch  to  correspond  with 
1|  inch  upon  the  rule,  the  linear  magnifying  power  is  150  dianieters:  if 
it  corresponcj  with  half  an  inch,  the  magnifying  power  is  50  diameters. 


174: 


THE  MICROSCOPE  AND  ITS  REVELATIONS. 


If,  again,  each  of  the  divisions  of  the  1-lOOOth  inch  Micrometer  corre- 
sponds to  0.6  of  an  inch  upon  the  rule,  the  magnifying  power  is  600  dia- 
meters; and  if  it  corresponds  to  1.2  inches,  the  magnifying  power  is 
1200  diameters.  In  this  mode  of  measurement,  the  estimate  ot  parts  of 
tenths  on  the  rule  can  only  be  made  by  guess;  but  greater  accuracy  may 
be  obtained  by  the  use  of  the  Diagonal  scale  (Fig.  67),  or  still  better,  by 
projecting  the  Micrometer-scale  with  the  Camera  Lucida  at  the  distance 
of  ten  inches  from  the  eye,  marking  the  intervals  on  paper,  taking  an 
average  of  these,  and  repeating  this  with  the  compasses  ten  times  along 
the  inch-scale.  Thus,  if  the  space  given  by  one  of  the  divisions  of  the 
1-lOOOth-inch  Micrometer,  repeated  ten  times  along  the  rule,  amounts 
to  6  inches  and  2^  tenths,  the  value  of  each  division  will  be  .625  of  an 
inch,  and  the  magnifying  power  625. — It  is  very  important,  whenever  a 
high  degree  of  accuracy  is  aimed  at  in  Micrometry,  to  bear  in  mind  the 
caution  already  given  (§  91)  in  regard  to  the  difference  in  magnifying 
power  produced  in  the  adjustment  of  the  Objective  to  the  thickness  of 
the  glass  that  covers  the  object. — The  superficial  Magnifying  power  is  of 
course  estimated  by  squaring  the  linear;  but  this  is  a  mode  of  statement 
never  adopted  by  Scientifio  observers. 


• 


PREPAEiiTIOJSr,  MOUJSTING,  AND  COLLECTION  OP  OBJECTS.  175 


CHAPTER  V. 

PREPARATION,  MOUNTING,  AND  COLLECTION  OF  OBJECTS. 

164.  Under  this  head  it  is  intended  to  give  an  account  of  those 
Materials,  Instrnments,  and  Appliances  of  various  kinds,  which  have  been 
found  most  serviceable  to  Microscopists  engaged  in  general  Biological  re- 
search, and  to  describe  the  most  approved  methods  of  employing  them 
in  the  preparation  and  mounting  of  Objects,  for  the  display  of  the 
minute  structures  thus  brought  to  our  knowledge.  Not  only  is  it  of  the 
greatest  advantage  that  the  discoveries  made  by  Microscopic  research 
should — as  far  as  possible — be  embodied  (so  to  speak)  in  'preparations,' 
which  shall  enable  them  to  be  studied  by  every  one  who  may  desire  to  do 
so;  but  it  is  now  universally  admitted  that  such  ^preparations'  often 
show  so  much  more  than  can  be  seen  in  the  fresh  organism,  that  no  ex- 
amination of  it  can  be  considered  as  complete,  in  which  the  methods  most 
suitable  to  each  particular  case  have  not  been  put  in  practice. — It  must 
be  obvious  that  in  a  comprehensive  Treatise  like  the  present,  such  di,  gen- 
eral treatment  of  this  subject  is  all  that  can  be  attempted,  excepting  in  a 
few  instances  of  peculiar  interest.  And  as  the  Histological  student  can 
find  all  the  guidance  he  needs  in  the  numerous  Manuals  now  prepared 
for  his  instruction,  the  Author  will  not  feel  it  requisite  to  furnish  him 
with  the  special  directions  that  are  readily  accessible  to  him  elsewhere. 

Section  1. — Materials,  Instruments^  and  Appliances. 

165.  Glass  Slides. — The  kind  of  Glass  best  suited  for  mounting  ob- 
jects, is  that  which  is  known  as  ^patent  plate;'  and  it  is  now  almost 
invariably  cut,  by  the  common  consent  of  Microscopists  in  this  country, 
into  slips  measuring  3  in.  by  1  inch.  For  objects  too  large  to  be  mounted 
on  these,  the  size  of  3  in.  by  1|  in.  may  be  adopted.  Such  slips  may  be 
purchased,  accurately  cut  to  size,  and  ground  at  the  edges,  for  so  little 
more  than  the  cost  of  the  glass,  that  few  persons  to  whom  time  is  an  ob- 
ject, would  trouble  themselves  to  prepare  them;  it  being  only  when  glass 
slides  of  some  unusual  dimensions  are  required,  or  when  it  is  desired  to 
construct  'built-up  cells'  (§  174),  that  a  facility  in  cutting  glass  with  a 
glazier's  diamond  becomes  useful.  The  glass  slides  prepared  for  use 
should  be  free  from  veins,  air-bubbles,  or  other  flaws,  at  least  in  the  cen- 
tral part  on  which  the  object  is  placed;  and  any  whose  defects  render 
them  unsuitable  for  ordinary  purposes,  should  be  selected  and  laid  aside  for 
uses  to  which  the  working  Microscopist  will  find  no  difficulty  in  putting 
them.  As  the  slips  vary  considerably  in  thickness,  it  will  be  advantage- 
ous to  separate  the  tliin  and  the  thick  from  those  of  medium  substance. 
The  first  may  be  employed  for  mounting  delicate  objects  to  be  viewed 


176 


THE  MICROSCOPE  AND  ITS  REVELATIONS. 


by  the  high  powers  with  which  the  Achromatic  Condenser  is  to  be  used, 
so  as  to  avoid  any  unnecessary  deflection  of  the  illuminating  pencil  by 
the  thickness  of  the  plate  which  it  has  to  traverse  beneath  the  object;  the 
second  should  be  set  aside  for  the  attachment  of  objects  which  are  to  be 
ground-down,  and  for  which,  therefore,  a  stronger  mounting  than  usual 
is  desirable;  and  the  third  are  to  be  used  for  mounting  ordinary  objects. 
Great  care  should  be  taken  in  washing  the  slides,  and  in  removing  from 
them  every  trace  of  greasiness  by  the  use  of  a  little  soda  or  potass  solu- 
tion. If  this  should  not  suffice,  they  may  be  immersed  in  the  solution 
recommended  by  Dr.  Seller,  composed  of  2  oz.  of  Bichromate  of  Potass, 
3  fl.  oz.  of  Sulphuric  Acid,  and  25  oz.  of  Water,  and  afterwards  thor- 
oughly rinsed.  (The  same  solution  may  be  advantageously  used  for 
cleansing  Cover-glasses,  §  132.)  Before  they  are  put  away,  the  slides 
should  be  wiped  perfectly  dry,  first  with  an  ordinary  '  glass-cloth,^  and 
afterwards  with  an  old  cambric  handkerchief.  And  before  being  used, 
each  slide  should  be  again  carefully  wiped,  so  as  to  remove  all  adherent 
dust.  Where  slides  that  have  been  already  employed  for  mounting 
preparations  are  again  brought  into  use,  great  care  should  be  taken  in 
completely  removing  all  trace  of  adherent  varnish  or  cement;  first  by 
scraping  (care  being  taken  not  to  scratch  the  glass),  then  by  using  an 
appropriate  solvent,  and  then  by  rubbing  the  slide  with  a  mixture  of 
equal  parts  of  alcohol,  benzole,  and  liquor  sodae,  finishing  with  clean 
water. 

166.  Thin  Glass, — The  older  Microscopists  were  obliged  to  employ 
thin  laminae  of  talc  for  covering  objects  to  be  viewed  with  lenses  of  short 
focus:  but  this  material,  which  was  in  many  respects  objectionable,  is 
now  only  employed  for  Objectives  of  exceptionally  short  focus  (such  as 
l-50th  or  1-75 th  inch),  being  entirely  superseded  for  other  purposes  by  the 
thin  glass  manufactured  by  Messrs.  Chance  of  Birmingham,  which  may  be 
obtained  of  various  degrees  of  thickness,  down  to  l-500th  of  an  inch.  This 
glass,  being  un-annealed,  is  very  hard  and  brittle;  and  much  care  and 
some  dexterity  are  required  in  cutting  it.  This  should  be  done  with  the 
tvriting  diamond;  and  it  is  advantageous  to  lay  the  thin  glass  upon  a  piece 
of  wetted  plate-glass,  as  its  tendency  to  crack  and  '  star  ^  is  thereby  di- 
minished. For  cutting  square  or  other  rectangular  covers,  nothing  but 
a  flat  rule  is  required.  The  cutting  of  rounds  by  unaccustomed  hands  is 
usually  attended  with  so  much  breakage,  that  it  is  really  a  saving  of 
money  as  well  as  of  time  to  purchase  them  from  the  dealers;  who  usually 
keep  them  in  several  sizes,  and  supply  any  others  to  order.  The  differ- 
ent thicknesses  are  usually  ranked  as  1,  2,  and  3;  the  first  being  used  for 
covering  objects  to  be  viewed  with  low  powers,  the  second  for  objects  to 
be  viewed  with  medium  powers;  and  the  third  for  objects  requiring  high 
powers.  The  thinnest  glass  is  of  course  most  difficult  to  handle  safely, 
and  is  most  liable  to  fracture  from  accidents  of  various  kinds;  and  hence 
it  should  only  be  employed  for  the  purpose  for  which  it  is  absolutely 
needed.  The  thickest  pieces,  again,  may  be  most  advantageously  em- 
ployed as  covers  for  large  Cells,  in  which  objects  are  mounted  in  fluid 
(§§  171-174)  to  be  viewed  by  the  low  powers  whose  performance  is  not 
sensibly  affected  by  the  aberration  thus  produced.  The  working  Micro- 
scopist  will  find  it  desirable  to  provide  himself  with  some  means  of  mea- 
suring the  thickness  of  his  cover-glass;  and  this  is  especially  needed  if  he 
is  in  the  habit  of  employing  Objectives  without  adjustment,  which  are 
corrected  to  a  particular  standard  (§  17).  A  small  screw-gauge  of  steel, 
made  for  measuring  the  thickness  of  rolled  plates  of  brass,  and  sold  at 


PREPARATION,  MOUNTING,  AND  COLLECTION  OF  OBJECTS.  177 


the  Tool-shops,  answers  this  purpose  yery  well;  but  Boss's  Lever  of  Con- 
tact (Fig.  119),  devised  for  this  express  purpose,  is  in  many  respects  pre- 
ferable. This  consists  of  a  small  horizontal  table  of  brass,  mounted  upon 
a  stand,  and  having  at  one  end  an  arc  graduated  into  20  divisions,  each 
of  which  represents 


Ross's  Lever  of  Contact. 


1-lOOOth  of  an  inch,  '  ^^ic;.li9. 

so  that  the  entire  arc 
measures  l-50th  of  an 
inch;  at  the  other  end 
is  a  pivot  on  which 
moves  a  long  and  deli- 
cate  lever  of  steel, 
whose  extremity  points 
to  the  graduated  arc, 
whilst  it  has  very  near 
its  pivot  a  sort  of  projecting  tooth,  which  bears  at*  against  a  ver- 
tical plate  of  steel  that  is  screwed  to  the  horizontal  table.  The  piece 
of  thin-glass  to  be  measured  being  inserted  between  the  vertical 
plate  and  the  projecting  tooth  of  the  lever,  its  thickness  in  thousandths 
of  an  inch  is  given  by  the  number  on  the  graduated  arc  to  which  the  ex- 
tremity of  the  lever  points.  Thus,  if  the  number  be  8,  the  thickness  of 
the  glass  is  .008  or  l-125tli  of  an  inch.^ — It  will  be  found  convenient  to 
sort  the  covers  according  to  their  thicknesses,  and  to  keep  the  sortings 
apart,  so  that  each  may  be  used  for  the  powers  to  which  it  is  the  most 
suitable.  For  Objectives  whose  angle  of  aperture  is  between  40°  and  75°, 
glass  of  .008  is  not  too  thick;  for  Objectives  of  between  75°  and  120°  of 
aperture,  the  thickness  may  range  from  .006  to  .004;  but  for  Objectives 
whose  angle  of  aperture  exceeds  120%  and  whose  focus  is  less  than  1-lOth 
of  an  inch,  only  covers  of  from  .004  to  .002  should  be  used. 

167.  On  account  of  the  extreme  brittleness  of  the  Thin-glass,  it  is 
desirable  to  keep  the  covers,  when  cut  and  sorted,  in  some  fine  and  soft 
powder,  such  as  Starch.  Before  using  a  cover,  however,  the  Microscopist 
should  be  careful  to  clean  it  thoroughly;  not  merely  for  the  sake  of 
removing  foulness  which  would  interfere  with  the  view  of  the  object,  but 
also  for  the  sake  of  getting  rid  of  adherent  starch -grains,  the  presence  of 
which  might  lead  to  wrong  conclusions;  and  also  to  free  the  surface  from 
that  slight  greasiness,  which,  by  preventing  it  from  being  readily  wetted 
by  water,  frequently  occasions  great  inconvenience  in  the  mounting  of 
objects  in  fluid.  The  thicker  pieces  may  be  washed  and  wiped  without 
much  danger  of  fracture,  if  due  care  be  employed;  but  the  thinner  require 
much  precaution;  and  in  cleansing  these,  a  simple  instrument  devised  by 
Mr.  W.  W.  Jones  will  be  found  very  useful.  This  consists  of  a  small 
tube  of  brass  about  an  inch  in  diameter  and  the  same  in  height  (a  stout 
pill-box  makes  a  good  substitute),  into  which  fits  loosely  a  weighted-plug, 
to  the  flat  bottom  of  which  is  cemented  a  piece  of  chamois  leather. 
Another  piece  of  soft  leather  is  stretched  upon  a  flat  tablet  of  wood  or 
plate-glass;  and  by  placing  the  cover-glass  (damped  by  the  breath)  under 
the  plug;  within  the  end  of  the  tube,  and  keeping  the  tube  well-down  on 
the  tablet,  the  glass  can  be  rubbed  between  the  two  leather  surfaces  with 
perfect  security,  the  weight  of  the  plug  affording  sufficient  pressure.' 

1  Another  form  of  gauge,  in  which  the  measurement  is  obtained  with  great 
precision  and  facility  by  the  sliding  of  a  wedge,  is  described  in  the  Journ.  of 
the  Roy.  Microsc.  Soc,"  Vol.  ii.  (1879),  p.  65. 

1  In  the  improved  form  of  this  little  instrument  made  by  Messrs.  Hunter  & 
12 


178 


THE  MICROSCOPE  AND  ITS  REVELATIONS. 


168.  Varnishes  and  Cements. — There  are  three  very  distinct  purposes 
for  which  Cements  that  possess  the  power  of  holding  firmly  to  Glass,  and 
of  resisting  not  merely  water  but  other  preservative  liquids,  are  required 
by  the  Microscopist;  these  being  (1)  the  attachment  of  the  glass  covers  to 
the  slides  or  cells  containing  the  object,  (2)  the  formation  of  thin  ^  cells,' 
of  cement  only,  and  (3)  the  attachment  of  the  '  glass-plate '  or  ^  tube-cells ' 
to  the  slides.  The  two  former  of  these  purposes  are  answered  by  liquid 
cements  or  varnishes,  which  may  be  applied  without  heat;  the  last  re- 
quires a  solid  cement  of  greater  tenacity,  which  can  only  be  used  in  the 
melted  state. — Among  the  many  such  Cements  that  have  been  recom- 
mended by  different  workers,  the  following  may  be  specially  named  as 
having  stood  the  test  of  a  large  experience,  both  as  to  general  utility  and 
permanent  value: — 

a.  Japanners*  Gold  size  — This,  which  may  be  obtained  at  every  Color-shop, 
is  (according  to  the  Author's  experience)  the  most  trustworthy  of  aU  cements  for 
closing-in  mounted  objects  of  almost  any  description.  It  takes  a  peculiarly  firm 
hold  of  glass;  and  when  dry  it  becomes  extremely  tough,  without  brittleness. 
When  new,  it  is  very  liquid  and  *  runs'  rather  too  freely;  so  that  it  is  often  advan- 
tageous to  leave  open  for  a  time  the  bottle  containing  it,  until  the  varnish  is 
somewhat  thickened.  By  keeping  it  still  longer  with  occasional  exposure  to  air, 
it  is  rendered  much  more  viscid:  and  though  such  *  old  '  Gold-size  is  not  fit  for 
ordinary  use,  yet  one  or  two  coats  of  it  may  be  advantageously  laid  over  tlie  films 
of  newer  varnish,  for  securing  the  thicker  covers  of  large  cells  (§§  171 — 4).  When- 
ever any  other  varnish  or  cement  is  used,  either  in  making  a  cell  or  in  closing  it 
in,  the  rings  of  these  should  be  covered  with  one  or  two  layers  of  Gold-size  ex- 
tending beyond  it  on  either  side,  so  as  to  form  a  continuous  film  extendmg  from 
the  marginal  ring  of  the  cover  to  the  adjacent  portion  of  the  glass  slide.' 

5.  Asphalts  Varnish.— This  is  a  black  varnish  made  by  dissolving  half  a 
drachm  Caoutchouc  in  mineral  naphtha,  and  then  adding  4  oz.  of  Asphaltum, 
using  heat  if  necessary  for  its  solution.  It  is  very  important  that  the  Asphaltum 
should  be  genuine,  and  the  other  materials  of  the  best  quality.  Some  use  Asphalte 
as  a  substitute  for  gold-size;  but  the  Author's  experience  leads  him  to  recommend 
that  it  should  only  be  employed  either  for  making  shallow  *  cement-cells '  170), 
CT  for  finishing-off  preparations  already  secured  with  gold-size.  For  the  former 
purpose  it  may  advantageously  be  slightly  thickened  by  evaporation. 

c.  Black  Japan. — The  varnish  sold  at  the  Color-shops  under  this  name,  may 
be  used  for  the  same  purposes  as  the  preceding.  When  it  is  used  for  making 
*  cement-cells,'  the  slides  to  which  it  has  been  applied  should  be  exposed  for  a  time 
to  the  heat  of  an  oven,  not  raised  so  high  as  to  cause  it  to  blister;  this  wiil  increase 
its  adhesion  to  the  glass  slide,  and  will  flatten  the  surface  of  the  rings. 

d.  Dammar  Cement ,  which  is  made  by  dissolving  gum  dammar  in  benzole,  and 
adding  about  one-third  of  gold-size,  has  the  advantage  of  drying  very  quickly; 
and  may  be  preferably  used  for  a  first  coat  when  glycerine  is  used  as  the  material 
for  mounting. 

e.  BelVs  Cement  may  be  recommended  on  the  same  grounds;  but  it  '  runs '  so 
freely,  that  for  ordinary  purposes  the  Author  much  prefers  gold-size  or  dam- 
mar. 

/.  Canada  Balsam  is  so  brittle  when  hardened  by  time,  that  it  cannot  be  safe- 
ly used  as  a  cement,  except  for  the  special  purpose  of  attaching  hard  specimens 
to  glass,  in  order  that  they  may  be  reduced  by  grinding,  etc.  Although  fresh 
soft  balsam  may  be  hardened  by  heating  it  on  the  slide  to  which  the  object  is  to 
be  attached,  yet  it  may  be  preferably  hardened  en  masse  by  exposing  it  in  a  shal- 
low vessel  to  the  prolonged  but  moderate  heat  of  an  oven,  until  so  much  of  its 
volatile  oil  has  been  driven  off  that  it  becomes  almost  (but  not  quite)  resinous  on 
cooling.^  If,  when  a  drop  is  spread  out  on  a  glass  and  allowed  to  become  quite 
cold,  it  is  found  to  be  so  hard  as  not  to  be  readily  idented  by  the  thumb-nail,  and 


Sands,  the  leather  is  not  cemented  to  the  bottom  of  the  plug,  but  merely  strained 
over  it,  so  as  to  be  easily  renewable. 

^  The  Author  has  fluid  preparations  mounted  with  Gold-size  nearly  forty  years 
ago,  which  have  remained  perfectly  free  from  leakage;  the  precaution  having 
been  taken  to  lay  on  a  fresh  coat  every  two  or  three  years. 


PREPARATION,  MOUNTING,  AND  COLLECTION   OF  OBJECTS.  179 

yet  not  so  hard  as  to  *  chip,'  it  is  in  the  best  condition  to  be  used  for  cementing. 
If  too  soft,  it  will  require  a  little  more  hardening  on  the  slide,  to  which  it  should 
be  transferred  in  tlie  liquid  state,  being  brought  to  it  by  the  heat  of  a  water-bath; 
if  too  hard,  it  may  be  dissolved  in  chloroform  or  benzole,  for  use  as  a  mounting 
*  medium  '  (§  205). 

g.  Shell-lac  Cement  is  made  by  keeping  small  pieces  of  picked  Shell-lac  in  a 
bottle  of  rectified  spirit,  and  shaking  it  from  time  to  time.  It  cannot  be  recom- 
mended as  a  substitute  for  any  of  the  preceding;  as,  when  dry  and  hard,  it  has 
little  hold  on  glass.  But  it  answers  very  well  for  making  cells  for  dry- mounting 
(§  167). — What  is  known  as  Liquid-glue  is  an  inferior  kind  of  the  same  cement, 
made  by  dissolving  inferior  shell-lac  or  some  commoner  resin,  in  naphtha.  It 
cannot  be  trusted  for  a  permanent  hold;  and  those  who  employ  it  are  likely  to  find 
themselves  disappomted  in  regard  to  the  durability  of  their  preparations.^ 

h.  Marine  Glue,  which  is  composed  of  Shell-lac,  caoutchouc,  and  naphtha,  is 
distinguished  by  its  extraordinary  tenacity,  and  by  its  power  of  resisting  solvents 
of  almost  every  kind.  Different  qualities  of  this  substance  are  made  for  the 
several  purposes  to  which  it  is  applied;  and  the  one  most  suitable  to  the  wants  of 
the  Microscopist  is  known  in  commerce  as  G  K  4.  The  special  value  of  this 
cement,  which  can  only  be  applied  hot,  is  in  attaching  to  glass  slides  the  glass  or 
metal  rings  which  thus  form  *  cells '  for  the  reception  of  objects  to  be  mounted  in 
fluid;  no  other  cement  being  comparable  to  it  either  for  tenacity  or  for  durability. 
The  manner  of  so  using  it  will  be  presently  described  (g  171)c 

i.  Various  colored  Varnishes  are  used  to  give  a  finish  to  mounted  preparations, 
or  to  mark  on  the  covering-glasses  of  large  preparations  the  parts  containing 
special  kinds  of  noteworthy  structure.  A  very  good  black  varnish  of  this  kind  is 
made  by  working  up  very  finely  powdered  lamp-black  with  gold-size.  For  red, 
sealing-wax  varnish  made  by  dissolving  red  sealing-w  ax  (the  best  is  alone  worth 
using)  in  rectified  spirit,  is  commonly  used;  but  it  is  very  liable  to  chip  and  leave 
the  glass,  when  hardened  by  time.  The  red  varnish  specially  prepared  for  Micro- 
scopic purposes  by  Messrs.  Thompson  &  Capper  (of  Liverj)ool)  seems  likely  to  stand 
better,  but  the  Author's  experience  of  it  has  been  short.  For  white, '  zinc  cement ' 
answers  well:  which  may  be  made  by  dissolving  1  oz.  of  gum  dammar  in  1  oz.  of 
oil  of  turpentine  by  the  aid  of  heat;  rubbing  up  1  drachm  of  oxide  of  zinc  with 
an  equal  quantity  of  oil  of  turpentine  (adding  the  latter  by  drop)  into  a  creamy 
mixture  perfectly  free  from  lumps  or  grii^  and  then  mixing  the  two  fluids,  which 
must  be  well  stirred  together,  and  strained  through  a  piece  of  fine  muslin  pre- 
viously wetted  with  turpentine.  Blue  or  green  pigments  may  be  worked-up  with 
this,  if  cements  of  those  colors  be  desired. 

k.  For  attaching  labels  and  covering  papers  to  slides  either  of  glass  or  wood, 
and  for  fixing-down  small-objects  to  be  mounted  '  dry '  (such  az  Foraminifera, 
parts  oi  Insects,  etc.),  the  Author  has  found  nothing;  preferable  to  ci  rather  thick 
mucilage  of  Gum  Arabic,  to  which  enough  Glycerine  has  been  added  to  prevent 
it  from  drying  hard,  with  a  few  drops  of  some  Essential  oil  to  prevent  the  develop- 
ment of  mouldc  The  following  formula  has  also  bf-a  recommended:— Dissolve  2 
oz.  of  Gum  Arabic  in  2  oz.  of  water,  and  then  add  l-4th  oz.  of  soaked  gelatine 
(for  the  solution  of  which  the  action  of  heat  will  be  required),  30  drops  of  gly- 
cerine, and  a  lump  of  camphor.— The  further  advantage  is  gained  by  the  addition 
of  a  slightly  increased  proportion  of  Glycerine  to  either  of  the  foregoing,  that  the 
gum  can  be  very  readily  softened  by  water;  so  that  covers  may  be  easily  removed 
(to  be  cleansed  if  necessary)  and  the  arrangement  of  objects  (where  many  are 
mounted  together,  §  175)  altered. 

169.  Cells  for  Dry-mounting. — ^Wliere  the  object  to  be  mounted  '  dry' 
{ix.  not  immersed  either  in  fluid  or  in  any  'medium')  is  so  thin  as  to 
require  that  the  cover  should  be  but  little  raised  above  the  slide  a  '  cement 
ceir  (§  170)  ansv^ers  this  purpose  very  well;  and  if  the  application  of  a 
gentle  warmth  be  not  injurious,  the  pressing  down  of  the  cover  on  the 
softened  cement  will  help  both  to  fix  it,  and  to  prevent  the  varnish  applied 
round  its  border  from  running  in.    Where  a  somewhat  deeper  cell  is 


'  From  the  appearance  and  smell  of  the  Hollis's  Glue  recommended  by  Dr. 
Heneage  Gibbs,  the  Author  cannot  but  believe  that  its  nature  is  essentially  the 
same  as  that  of  ordinary  '  liquid  glue,'  and  that  it  is  therefore  liable  to  the  same 
objection. 


180 


THE  MICROSCOPE  AND  ITS  EEYELATIONS. 


required,  it  can  be  made  in  the  manner  suggested  by  Prof.  H.  L.  Smitli 
(U.S.)  specially  for  the  mounting  of  Diatoms.  A  sheet  of  thin  writing- 
l)aper  dipj^ed  into  thick  shell-lac  varnish  is  hung  up  to  dry;  and  rings 
are  then  cut  out  from  it  by  punches  of  two  different  sizes.  One  of  these 
rings  being  laid  on  a  glass  slide,  and  the  cover,  with  the  object  dried 
upon  it,  laid  on  the  ring,  it  is  to  be  held  in  its  place  by  the  forceps  or 
spring-clip,  and  the  slide  gently  warmed  so  as  cause  a  slight  adhesion  of 
the  cover  to  the  ring,  and  of  the  ring  to  the  slide;  and  this  adhesion  may 
then  be  rendered  complete,  by  laying  another  glass  slide  on  the  cover,  and 
pressing  the  two  slides  together,  with  the  aid  of  a  continued  gentle  heat. 
— Still  deeper  cells  may  be  made  with  rings  punched  out  of  tin-foil  of 
various  thicknesses;  and  cemented  with  shell-lac  varnish  on  either  side. 
And  if  yet  deeper  cells  are  needed,  they  may  be  made  of  turned  rings  of 
vulcanite  or  ebonite,  cemented  in  the  same  manner. — It  is  always  safer 
to  protect  such  dry  mounts  by  attaching  paper  covers  to  the  slides;  as  the 
tendency  of  the  rings  to  start  at  any  ^jar,^when  the  shell  lac  lias  re- 
acquired its  resinous  hardness,  is  thereby  greatly  diminished. — Small 
objects,  such  as  Diatoms  and  Polycysiina,  which  are  to  be  viewed  by 
Lieberktihn  illumination  (§  115),  should  be  mounted  on  disks  punched 
out  of  thin  black  card-board,  whose  diameter  scarcely  exceeds  the  field 
of  the  Objective  under  which  they  are  to  be  shown;  and  the  protecting 
cell  should  be  large  enough  to  allow  an  ample  opening  for  the  light-rays 
to  pass  up  from  the  mirror  to  the  speculum,  between  the  inner  edge  of  its 
ring  and  the  outer  margin  of  the  disk. 

170.  Cement-Cells, — Cells  for  mounting  tliin  objects  in  any  watery 
medium,  may  be  readily  made  with  Asphalte  or  Black  Japan  varnish,  by 
the  use  of  Mr.  Shadbolt's  '  Turn-table'  (§  176)  or  one  of  its  modifications. 

The  glass  slide  being  placed  under 
'Bm.WDs,  its  springs,  in  such  a  manner  that  its 

two  edges  shall  be  equidistant  from 
^   the  centre  (a  guide  to  which  posi- 
tion is  afforded  by  the  circles  traced 
on  the  brass),  and  its  four  corners 
equally  projecting  beyond  tho  cir- 
cular margin  of  the  plate,  a  camel  s 
hair  pencil  dipped  in  the  varnish 
"B  is  held  in  the  right  hand,  so  thvX 
its  point  comes  into  contact  with 
the  glass  over  whichever  of  the 
circles  may  be  selected  as  the  guide 
to  the  size  of  the  ring.    The  turn- 
table being  made  to  rotate  by  the 
^  application  of  the  left  fore-finger 
to  the  milled-head  beneath,  a  ring 
of  varnish  of  a  suitable  breadth  is 
made  upon  the  glass;  and  if  this  be 
set  aside  in  a  horizontal  position, 
it  will  be  found,  when  hard,  to 
^  present  a  very  level  surface.    If  a 
greater  thickness  be  desired  than 
,  a  sino^le  application  will  conveni- 

Tube-Cells,  Round  and  Quadrang-ular.  l^   ^     ^  ^  ^  i  i  i, 

V  au^uiai.  ently  make,  a  second  layer  may  be 
afterwards  laid  on.  It  will  be  found  convenient  to  make  a  considerable 
number  of  such  cells  at  once,  and  to  keep  a  stock  of  them  ready  prepared 


PREPARATION,  MOUNTING,  AND  COLLECTION  OF  OBJECTS.  181 


for  use.  If  the  surface  of  any  ring  should  not  be  sufficiently  level  for  a 
covering-glass  to  lie  flat  upon  it,  a  slight  rubbing  upon  a  piece  of  fine 
emery-paper  laid  upon  a  flat  table  (the  ring  being  held  downwards)  will 
make  it  so. 

171.  Ring-cells. — For  mounting  objects  of  greater  thickness,  it  is  de- 
sirable to  use  cells  made  by  cenenting  rings,  either  of  glass  or  metal,  to 
the  glass  slides,  with  marine  glue.  Glass-rings  of  any  size,  diameter, 
thickness,  and  breadth  are  made  by  cutting  transverse  sections  of  thick 
walled  tubes;  the  surfaces  of  these  sections  being  ground  flat  and  parallel. 
Not  only  may  round  cells  (Fig.  120  A,  b)  of  various  sizes  be  made  by 
this  simple  method,  but,  by  flattening  the  tube  (when  hot)  from  which 
they  are  cut,  the  sections  may  be  made  quadrangular  or  square,  or  oblong 
(c,  d).  For  intermediate  thicknesses  between  cement-cells  and  glass  ring- 
cells,  the  Author  has  found  no  kind  so  convenient  as  the  rings  (sold  by 
Mr.  Collins)  stamped  out  of  tin,  of  various  thicknesses.  These,  after 
being  cemented  to  the  slides,  should  have  their  surfaces  made  perfectly 
flat  by  rubbing  on  a  piece  of  fine  grit  or  a  corundum-file,  and  then 
smoothed  on  a  "Water  of  Ayr  stone;  to  such  surfaces  the  glass  covers 
will  be  found  to  adhere  with  great  tenacity. 

*  The  Glass  Slides  and  Cells  v^hich  aio  to  be  attached  to  each  other,  must  first  be 
heated  on  the  Mounting  piate;  and  some  small  cuttings  of  Marine  glue  are  then 
to  be  placed  either  upon  that  surface  of  the  cell  which  is  to  be  attached,  or  upon 
that  portion  of  the  slide  on  vs^hich  it  is  to  lie,  the  former  being  perhaps  prefer- 
able. When  they  begin  to  melt,  they  may  be  worked  over  the  surface  of  attach- 
ment by  means  of  a  needle  point;  and  m  this  manner  the  melted  glue  may  be 
uniformly  spread,  care  being  taken  to  pick  out  any  of  the  small  gritty  particles 
which  this  cement  sometimes  contains.  When  the  surface  of  attachment  is  thus 
completely  covered  with  liquefied  glue,  the  cell  is  to  be  taken  up  with  a  pair  of 
forceps,  turned  over,  and  deposited  in  its  proper  place  on  the  slide;  and  it  is  then 
to  be  firmly  pressed  down  with  a  stick  (such  as  the  handle  of  the  needle),  or  with 
a  piece  of  flat  wood,  so  as  to  squeeze  out  any  superfluous  glue  from  beneath.  If 
any  air-bubbles  should  be  seen  between  the  cell  and  the  slide,  these  should  if  pos- 
sible be  got  rid  of  by  pressure,  or  by  slightly  moving  the  cell  from  side  to  side; 
but  if  their  presence  results,  as  is  sometimes  the  case,  from  deficiency  of  cement 
at  that  point,  the  cell  must  be  lifted  off  .again,  and  more  glue  applied  at  the 
required  spot.  Sometimes,  in  spite  of  care,  the  glue  becomes  hardened  and  black- 
ened by  overheating;  and  as  it  will  not  then  stick  well  to  the  glass,  it  is  pref era- 
able  not  to  attempt  to  proceed,  but  to  lift  off  the  cell  from  the  slide,  to  let  it  cool, 
scrape  off  the  overheated  glue,  and  then  repeat  the  process.  When  the  cement- 
ing has  been  satisfactorily  accomplished,  the  slides  should  be  allowed  to  cool 
gradually  in  order  to  secure  the  firm  adhesion  of  the  glue;  and  this  is  readily  ac- 
complished, in  the  first  instance,  by  pushing  each,  as  it  is  finished,  towards  one 
of  the  extremities  of  the  plate.  If  two  plates  are  in  use,  the  heated  plate  may 
then  be  readily  moved  away  upon  the  ring  which  supports  it,  the  other  being 
brought  down  in  its  place,  and  as  the  heated  plate  will  be  some  little  time  in 
cooling,  the  firm  attachment  of  the  cells  will  be  secured.  If,  on  the  other  hand, 
there  be  only  a  single  plate,  and  the  operator  desire  to  proceed  at  once  in  mount- 
ing more  cells,  the  slides  already  completed  should  be  carefully  removed  from  it, 
and  laid  upon  a  wooden  surface,  the  slow  conduction  of  which  will  prevent  them 
from  cooling  too  fast.  Before  they  are  quite  cold,  the  superfluous  glue  should  be 
scraped  from  the  glass  with  a  small  chisel  or  awl;  and  the  surface  should  then  be 
carefully  cleansed  with  a  solution  of  potash,  which  may  be  rubbed  upon  it  with  a 
piece  of  rag  covering  a  stick  shaped  like  a  chisel.  The  cells  should  next  be 
washed  with  a  hard  brush  and  soap  and  water,  and  may  be  finally  cleansed  by 
rubbing  with  a  little  weak  spirit  and  a  soft  cloth.  In  cases  in  which  apxjearance 
is  not  of  much  consequence,  and  especially  in  those  in  which  the  cell  is  to  be 
used  for  mounting  large  opaque  objects,  it  is  decidedly  preferable  not  to  scrape 
off  the  glue  too  closely  round  the  edges  of  attachment;  as  the  *hold'  is  much 
firmer,  and  the  probability  of  the  penetration  of  air  or  fiuid  much  less,  if  the  im- 
mediate margin  of  glue  be  left  both  outside  and  inside  the  cell. — To  those  to 
whom  time  is  of  value,  it  is  recommended  that  all  cells  which  require  Marine- 
glu^  cementing  be  purchased  from  the  dealers  in  Microscopic  apparatus. 


182 


THE  MICROSCOPE  AND  ITS  REVELATIONS. 


172.  Plate-Glass  Cells. — Where  large  sliallow  cells  vnth  flat  bottoms 
are  required  (as  for  mounting  Zoophytes,  small  Medusce,  etc.),  they  may 
be  made  by  drilling  holes  in  pieces  of  plate  glass  of  various  sizes,  shapes, 
and  thicknesses  (Fig.  121,  a),  which  are  then  cemented  to  glass  slides  with 
marine  glue.  By  drilling  two  holes  at  a  suitable  distance  and  cutting 
out  the  piece  between  them,  any  required  elongation  of  the  cavity  may 
be  obtained  (b,  c,  d). 

173.  Sunk  Cells.— 
"Eict- 121.                        name  is  given  to  round  or  oval 

hollows  excavated  by  grinding 
^  in  the  substance  of  glass  slides, 
which,  for  this  purpose,  should 
be  thicker  than  ordinary.  Such 
cells  have  the  advantage  not 
only  of  comparative  cheapness, 
^  but  also  of  durability,  as  they 
are  not  liable  to  injury  by  a  sud- 
den jar,  such  as  sometimes  causes 
the  detachment  of  a  cemented 
D  plate  or  ring.  For  objects  whose 
shape  adapts  them  to  the  form 
and  depth  of  the  cavity,  such 
cells  will  be  found  very  conve- 
nient; thus  the  Author  has  a 
p  series  of  young  Coinatulce  (Fig 
378)  thus  mounted,  which  are 
Plate-Glass  Cells.  extremely  well  displayed,  alike 

on  their  upper  and  on  their 
Ekx  .  under  surfaces.     It  naturally 

suggests  itself  as  an  objection 
to  the  use  of  such  cells,  that  the 
concavity  of  their  bottom  must 
^  so  deflect  the  light  rays,  as  to 
distort  or  obscure  the  image; 
but  as  the  cavity  is  filled  either 
with  water  or  some  other  liquid 
of  higher  refractive  power;  the 
deflection  is  so  slight  as  to  be 
I  practically  inoperative.  Before 
mounting  objects  in  such  cells, 
the  Microscopist  should  see  that 
their  concave  surfaces  are  free 
from  scratches  or  roughness. 

174.  Built-up  Cfe«^\— When 
cells  are  required  of  forms  or 
dimensions  not  otherwise  pro- 
curable, they  may  be  huilt  up  of 
separate  pieces  of  glass  cemented 
together.  Large  shallow  Cells, 
suitable  for  mounting  Zoophytes 

Sunk  Cells.  similar  flat  objects,  may  be 

easily  constructed  after  the 
following  method: — A  piece  of  plate-glass,  of  a  thickness  that  shall  give 
the  desired  depth  to  the  cell,  is  to  be  cut  to  the  dimensions  of  its  outside 


PREPARATION,  MOUNTING,  AND  COLLECTION  OF  OBJECTS.  183 


wall;  and  a  strip  is  then  to  be  cut  off  with  the  diamond  from  each  of  its 
edges,  of  such  breadth  as  shall  leave  the  interior  piece  equal  its  dimen- 
sions to  the  cavity  of  the  cell  that  is  desired.  This  piece  being  rejected, 
the  four  strips  are  then  to  be  cemented  upon  the  glass  slide  in  their 
original  position,  so  that  the  diamond-cuts  shall  fit  together  with  the  most 
exact  precision;  and  the  upper  surface  is  then  to  be  ground  flat  with  emery 
upon  a  pewter  plate,  and  left  rough. — The  perfect  construction  of  largo 
deep  cells  of  this  kind  (Fig.  123,  A,  b),  however,  requires  a  nicety  of 
workmansliip  which  few  amateurs  possess,  and  the  expenditure  of  more 
time  than  Microscopists  generally 

have  to  spare;  and  as  it  is  con-  •      rrc.  i2.:>\ 

sequently  preferable  to  obtain 
them  ready-made,  directions  for 
making  them  need  not  be  here 
given. 

175.  Wooden  Slides  for  Opaque 
Objects. — Such  ^dry^  objects  as 
Foraminifera,  the  capsules  of 
Mosses y  parts  of  Insects,  and  the 
like,  may  be  conveniently  mounted 
in  a  very  simple  form  of  wooden  ^ 
slide  (first  devised  by  the  Author 
and  now  come  into  general  use), 
which  also  serves  as  a  jirotective 
'cell.'    Let  a  number   of  slips  Built-up ceiis. 

of  mahogany  or  cedar  be  pro- 
vided, each  of  the  3-inch  by  1-inch  size,  and  of  any  thickness  that  may 
be  found  convenient,  with  a  corresponding  number  of  slips  of  card  of  the 
same  dimensions,  and  of  pieces  of  deadAA^ick  paper  rather  larger  than  the 
aperture  of  the  slide.  A  piece  of  this  paper  being  gummed  to  the  middle 
of  the  card,  and  some  stiff  gum  having  been  previously  spread  over  one  side 
of  the  wooden  slide  (care  being  taken  that  there  is  no  superfluity  of  it 
immediately  around  the  aperture),  this  is  to  be  laid  down  upon  the  card, 
and  subjected  to  pressure.  ^  An  extremely  neat  '  cell '  will  thus  be  formed 
for  the  reception  of  the  object  (Fig.  124),  the  depth  of  which  will  bo 
determined  by  the  thickness  of  the  slide,  and  the  diameter  by  the  size  of 
the  perforation;  and  it  will  be  found  convenient  to  provide  slides  of 
various  thicknesses,  with  apertures  of  different  sizes.  The  cell  should 
always  be  deep  enough  for  its  wall  to  rise  above  the  object;  but,  on  the 
other  hand,  it  should  not  be  too  deep  for  its  walls  to  interfere  with  the 
oblique  incidence  of  the  light  upon  any  object  that  may  be  near  its 
periphery.  The  object,  if  flat  or  small,  may  be  attached  by  Gum-muci- 
lage (§  168  ^);  if,  however,  it  be  large,  and  the  part  of  it  to  be  attached 
have  an  irregular  surface,  it  is  desirable  to  form  a  ^bed'  to  this  by  gum 
thickened  with  starch.  If,  on  the  other  hand,  it  should  be  desired  to 
mount  the  object  edgeways  (as  when  the  mouth  of  a  Foraminifer  is  to  be 
brought  into  view),  the  side  of  the  object  may  be  attached  with  a  little 
gum  to  the  loall  of  the  cell. — The  complete  protection  thus  given  to  the 
Object  is  the  great  recommendation  of  this  method.    But  this  is  by  no 

^  It  will  be  found  a  very  convenient  plan  to  prepare  a  large  number  of  such 
Slides  at  once:  and  this  may  be  done  in  a  marvellously  short  time,  if  the  slips  of 
card  have  been  previously  cut  to  the  exact  size  in  a  bookbinder's  press.  The 
slides,  v^^hen  put  together,  should  be  placed  in  pairs,  back  to  back;  and  every  pair 
should  have  each  of  its  ends  embraced  by  a  Spring-press  ;Fig.  129)  until  dry. 


184 


THE  MICROSCOPE  AND  ITS  REVELATIONS. 


means  its  only  convenience.  It  allows  the  slides  not  only  to  range  in  the 
ordinary  Cabinets,  but  also  to  be  laid  one  against  or  over  another,  and  to 
be  packed  closely  in  cases,  or  secured  by  elastic  bands;  which  plan  is 
extremely  convenient  not  merely  for  the  saving  of  space,  but  also  for 
preserving  the  objects  from  dust.  Should  any  more  special  protection  be 
required,  a  thin  glass  cover  may  be  laid  over  the  top  of  the  cell,  and 
secured  there  either  by  a  rim  of  gum  or  by  a  perforated  paper  cover 
attached  to  the  slide;  and  if  it  should  be  desired  to  pack  these  covered 
slides  together,  it  is  only  necessary  to  interpose  guards  of  card  somewhat 
thicker  than  the  glass  covers. 

176.  Turn-table. — This  simple  instrument  (Fig.  125),  devised  by  Mr. 


Wooden  Slide  for  Opaque  Objects. 


Shadbolt's  Turn-table  for  making  Cement-Cells. 


Shadbolt,  is  almost  indispensable  to  the  Microscopist  who  desires  to  pre- 
serve preparations  that  are  mounted  in  any  ^  medium  ^  beneath  circular 
covers;  since  it  not  only  serves  for  the  making  of  those  ^Cement-cells' 
(§  170)  in  which  thin  transparent  objects  can  be  best  mounted  in  any 
kind  of  ^  medium'  but  also  enables  him  to  apply  his  varnish  for  the  secur- 
ing of  circular  cover-glasses  not  only  with  greater  neatness  and  quickness, 
but  also  with  greater  certainty  than  he  can  by  the  hand  alone.  As  the 
method  of  using  it  for  the  latter  purpose  is  essentially  the  same  as  that 

already  described  under  the  former 
head,  it  need  not  be  here  repeated; 
the  only  special  precaution  to  be  ob- 
served, being  that  the  cover-glass, 
not  the  slide,  should  be  ^centered;' 
which  can  be  readily  done,  if  several 
concentric  circles  have  been  turned 
on  the  rotating-table,  by  making  the 
cover-glass  correspond  with  the  one 
having  its  own  diameter. — A  number 
of  ingenious  modifications  have  been 
devised  in  this  simple  instrument, 
with  the  view  of  securing  exact  cen- 
tering; the  simplest  of  them  (which 
has  the  advantage  of  being  applic- 
able at  a  trifling  expense  to  any  ex- 
isting turn-table)  being  that  of  Mr. 
0.  S.  Rolfe.^  But  as  it  is  often  re- 
quisite to  use  this  instrument  with 
slides  not  accurately  cut  to  size  and 
shape,  or  of  greater  breadth  than  the  ^regulation '  1-inch,  the  Author  is  dis- 
posed to  prefer  the  form  devised  by  Mr.  Dunning  ^  (Fig.  126).   The  circular 


Dunning's  turn-table. 


^ Journal  of  the  Quekett  Microscopical  Club,"  Vol.  v.,  p.  249. 
2  Op.  cit.,  vol.  vi.,  p.  81. 


PREPARATION,  MOUNTING,   AND  COLLECTION  OF  OBJECTS. 


185 


table,  made  rather  thicker  than  usual,  has  a  dovetail  groove  ploughed  out 
across  its  diameter,  in  which  work  two  sliding  guides  A,  a,  the  ends  of 
which  are  cut  and  '  sprung/  so  as  to  have  a  sufficiently  firm  hold.  These 
guides  carry  the  two  clips  b,  b;  one  of  which  is  fixed  at  right  angles  to 
its  guide,  whilst  the  other  is  pivoted,  in  order  that  it  may  adjust  itself 
to  any  irregularity  in  the  form  of  the  slide. — When  Cement-cells  arc 
being  made  either  with  this  or  the  ordinary  Turn-table,  it  is  convenient 
to  mark  the  centre  of  each  slide  with  a  dot  of  ink  on  its  under  surface; 
this  may  be  easily  applied  in  its  right  place  by  laying  on  it  a  slip  of  card 
cut  to  the  regulation  size,  with  a  small  central  perforation;  and  by  so 
laying  down  the  slide  that  the  dot  lies  on  the  centre  of  the  rotating  plate, 
much  trouble  may  afterwards  be  saved. 

177.  Moimting  Plate  and  Water-hath, — Whenever  heat  has  to  be 
applied  either  in  the  cementing  of  Cells  or  in  the  mounting  of  Objects, 
it  is  desirable  that  the  slide  should  not  be  exposed  direct  to  the  ilame, 
but  that  it  should  be  laid  upon  a  surface  of  regulated  temperature.  As 
cementing  with  Marine  Glue  or  hardened  Canada  Balsam  requires  a  heat 
above  that  of  boiling  water,  it  must  be  supplied  by  a  plate  of  metal;  and 
the  Author's  experience  leads  him  to  recommend  that  this  should  be  a 
piece  of  iron  not  less  than  six  inches  square  and  half  an  inch  thick;  and 
that  it  should  be  supported,  not  on  legs  of  its  own,  but  on  the  ring  of  a 
Eetort-stand,  so  that  by  raising  or  lowering  the  ring,  any  desired  amount 
of  heat  may  be  imparted  to  it  by  the  lamp  or  gas-flame  beneath.  The 
advantage  of  a  plate  of  this  size  and  thickness  consists  in  the  gradational 
temperature  which  its  different  parts  afford,  and  in  the  slowness  of  its 


cooling  when  removed  from  the  lamp.  When  many  cells  are  being 
cemented  at  once,  it  is  convenient  to  have  two  such  plates,  that  one  may 
be  cooling  while  the  other  is  being  heated. — The  Eetort-stand  also  serves 
for  the  support  of  the  Water-bath,  which  affords  the  heat  required  for 
liquefying  and  mixing  the  fats  employed  in  the  imbedding  process  (§  189), 
for  melting  the  glycerine  jelly  or  other  media  used  in  mounting,  and  for 
a  variety  of  other  purposes.  A  circular-bottomed  flat  tin  vessel,  6  inches 
in  diameter  and  2^  inches  deep,  with  a  handle  like  that  of  a  saucepan, 
and  two  covers, — one  a  flat  plate  of  8  inches  square  (its  edges  guarded  by 
being  turned  over  wire)  for  slides  to  lie  upon,  having  a  hole  large  enough 
to  admit  a  small  bottle  of  cement  or  medium, — the  other  fitting  the 
vessel,  but  with  an  opening  large  enough  for  a  porcelain  basin, — will 
answer  every  purpose. 

178.  Slider- Forceps,  Spring  Clip,  and  Spring-Press. — For  holding 
slides  to  which  heat  is  being  applied,  especially  while  cementing  objects 
to  be  ground-down  into  thin  sections,  the  wooden  Slider-Forceps  (Fig. 
127)  will  be  found  extremely  convenient.  This,  by  its  elasticity,  affords 
a  secure  grasp  to  a  slide  of  any  ordinary  thickness,  the  wooden  blades 
being  separated  by  pressure  upon  the  brass  studs;  while  the  lower  stud, 
with  the  bent  piece  of  brass  at  the  junction  of  the  blades,  affords  a  level 
support  to  the  forceps,  which  thus,  while  resting  upon  the  table,  keeps 


Slider-Forceps. 


186 


THE  MICROSCOPE  AND  ITS  REVELATIONS. 


the  heated  glass  from  contact  with  its  surface.  For  holding-down  cover- 
glasses  whilst  the  balsam  or  other  medium  is  cooling,  if  the  elasticity  or 
the  object  should  tend  to  make  them  spring-up,  the  wire  Spring-Clip 
(Fig.  128),  sold  at  a  cheap  rate  by  dealers  in  Microscopic  apparatus,  will 
be  found  extremely  convenient.  Or,  if  a  stronger  pressure  be  required, 
recourse  may  be  had  to  a  simple  Spring- Press  made  by  a  light  alteration 
of  the  '  American  clothes  peg '  which  is  now  in  general  use  in  this  country 
for  a  variety  of  purposes;  all  that  is  necessary  being  to  rub  down  the 
opposed  surfaces  of  the  ^clip^  with  a  flat  file,  so  that  they  shall  be  parallel 
to  each  other  when  an  ordinary  slide  with  its  cover  is  interposed  between 
them  (Fig.  129).    One  of  these  convenient  little  implements  may  also  be 


Spring-Clip.  Spring-Press. 


easily  made  to  serve  the  purpose  of  a  Slider-forceps,  by  cutting  back  the 
upper  edge  of  the  clip,  and  filing  the  lower  to  such  a  plane  that  when  it 
rests  on  its  fiat  side,  it  shall  hold  the  slide  parallel  to  the  surface  of  the 
table,  as  in  Fig.  127. 

179.  Ifounting  Instrument, — A  simple  mode  of  applying  graduated 
pressure  concurrently  with  the  heat  of  a  lamp,  which  will  be  found  very 
convenient  in  the  mounting  of  certain  classes  of  objects,  is  afforded  by 
the  Mounting  instrument  devised  by  Mr.  James  Smith.  This  consists  of 
a  plate  of  brass  turned  up  at  its  edges,  of  the  proper  size  to  allow  the 
ordinary  glass  slide  to  lie  loosely  in  the  bed  thus  formed,  this  plate  has  a 
large  perforation  in  its  centre,  in  order  to  allow  heat  to  be  directly  applied 
to  the  slide  from  beneath,  and  it  is  attached  by  a  stout  wire  to  a  handle 
(Fig.  130).    Close  to  this  handle  there  is  attached  by  a  joint  an  upper 


Smith  Mounting  Instrument. 

wire,  which  lies  nearly  parallel  to  the  first,  but  makes  a  downward  turn 
just  above  the  centre  of  the  slide-plate,  and  is  terminated  by  an  ivory 
knob;  this  wire  is  pressed  upwards  by  a  spring  beneath  it,  whilst,  on  the 
other  hand,  it  is  made  to  approximate  the  lower  by  a  milled-head  turning 
on  a  screw,  so  as  to  bring  its  ivory  knob  to  bear  with  greater  or  less  force 
on  the  covering  glass.  The  special  use  of  this  arrangement  will  be  ex- 
plained hereafter  (§  210). 

^  180.  Dissecting  Apparatus, — The  mode  of  making  a  dissection  for 
Microscopic  purposes  must  be  determined  by  the  size  and  character  of  the 
object.    Generally  speaking,  it  will  be  found  advantageous  to  carry  on 


PREPARATION,  MOUNTING,   AND  COLLECTION  OF  OBJECTS.  187 


the  dissection  under  Water,  with  which  Alcohol  should  be  mingled  where 
the  substance  has  been  long  immersed  in  spirit.  The  size  and  depth  of 
the  vessel  should  be  proportioned  to  the  dimensions  of  the  object  to  be 
dissected;  since,  for  the  ready  access  of  the  hands  and  dissecting-instru- 
ments,  it  is  convenient  that  the  object  should  neither  be  far  from  its 
walls,  nor  lie  under  any  great  depth  of  water.  Where  there  is  no  occa- 
sion that  the  bottom  of  the  vessel  should  be  transparent,  no  kind  of  Dis- 
secting trough  is  more  convenient  than  that  which  every  one  may  readily 
make  for  himself,  of  any  dimension  he  may  desire,  by  taking  a  piece  of 
sheet  Gutta-percha  of  adequate  size  and  stoutness,  warming  it  sufficiently 
to  render  it  flexible,  and  then  turning-up  its  four  sides,  drawing  out  one 
corner  into  a  sort  of  spout,  which  serves  to  pour  away  its  contents  when 
it  needs  emptying.  The  dark  color  of  this  substance  enables  it  to  fur- 
nish a  back-ground,  which  assists  the  observer  in  distinguishing  delicate 
membranes,  fibres,  etc.,  especially  when  magnifying  lenses  are  employed; 
and  it  is  hard  enough  (without  being  too  hard)  to  allow  of  pins  being 
fixed  into  it,  both  for  securing  the  object,  and  for  keeping  apart  such 
portions  as  it  is  useful  to  put  on  the  stretch.  When  glass  or  earthenware 
troughs  are  employed,  a  piece  of  sheet-cork  loaded  with  lead  must  be  pro- 
vided, to  answer  the  same  purposes.  In  carrying-on  dissections  in  such 
a  trough,  it  is  frequently  desirable  to  concentrate  additional  light  upon 
the  part  which  is  being  operated-on,  by  means  of  the  smaller  Condensing- 
lens  (Fig.  86);  and  when  a  low  magnifying  power  is  wanted,  it  may  be 
supplied  either  by  a  single  lens  mounted  after  the  manner  of  Eoss's 
Simple  Microscope  (Fig.  31,  b),  or  by  a  pair  of  Spectacles  mounted  with 
the  ^semi-lenses,^  ordinarily  used  for  Stereoscopes.*  Portions  of  the  body 
under  dissection  being  floated-ofl  when  detached,  may  be  conveniently 
taken  up  from  the  trough  by  placing  a  slip  of  glass  beneath  them  (which 
is  often  the  only  mode  in  which  delicate  membranes  can  be  satisfactorily 
spread  out);  and  may  be  then  placed  under  the  Microscope  for  minute 
examination,  being  first  covered  with  thin  glass,  beneath  the  edges  of 
which  is  to  be  introduced  a  little  of  the  liquid  wherein  the  dissection  is 
being  carried-on.  Where  the  body  under  dissection  is  so  transparent, 
that  more  advantage  is  gained  by  transmitting  light  through  it  than  by 
looking  at  it  as  an  opaque  object,  the  trough  should  have  a  glass  bot- 
tom, and  for  this  purpose,  unless  the  body  be  of  unusual  size,  some  of 
the  Glass  Cells  already  described  (Figs.  121-123)  will  usually  answer  very 
well.  The  finest  dissections  may  often  be  best  made  upon  ordinary  slips 
of  glass;  care  being  taken  to  keep  the  object  sufficiently  surrounded  by 
fluid.  For  work  of  this  kind  no  simple  instrument  is  more  generally 
serviceable  than  the  Laboratory  Dissecting  Microscope  (Fig.  35),  which 
will  carry  any  power  from  3-inch  to  a  l-4th  inch;  whilst  the  Stephenson 
Erecting  Binocular  (Fig.  47)  may  be  used  with  the  like  supports  for  the 
hands,  when  a  higher  power  is  preferred. 

181.  The  Listruments  used  in  Microscopic  dissection  are  for  the  most 
part  of  the  same  kind  as  those  which  are  needed  in  ordinary  minute  Au- 
atomical  research,  such  as  scalpels,  scissors,  forceps,  etc.;  the  fine  instru- 


^  The  author  can  strongly  recommend  these  Spectacles,  as  useful  in  a  great 
variety  of  manipulations  which  are  best  performed  under  a  low  magnifying 
power,  with  the  conjoint  use  of  both  eyes. — Where  a  higher  power  is  needed,  re- 
course may  be  advantageously  had  to  Messrs.  Beck's  3-inch  Achromatic  Binocu- 
lar Magnifier,  which  is  constructed  on  the  same  principle,  allowing  the  object  to 
be  brought  very  near  the  eyes,  without  requiring  any  uncomfortable  convergence 
of  their  axes. 


188 


THE  MICROSCOPE  AND  1TB  REVELATIONS. 


ments  used  in  operations  upon  the  eye,  however,  will  commonly  be  found 
most  suitable.  A  pair  of  delicate  scissors,  curved  to  one  side,  is  extremely 
convenient  for  cutting  open  tubular  parts;  these  should  have  their 
points  blunted;  but  other  scissors  should  have  fine  points.  A  pair  of 
very  fine* pointed  Scissors  (Fig.  131),  one  leg  of  which  is  fixed  in  a  light 
handle,  and  the  other  kept  apart  from  it  by  a  spring,  so  as  to  close  by  the 
pressure  of  the  finger  and  to  open  of  itself,  will  be  found  (if  the  blades 
be  well  sharpened)  much  superior  to  any  kind  of  knives,  for  cutting 
through  delicate  tissues  with  as  little  disturbance  of  them  as  possible. — 
A  pair  of  small  straight  Forceps  with  fine  points,  and  another  pair  of 
curved  forceps  will  be  found  useful  in  addition  to  the  ordinary  dissecting 
forceps. 

182.  Of  all  the  instruments  contrived  for  delicate  dissections,  however, 
none  are  more  serviceable  than  those  which  the  Microscopist  may  make 
for  himself  out  of  ordinary  needles.    These  should  be  fixed  in  light 


rjG.  T31.  Fig.  132, 


Spring-Scissors.  Curved  Scissors  for  Cutting  thin  Sections. 


wooden  handles'  (the  cedar  sticks  used  for  camel-hair  pencils,  or  the 
handles  of  steel-j)enholders,  or  small  Porcupine-quills,  will  answer  ex- 
tremely well),  in  such  a  manner  that  that  their  points  should  not  project 
far,^  since  they  will  otherwise  have  too  much  ^spring;'  much  may  be  done 
by  their  mere  tearing  action;  but  if  it  be  desired  to  use  them  as  cutting 
instruments,  all  that  is  necessary  is  to  harden  and  temper  them,  and  then 
give  them  an  edge  upon  a  hone.  It  will  sometimes  be  desirable  to  give  a 
finer  j^oint  to  such  needles  than  they  originally  possess;  this  also  may  be 
done  upon  a  hone.  A  needle  with  its  point  bent  to  a  right  angle,  or 
nearly  so,  is  often  useful;  and  this  may  be  shaped  by  simply  heating  the 
point  in  a  lamp  or  candle,  giving  to  it  the  required  turn  with  a  pair  of 
pliers,  and  then  hardening  the  point  again  by  reheating  it  and  plunging 
it  into  cold  water  or  tallow. 

183.  Section-cutting. — The  young  Microscopist  will  do  well  to  practise 
the  cutting  of  thin  Sections  of  soft  Vegetable  and  animal  substances  with 
a  sharp  razor:  considerable  practice  is  needed,  however,  to  make  effect- 
ual use  of  it;  and  some  individuals  acquire  a  degree  of  dexterity  which 


^  The  handles  of  ladies  Crochet-needles  have  been  recommended  for  this  pur- 
pose; and  although  they  afford  the  facility  of  lengthening  of  shortening  the  act- 
ing point  of  the  needle  at  will,  and  also  of  carrying  a  reserve  store  of  needles  at 
the  other  end,  yet  the  Author  would  decidedly  recommend  the  use  of  the  wooden 
handles,  of  which  it  will  be  found  convenient  always  to  have  several  at  hand, 
mounted  with  needles  of  different  sizes. 

2  The  following  is  the  mode  in  which  the  Author  has  found  it  convenient  to 
mount  his  needles  for  this  and  other  purposes : — The  needle  being  held  firmly  in 
a  pair  of  pliers  grasped  by  the  right  hand,  its  point  may  be  forced  into  the  end  of 
a  cedar  or  other  stick  held  in  the  left,  until  it  has  entered  to  the  depth  of  half  an 
inch  or  more;  the  needle  is  then  cut  off  to  the  desired  length  (the  eye-end  being 
thus  got  rid  of);  and  being  then  drawn  out  of  the  stick,  the  truncated  end  is  forced 
into  the  hole  previously  made  by  the  point,  until  it  cannot  be  made  to  penetrate 
farther,  when  it  will  be  found  to  be  very  securely  fixed.  The  end  of  the  handle 
which  embraces  it  may  then  be  bevelled-away  round  its  point  of  insertion. 


PREPARATION  J  MOdNTING,  AND  COLLECTION  OF  OBJECTS. 


189 


Tig. 


others  never  succeed  in  attaining.  The  making  of  hand-sections  will  be 
greatly  facilitated  by  the  previous  use  of  the  hardening  and  imbeddincr 
processes  robe  hereafter  described  (§§  189,  199);  but  the  best  of  them 
rarely  equal  good  sections  cut  by  a  Slicrotome. — For  the  preliminary 
examination  of  any  soft  structure,  such  a  pair  of  Scissors  as  is  represented 
in  Fig  132  will  often  be  found  very  useful;  since,  owmg  to  the  cur- 
vature of  the  blades,  the  two  extremities  of  a  section  taken  from  a  flat 
surface  will  generally  be  found  to  thin  away,  although  the  middle  of  it  may 
be  too  tliick  to  exhibit  any  structure.  The  two-bladed  Knife  contrived 
by  Prof.  Valentin  was  formerly  much  used  for  cutting  microscopic  sec- 
tions of  soft  tissues:  but  as  such  sections  can  be  cut  far  more  ett'ectively 
by  the  methods  to  be  presently  described,  a  mere  mention  of  this  instru- 
ment will  here  suffice. 

184.  Microtome.  —There  is  a  large  class  of  substances,  of  moderate  hard- 
ness, both  Animal  and  Vegetable,  of  which  extremely  thin  and  uniform 
slices  can  be  made  by  a  sharp-cutting  instrument,  if  they  be  properly  held 
and  supported,  and  the  thickness  of  the  section  be  regulated  by  a  mechan- 
ical contrivance;  such  are,  in  particular,  the  Stems  and  Eoots  of  Plants, 
and  the  Horns,  Hoofs,  Cartilages,  and  similarly  firm  structures  of  Animals. 
Various  costly  machines  have  been  devised  for  this  purpose,  some  of 
them  characterized  by  great  ingenuity  of  contrivance  and  beauty  of  work- 
manship, but  most  of  the  purposes 
to  which  these  are  adapted  will  be 
found  to  be  answered  by  a  very  simple 
and  inexpensive  little  instrument,  I 
which  may  either  be  held  in  the  hand, 
or  (as  is  preferable)  may  be  firmly 
attached  by  means  of  a  T-shaped  piece 
of  wood  (Fig.  133),  to  the  end  of  a 
table  or  work-bench.  This  instrument 
essentially  consists  of  an  upright  hol- 
low cylinder  of  brass,  with  a  kind  of 
piston  which  is  pushed  from  below 
upwards  by  a  fine-threaded  or  '  micro- 
meter '  screw  turned  by  a  large  milled- 
head,  at  the  upper  end  the  cylinder 
terminates  in  a  brass  table,  which  is 
T)laned  to  a  flat  surface,  or  (which  is 
preferable)  has  a  piece  of  plate-glass  Simple  Microtome, 

cemented  to  it,  to  form  its  cutting  bed.  At  one  side  is  seen  a  small  milled- 
head,  which  acts  upon  a  ^binding  screw,'  whose  extremity  projects  into 
the  cavity  of  the  cylinder,  and  serves  to  compress  and  steady  anything 
that  it  holds.  For  this  is  now  generally  substituted  a  pair  of  screws,  work- 
ing through  the  side  of  the  cylinder,  as  in  Fig.  120.  A  cylindrical  stem  of 
wood,  a  piece  of  horn,  whalebone,  cartilage,  etc.,  is  to  be  fitted  to  the 
interior  of  the  cylinder  so  as  to  project  a  little  above  its  top,  and  is  to  be 
steadied  by  the  ^binding  screw; ^  it  is  then  to  be  cut  to  a  level  by  means 
of  a  sharp  knife  or  razor  laid  flat  upon  the  table.  The  large  milled-head 
is  next  to  be  moved  through  such  a  portion  of  a  turn  as  may  very  slightly 


^  It  is  difficult  to  convey  by  a  drawing  the  idea  of  the  real  curvature  of  this 
instrument,  the  blades  of  which,  when  it  is  held  tti  front  view,  curve — not  to 
either  side— but  towards  the  observer;  these  scissors  being,  as  the  French  instru- 
ment-makers say,  courb4s  sur  leplat. 


190 


THE  MICROSCOPE  AND  ITS  REVELATIONS. 


elevate  the  substance  to  be  cut,  so  as  to  make  it  project  in  an  almost  insen- 
sible degree  above  the  table,  and  this  projecting  part  is  to  be  sliced  off 
with  a  knife  previously  dipped  in  water.  For  many  purposes  an  ordinary 
razor  will  answer  sufficiently  well;  but  thinner  and  more  uniform  sections 
can  be  cut  by  a  special  knife  having  its  edge  parallel  to  its  back,  its  sides 
slightly  concave,  and  its  back  with  a  uniform  thickness  of  rather  less  than 
l-4th  inch.  Such  a  knife  should  be  4  or  5  inches  long,  and  7-8ths  inch 
broad;  and  should  be  set  in  a  box-wood  handle  about  4  inches  long  (Dr. 
S.  Marsh).  The  motion  given  to  its  edge  should  be  a  combination  of 
drawing  'd>ndi  pressing,  (It  will  be  generally  found  that  better  sections  are 
made  by  working  the  knife  /rom  the  operator,  than  toiuards  him).  When 
one  slice  has  been  thus  taken  off,  it  should  be  removed  from  the  blade 
by  dipping  it  into  water,  or  by  the  use  of  a  camel-hair  brush;  the 
milled-head  should  be  again  advanced,  and  another  section  taken:  and 
so  on.  Different  substances  will  be  found  both  to  lear  and  to  require 
different  degrees  of  thickness;  and  the  amount  that  suits  each  can  only 
be  found  by  trial.  It  is  advantageous  to  have  the  large  milled-head  gradu- 
ated, and  furnished  with  a  fixed  index;  so  that  this  amount  having  been 
once  determined,  the  screw  shall  be  so  turned  as  to  always  produce  the 
exact  elevation  required. — Where  the  substance  of  which  it  is  desired  to 
obtain  sections  by  this  instrument  is  of  too  small  a  size  or  of  too  soft  a 
texture  to  be  held  firmly  in  the  manner  just  described,  it  may  be  placed 
between  the  two  vertical  halves  of  a  cork  of  suitable  size  to  be  pressed  into 
the  cylinder;  and  the  cork,  with  the  object  it  grasps,  is  then  to  be  sliced  in 
the  manner  already  described,  the  small  section  of  the  latter  being  care- 
fully taken-off  the  knife,  or  floated-away  from  it,  on  each  occasion,  to 
prevent  it  from  being  lost  among  the  lamellae  of  cork  which  are  removed 
at  the  same  time.  Vertical  sections  of  many  Leaves  may  be  successfully 
made  in  this  way;  and  if  their  texture  be  so  soft  as  to  be  injured  by  the 
pressure  of  a  cork,  they  may  be  placed  between  two  half-cylinders  of  car- 
rot or  elder-pith. 

185.  Hailes^s  Microtome, — The  foregoing  simple  form  of  Microtome 
has  received,  at  various  hands,  numerous  modifications  of  detail,  without 
any  essential  change  in  its  plan  of  construction.  Its  chief  defect  is,  that 
as  the  body  to  be  cut  is  directly  acted-on  by  the  screw  at  the  bottom  of 
the  cylinder,  its  motion  (if  it  be  tightly  held  by  the  binding  screws)  is  apt 
to  be  jerky  and  irregular.  To  remedy  this  defect,  Mr.  II.  P.  Hailes  has 
devised  an  improved  model,  the  essential  feature  of  which  is  that  the  body 
to  be  cut  is  secured  within  an  inner  tube,  which,  sliding  freely  within  the 
outer  cylinder,  is  raised  smoothly  and  equally  by  the  micrometer  screw 
attached  to  the  base  of  the  latter,  as  shown  in  Fig.  134  (1,  2).  The  cut- 
ting-bed formed  by  the  flange  B,  is  provided  with  two  slips  h  of  hardened 
steel,  on  which,  in  ordinary  section-cutting,  the  knife  or  razor  slides 
horizontally,  as  in  the  ordinary  Microtome.  But  by  the  addition  shown 
in  3,  4,  this  instrument  can  also  be  effectively  adapted  for  cutting  thin 
sections  of  substances  hard  enough  to  require  the  use  of  the  saw.  At  the 
back  of  the  cutting-bed,  there  can  be  secured  (by  means  of  the  screw  and 
and  steadying-pins)  a  metal  block,  h\  which  carries  two  guides  of 
hard  steel;  and  these,  when  thus  attached,  lie  over  the  two  similar  strips 
fixed  on  the  cutting-bed.  By  passing  the  blade  of  a  fine  saw  between  the 
movable  guides  and  the  fixed  strips,  and  screwing  down  the  former 
(which  are  raised  by  a  spring)  as  far  as  will  confine  the  saw  without  im- 
peding its  working,  sections  of  Bone,  Teeth,  etc.,  may  be  cut  as  thin  as 
the  nature  of  the  substance  will  allow,  and  with  a  uniformity  that  with- 


PREPARATION,  MOUNTING,  AND  COLLECTION   OF  OBJECTS.  191 


out  such  guidance  cannot  be  attained. — When  the  Microtome  is  employed 
for  this  last  purpose,  the  saw  may  be  most  conveniently  worked  vertically; 
and  this  is  readily  done  by  detaching  the  instrument  from  the  table,  and 
holding  it  down  upon  its  clamp-side,  which  is  so  shaped  as  to  afford  a 
level  support. 

186.  In  what  is  known  as  the  Strashicrg  Microtome^  invented  by 
Prof.  Schieflerdecker,  the  substance  to  be  cut  is  fixed  in  the  cylinder  by 
binding-screws,  while  the  circular  cutting-bed,  instead  of  being  fixed  on 
the  upper  end  of  the  cylinder,  is  made  to  screw  upon  it,  so  as  to  be  raised 
or  lowered  by  turning  it  round.  Thus,  after  a  section  has  been  taken,  a 
slight  lowering  of  the  cutting-bed,  measured  by  the  graduation  of  its 
margin,  prepares  it  for  the  cutting  of  the  next.^ — The  simplicity  of  this 


5  fni 

J  ^  1 

Ilailes'G  Microtome. 

The  two  upper  figures  show  the  instrument  (1)  as  seen  from  the  side,  (2)  as  seen  in  section  a, 
outer  cyUnder,  carryin,-  u;.;  :r  flan?;-e  c,  on  whose  surface  he  two  strips  of  hard  steel,  6,  h;  this 
flange  fixed  t^*  the  bar  c,  v/hich  carries  a  clar_ip  and  screw  for  attaching  the  Microtome  to  a  table: 
in  the  sec'onalfi^n  (2)  is  seen  the  inne.- tubo  c,  within  which  the  substance  to  be  cutis  fixed 
by  tht  twc  binding  screws,  r,  c,  which  work  through  a  slot  in  the  outer  cylinder;  to  the  bottom  of 
the  inner  tube  is  fixed  a  block  d.  thror.(;:h  which  works  the  micrometer-screw  e,  turned  by  the 
milled-hdad  o  in  the  bracket  f  attached  to  the  bottom  of  the  outer  cylinder,  and  having  a  graduated 
collar/. 

The  two  lower  figures  show  the  additional  Saw-guide,  seen  from  the  side  at  3,  and  from  above 
at  4  :— &i,  metal  block  with  a  screw  to  secure  it  on  cutting-bed;  h\  b^,  steel  guides. 

instrument,  which  is  made  to  be  held  in  one  hand  whilst  the  section  is 
cut  with  the  other,  is  its  great  recommendation. 

187.  Imledding  and  Freezing  Microtomes,  — Y or  making  thin  sections 
of  soft  tissues,  however,  preference  is  now  generally  given  to  Microtomes 
in  which  the  substance  to  be  cut  is  so  imbedded  in  some  material  that 
fills  the  cylinder,  that  it  does  not  need  to  be  fixed  by  binding-screws, 
being  pushed  upwards  by  the  action  of  the  micrometer-screw  beneath 

^ Quart.  Journ.  of  Microsc.  Science,"  Vol.  xvii.  (1877),  p.  35.— Another  Micro- 
tome, suggested  by  the  preceding,  is  described  by  Mr.  W.  Teesdale  in  "Journ.  of 
Roy.  Microsc.  Soc,"  Vol.  iii.  (1880),  p.  1035. 


192 


THE  MICROSCOPE  AND  ITS  REVELATIONS. 


upon  the  imbedding  plug.  This  plug  may  be  either  a  cylinder  of  carrot, 
turnip,  potatoe,  or  elder-pith,  cut  to  fit  the  well  of  the  Microtome,  and 
excavated  to  receive  the  substance  to  be  cut;  or  it  may  be  a  cast  of  the 
interior,  made  either  by  pouring  into  it  paraffine  or  some  similar  sub- 
stance liquefied  by  heat  (§  189),  or  by  filling  it  with  thick  gum-mucilage 
which  is  then  rendered  dense  by  cold  (§  191).  The  latter  plan  was  first 
devised  by  Prof.  Kutherford,  whose  Freezing  Microtome,  in  which  the 
upper  part  of  the  cylinder  is  surrounded  by  a  well  filled  with  a  freezing- 
mixture,  has  now  come  into  general  use. — The  substitution  of  ether-spray 
for  ice-congelation  was  suggested  by  Mr.  Bevan  Lewis;  and  an  improved 
model,  which  can  be  used  either  as  a  Freezing  or  as  an  Imbedding 
Microtome,  has  been  devised  by  Messrs.  Beck.  An  ingenious  method  of 
so  attaching  the  cutting-blade  by  a  ^  parallel  motion,'  as  to  make  its  edge 
at  the  same  time  move  tangentially  and  transversely  to  the  plane  of 
section,  has  been  devised  by  Prof.  Seiler,  of  Philadelphia,  and  has  found 
much  approval,  as  well  in  this  country  as  in  the  United  States.^ 


Rivet-Leiser  Microtome;— a,  as  seen  from  the  front;  b,  as  seen  from  behind. 

188.  Rivet-Leiser  Microtome, — For  the  cutting  of  very  thin  sections 
of  soft  Animal  or  Vegetable  substances  which  may  be  advantageously 
imbedded  in  paraffine  or  some  other  hard  fat  (§  189),  no  instrument  is 
more  eifective  than  that  represented  in  Fig.  135,  which  is  known  as  the 
'  Leipzig  ^  or  '  Kivet-Leiser '  Microtome.  This  has  for  its  base  an  oblong 
solid  metal  plate,  from  which  rises  a  vertical  plate,  of  which  the  upper 
edge  is  inclined  at  a  gentle  angle.  From  either  side  of  this  vertical 
plate,  there  projects  a  smoothly-planed  plate,  like  a  shelf  sloping 
inwards;  but  while  the  edge  of  one  of  these  shelves  is  parallel  to  the  base, 
that  of  the  other  is  parallel  to  the  inclined  margin  of  tho  vertical  plate. 
On  the  former  slides  a  carrier  bearing  a  Knife,  the  position  of  which  can  be 
adjusted  and  fixed  by  means  of  a  binding-screw  that  works  through  a  slot 


1  "  Journ.  of  Roy.  Microsc.  Soc,"  Vol.  ii.  (1879),  p.  329. 


PREPARATION,  MOUNTING,  AND  COLLECTION  OF  OBJECTS.  193 


in  its  handle;  whilst  on  the  latter  there  slides  an  Object-carrier,  consist- 
ing of  a  clamp,  whose  opening  is  controlled  by  a  binding-screw,  for  hold- 
ing the  block  of  paraffine  in  which  the  substance  to  be  cut  is  imbedded. 
From  this  description,  it  will  be  obvious  that  when  the  carrier  that  bears 
the  knife  (as  seen  at  a)  is  slid  from  one  end  of  its  shelf  to  the  other,  the 
knife  always  remains  on  the  same  level;  but  that  when  the  Object-carrier 
is  similarly  slid  (from  right  to  left  in  Fig.  b),  it  gradually  rises,  always 
keeping  at  the  same  height  in  relation  to  the  inclined  edge  of  the  vertical 
plate.  This  edge  being  graduated,  and  a  ^ vernier^  being  engraved  on 
the  carriage,  the  progressive  elevation  of  the  surface  from  which  the 
section  is  to  be  taken  can  be  measured  with  the  most  minute  exactness; 
as  the  substitution  of  the  inclined  plane  for  the  screw  altogether  does 
away  with  the  '  lost  time '  from  which  the  action  of  the  latter  is  seldom 
entirely  free.  The  manner  in  which  the  knife  is  attached  to  its  carriage, 
enables  it  to  be  so  fixed  as  to  give  any  proportion  that  may  be  desired 
between  the  sliding  and  the  pressing  cut. — The  simple  model  here 
described  is  extensively  used  on  the  Continent;  and  the  Author  can 
indorse  its  reputation  from  large  personal  experience.  Certain  modifica- 
tions have  been  recently  made  in  it,  however  which  must  not  be  passed 
without  notice.  One  of  these  re- 
lates to  the  mode  in  which  the  Eto^ISs;?, 
block  of  paraffine  is  held  in  its  car- 
rier, so  that  the  position  of  the 
body  imbedded  in  it  may  be  varied, 
without  taking  the  block  out  of 
the  clamp.  The  screw  a  (Fig. 
136),  working  through  the  fixed 
piece  h,  brings  the  movable  piece  c 
(which  is  guided  by  two  pins  that 
work  through  h)  against  the  fixed 
piece  d,  and  thus  secures  the  body 
to  be  cut.  The  clamp  is  connected 
by  means  of  the  bent  arm  e  with  the 
block  /,   the  upper    surface  of 

which  is  rounded ;  and  on  this  it  can     improved  object  Carrier  for  the  Rivet-LeiseP 
,  T  .  '  n  1  J      i  1  Microtome. 

be  moved  m  a  plane  parallel  to  the 

middle  plate  of  the  instrument,  so  as  to  take  a  position  more  or  less 
oblique  in  which  it  may  be  fixed  by  the  binding-screw  g.  The  block  / 
again  is  connected  with  the  fixed  block  li,  by  a  pivot  passing  through  the 
latter;  and  on  this  it  may  be  rotated  in  a  plane  at  right  angles  to  the 
middle  plate,  being  fixed  in  any  position  by  the  binding-screw  i.  By 
the  combination  of  these  two  movements,  the  object  can  be  placed  (and 
then  fixed)  in  such  a  position  that  the  sectional  plane  shall  traverse  it 
in  any  desired  direction. — The  knife-carrier  is  also  furnished  with  screws 
that  etiable  the  inclination  of  the  blade  to  be  regulated  with  great  pre- 
cision. And,  if  desired,  the  object-carrier  may  be  advanced  up  its 
incline  by  a  screw  traversing  the  entire  length  of  the  instrument,  instead 
of  by  hand;  an  addition,  however,  which  seems  to  the  Author  quite 
unnecessary,  and  certainly  not  worth  its  cost.  ^— This  Microtome  can  bo 
made  in  hard  wood  at  a  lower  cost  than  in  metal,  and  with  very  little 
sacrifice  (if  any)  of  efficiency;  and  it  has  lately  been  recommended  that 
the  body  of  the  instrument  should  be  divided  longitudinally,  and  its  two 


''♦Joum.  of  Roy.  Microsc.  See,"  Vol.  ii,  (1880),  p.  334. 
13 


194 


THE  MICROSCOPE   AND  ITS  REVELATIONS. 


halves  attached  at  one  end,  but  made  to  diverge  at  the  other  at  any 
angle,  being  there  fixed  by  a  clamping  screw.  ^ 

Section"  2. — Preparation  and  Mounting  of  Objects. 

189.  Imiedding  Processes. — The  preparation  of  soft  Organic  sub- 
stances for  Section-cutting  by  ^imbedding/  may  be  made  in  two  modes> 
the  choice  between  which  will  depend  upon  the  consistence  of  the  sub- 
stance. If  (1)  it  be  compact,  like  a  piece  of  liver  or  kidney,  it  only 
needs  to  be  surrounded  by  the  imbedding  mass,  which  will  afford  it  as  a 
iuliole  the  requisite  support.  But  if  (2)  it  be  partly  occupied,  like  a  piece 
of  lung,  by  interstitial  cavities,  it  must  be  penetrated  by  the  imbedding 
substance,  so  that  every  part  may  be  duly  supported. — For  simple  im- 
bedding, nothing  is  so  suitable  as  the  firmer  fats;  which  must  not,  how- 
ever, be  so  hard  as  to  be  brittle.  Thus,  if  white  Wax  be  used,  it  should 
be  melted  with  an  equal  weight  of  olive  oil;  if  Paraffine  or  Spermaceti,  it 
should  be  melted  with  about  one-fifth  of  its  weight  of  lard  or  soft  tallow. 
The  latter  is  generally  to  be  preferred,  as  shrinking  less  in  cooling;  the 
cylinder  formed  by  the  hardened  wax  being  liable  to  become  loose  in  the 
well  of  the  Microtone.  Either  mixture  being  kept  in  stock,  carefully 
secluded  from  dust,  a  small  quantity  of  it  should  be  melted  for  use  in  a 
porcelain  basin  floated  in  a  water-bath.  To  avoid  injury  to  the  tissue, 
its  temperature  should  not  be  raised  more  than  is  requisite  for  its  thor- 
ough liquefaction.  The  substance  to  be  cut,  having  been  previously 
hardened  (§  199),  should  be  taken  out  of  the  spirit  in  which  it  is  pre- 
served; and  a  piece  of  suitable  size  having  been  cut  off,  this  should  bj 
placed  on  blotting-paper,  so  that  the  spirit  may  drain  away,  and  its  sur- 
face may  become  dry.  It  is  then  to  be  dipped  (as  recommended  by  Dr, 
Sylvester  Marsh),  in  a  very  weak  solution — 20  grains  to  the  ounce — of 
Gum  Arabic,  care  being  taken  in  doing  so  not  to  squeeze  out  the  spirit  so 
as  to  remoisten  the  surface;  and  the  superfluous  liquid  being  then  again 
removed  by  blotting-paper,  the  surface  will  in  a  few  minutes  become  dry 
and  glazed  with  a  thin  film  of  gum,  the  use  of  which  is  to  keep  the 
imbedding  substance  from  adhering  to  it.  The  plug  of  the  Microtome 
(which  may  advantageously  have  a  large-headed  screw  inserted  into  its 
upper  side,  to  furnish  a  '  hold  ^  for  the  imbedding  substance)  being  set 
at  the  depth  of  about  an  inch  beneath  the  cutting-bed,  melted  wax  or 
paraffine  is  to  be  poured  into  it  to  about  half  this  depth;  and  the  sub- 
stance to  be  cut  being  then  held  in  the  tube  in  the  best  position  (which 
is  not  its  centre,  but  nearer  the  side  next  the  operator),  the  imbedding 
material  is  to  be  slowly  poured  in,  until  the  imbedded  substance  is 
entirely  covered,  and  the  cavity  completely  filled.  When  the  imbedding 
material  has  become  quite  solidified  by  cooling,  the  cutting  of  sections 
may  be  proceeded  with. 

190.  When,  however,  it  is  necessary  that  the  substance  to  be  cut 
should  be  entirely  penetrated  by  the  imbedding  material,  a  much  longei 
preparatory  process  is  necessary.  In  many  cases  in  which  the  sections 
are  required  to  display  rather  the  goieral  than  the  minute  structure, 
satisfactory  results  may  be  obtained  by  keeping  the  substance  (previously 
steeped  in  pure  water)  immersed  for  a  lengthened  period  at  a  gentle 
warmth,  either  in  a  strong  mucilage  of  Gum  Arabic,  or  in  a  solution  of 
Gelatine  that  will  ^set'  on  cooling,  its  cavities  having  been  laid  open 


1  Brandt  in    Zeitschrift  fiir  Mikrosk.,"  Bd.  ii.  (1880),  p.  172. 


PREPARATION,  MOUNTING,  AND  COLLECTION  OF  OBJECTS. 


195 


sufficiently  for  the  gradual  penetration  of  the  liquid  to  their  interior. 
The  entire  mass  being  then  exposed  to  the  air,  the  slow  evaporation  of 
its  water  will  at  last  reduce  it  to  a  consistence  sufficiently  firm  to  enable 
sections  of  it  to  be  taken;  or  the  water  may  be  drawn  out  by  steepino-  in 
Alcohol.  This  plan  has  been  found  to  answer  for  the  entire  bodied  of 
Insects,  Stems  of  herbaceous  Plants,  and  the  like. — But  when  the  sec- 
tions are  to  be  cut  of  the  extreme  thinness  required  for  showing  minute* 
histological  detail,  it  is  much  better  to  use  either  Paraffine  slightly  soft-" 
ened  with  lard,  or  Cacao-butter,  which  last  has  been  much  recommended 
for  the  imbedding  of  structures  of  extreme  delicacy.  The  material  to  be 
cut  must  be  first  dehydratedy  or  deprived  of  its  Water;  which  is  done  by 
letting  it  lie  for  a  time  in  ordinary  Spirit,  then  transferring  it  to  Recti- 
fied spirit,  and  at  last  treating  it  with  absolute  Alcohol.  From  this  it  is 
to  be  transferred  to  some  volatile  oil;  oil  of  bergamot  being  used  for 
delicate  objects;  oil  of  turpentine  answering  sufficiently  well  for  larger 
bodies.  When  this  has  completely  replaced  the  spirit,  the  body  is  to  be 
immersed  for  some  little  time  in  a  hot  saturated  solution  of  paraffine  in 
oil  of  turpentine.  When  it  has  lain  sufficiently  long  in  this  to  be  thor- 
oughly penetrated,  it  is  to  be  immersed  in  the  melted  paraffine,  which 
should  not  be  more  heated  than  is  necessary  to  keep  it  quite  liquid;  and 
it  should  be  moved  about  in  this  for  some  little  time,  an  occasional  gentle 
squeeze  being  given  to  it  with  the  forceps,  so  that  the  solution  may  be 
replaced  as  completely  as  possible  by  the  liquefied  paraffine.  When 
hardened  by  cooling,  the  substance  thus  prepared  may  be  '  imbedded '  in 
any  ordinary  cylinder  Microtome,  in  the  manner  already  described;  the 
coating  with  gum  being  of  course  omitted.  But  if  the  sections  are  to  be 
made  either  with  the  Rivet-Leiser  Microtome,  or  by  hand,  it  is  necessary 
to  provide  a  mould  into  which  the  imbedding;  material  can  be  poured. 
This  may  be  made  of  cylindrical  form,  by  twisting  a  strip  of  paper  round 
the  end  of  a  small  ruler;  or  a  brick-shaped  block  may  be  cast  in  a  mould 
made  by  turning  up  the  edges  of  a  suitably-sized  piece  of  paper,  and 
pinning  together  the  cross-folds  at  the  two  ends.  But  it  is  generally 
more  convenient  to  use  for  this  purpose  small  boxes  of  tin  2  inches  long, 
and  3-4ths  of  an  inch  in  breadth  and  depth,  with  removable  bottoms.  A 
small  piece  of  filtering  paper  being  placed  between  the  bottom  and  the 
sides  of  the  box,  and  the  substance  to  be  imbedded  being  held  in  it  in  the 
most  suitable  position,  the  paraffine  is  poured  in  until  the  box  is 
completely  filled,  and  this  is  set  aside  to  cool.  When  the  paraffine  has 
perfectly  solidified,  the  box  it  to  be  lifted  off  its  bottom;  and  the  block, 
being  pushed  out  of  it,  is  thm  ready  for  cutting.— In  using  the  section- 
knife,  care  should  be  taken  to  keep  it  constantly  wetted  with  methylated 
spirit;  and  it  is  desirable  that  each  section  should  be  removed  from 
it  before  another  is  taken.  When,  for  the  study  of  the  anatomy  of  an 
animal,  sections  are  being  taken  m  series,  and  it  is  important  that  their 
order  should  be  preserved,  a  set  of  watch-glasses  should  be  previously 
provided,  each  about  half  filled  with  spirit,  and  the  sections  successively 
taken  should  be  dropped  singly  into  them;  care  being  taken  in  the 
arrangement  of  the  glasses  to  maintain  the  relative  position  of  the 
sections.  In  order  to  dissolve  out  the  imbedding  material,  the  sections 
should  be  soaked  in  oil  of  turpentine  with  about  one-fourth  part  of 
creasote;  and  if  its  structure  is  suitable  for  examination  with  high  powers, 
it  may  be  cleared  by  a  short  immersion  in  oil  of  cloves.  They  are  then 
to  be  mounted  either  in  Canada  balsam  solution  (§  209)  or  in  Dammar 
cement. 


196 


THE  MICROSCOPE  AND  ITS  REVELATIONS. 


191.  When  the  freezing  process  is  employed,  the  substance  to  be  cut 
(which  may  either  be  fresh,  or  have  been  hardened  by  some  of  the 
processes  to  be  hereafter  described,  §  199)  must  be  thoroughly  penetrated 
by  a  thick  solution  of  gum;  for  this,  when  frozen,  does  not  become 
crystalline,  and  may  be  cut  like  cheese.    If  the  substance  to  be  cut  has 

^  been  immersed  in  p^lcohol,  this  must  be  completely  removed  in  the  first 
'instance  by  immersion  in  water  for  from  six  to  twenty-four  hours, 
according  to  the  size  of  the  mass;  for  the  gum  will  not  penetrate  any  part 
which  is  still  alcoholized.  And  the  substance  should  be  then  immersed 
in  the  gum-solution  for  from  twelve  to  twenty-four  hours  before  it  is 
frozen;  in  order  that  every  part  may  be  permeated  by  the  gum,  and  no 
water  be  left  to  form  crystals  of  ice.  If  the  freezing  Microtome  of 
Prof.  Rutherford  *  be  employed,  the  freezing-box  should  be  filled  with 
alternate  spoonsful  of  salt  and  either  snow  or  finely  powdered  ice,  which 
are  to  be  stirred  round  the  well  previously  filled  with  the  gum  solution. 
With  the  Ether-spray  Microtome,  the  freezing  is  produced  by  the  rapid 
evaporation  of  the  liquid  injected  into  the  freezing  chamber.  In  either 
case,  the  substance  to  be  cut  is  to  be  introduced  into  the  well,  as  soon  as 
the  gum  begins  to  harden  at  its  periphery;  and  should  be  held  in  place 
until  fixed  by  the  advancing  congelation.  In  cutting  the  sections,  no 
wetting  of  the  knife  is  necessary;  as  it  is  kept  sufficiently  wetted  by  the 
thawing  gum.  The  sections  should  be  placed  in  methylated  spirit  diluted 
with  twice  its  volume  of  water;  and  this  soon  not  only  dissolves  out  the 
gum,  but  removes  any  air-bubbles  the  section  may  contain.  If  the  section 
is  to  be  at  once  mounted  (which  should  always  be  done  if  it  is  very 
delicate  and  liable  to  be  spoiled  by  manipulation)  it  should  be  placed  on 
a  slide  before  it  has  thawed,  and  washed  by  forming  around  it  a  little 
pool  of  dilute  spirit,  which  may  be  readily  changed  two  or  three  times 
by  the  glass  syringe  (§  127).  Sections  cut  by  the  freezing  process  may 
for  the  most  part  be  mounted  in  glycerine  jelly,  for  which  no  other 
preparation  will  be  needed  than  the  use  (if  desired)  of  the  Staining 
process  hereafter  to  be  described  (§  202).  But  if,  for  the  sake  of 
rendering  the  sections  more  transparent,  mounting  them  in  Canada 
balsam  or  Dammar  is  preferred,  they  must  be  treated  first  with  strong 
spirit,  then  with  absolute  alcohol,  and  then  with  either  oil  of  cloves  or 
oil  of  turpentine. — It  is  claimed  by  Dr.  Rutherford  as  the  special  advan- 
tage of  the  freezing  process,  that  "  delicate  organs,  such  as  the  retina, 
the  embryo,  villi  of  the  intestines,  lung,  trachea  with  its  ciliated  epithe- 
lium, may  all  be  readily  cut  without  fear  of  their  being  destroyed  by  the 
imbeddinfic  agent. When  imbedded  in  paraffine,  very  delicate  structures 
are  more  liable  to  damage;  the  villi  of  the  intestine,  for  instance,  being 
often  denuded  of  their  epithelium,  and  sometimes  themselves  torn. 

192.  Grinding  and  Polishing  Sections  of  Hard  Substances, — Sub- 
stances which  are  too  hard  to  be  sliced  in  a  Microtome — such  as  Bones, 
Teeth,  Shells,  Corals,  Fossils  of  all  kinds,  and  even  some  dense  Vegetable 
Tissues — can  only  be  reduced  to  the  requisite  thinness  for  Microscopical 
examination,  by  grinding-down  thick  sections  until  they  become  so  thin 
as  to  be  transparent.    General  directions  for  making  such  preparations 


^  This  instrument  has  received  various  improvements  since  it  was  first  devised, 
and  should  be  obtained  from  Mr.  Gardner,  South  Bridge,  Edinburgh, — the  maker 
recommended  by  its  inventor.  It  may  be  employed  also  as  an  ordinary  *  imbed- 
ding' Microtome,  when  the  'imbedding'  is  thought  preferable  to  the  freezing 
process. 


PREPARATION,  MOUNTING,  AND  COLLECTION  OF  OBJECTS.  197 

will  be  here  given;^  but  those  special  details  of  management  which  par- 
ticular substances  may  require,  will  be  given  when  these  are  respectively 
described. — The  first  thing  to  be  done  will  usually  be  to  procure  a  section 
of  the  substance,  as  thin  as  it  can  be  safely  cut.  Most  substances  not  sili- 
ceous may  be  divided  by  the  fine  Saws  used  by  artisans  for  cutting  brass; 
and  these  may  be  best  worked  either  by  a  mechanical  arrangement  such 
as  that  devised  by  Dr.  Matthews/  or,  if  by  hand,  between  '  guides,'  such 
as  are  attached  for  this  purpose  to  Hailes's  and  some  other  Microtomes. 
But  there  are  some  bodies  (such  as  the  Enamel  of  Teeth,  and  Porcellane- 
ous Shells),  which,  though  merely  calcareous,  are  so  hard  as  to  make  it 
very  difiicult  and  tedious  to  divide  them  in  this  mode;  and  it  is  much  the 
quicker  operation  to  slit  them  with  a  disc  of  soft  iron  (resembling  that 
used  by  the  Lapidary)  charged  at  its  edge  with  diamond-dust,  which  disc 
may  be  driven  in  an  ordinary  lathe.  Where  waste  of  material  is  of  no  ac- 
count, a  very  expeditious  method  of  obtaining  pieces  fit  to  grind  down, 
is  to  detach  them  from  the  mass  with  a  strong  pair  of  ^  cutting  pincers, 
or,  if  they  be  of  small  dimensions,  with  ^  cutting  pliers;'  and  a  flat  sur- 
face must  then  be  given  to  it,  either  by  holding  them  to  the  side  of  an 
ordinary  grindstone,  or  by  rubbing  on  a  plate  of  lead  (cast  or  planed  to  a 
perfect  level)  charged  with  emery,  or  by  a  strong-toothed  file;  the  former 
being  the  most  suitable  for  the  hardest  substances,  the  latter  for  the 
toughest.  There  are  certain  substances,  especially  Calcareous  Fossils  of 
Wood,  Bone,  and  Teeth,  in  which  the  greatest  care  is  required  in  the 
performance  of  these  preliminary  operations,  on  account  of  their  extreme 
friability;  the  vibration  produced  by  the  working  of  the  saw  or  the  file, 
or  by  grinding  on  a  rough  surface,  being  sufiicient  to  disintegrate  even  a 
thick  mass,  so  that  it  falls  to  pieces  under  the  hand;  such  specimens, 
therefore,  it  is  requisite  to  treat  with  great  caution,  dividing  them  by  the 
smooth  action  of  the  wheel,  and  then  rubbing  them  down  upon  nothing 
rougher  than  a  very  fine  ^grit,'  or  on  the  ^corundum-files '  now  sold  in 
the  tool-shops,  which  are  made  by  imbedding  corundum  of  various 
degrees  of  fineness  in  a  hard  resinous  substance.  Where  (as  often  hap- 
pens) such  specimens  are  suflBciently  porous  to  admit  of  the  penetration 
of  Canada  Balsam,  it  will  be  desirable,  after  soaking  them  in  turpentine 
for  a  while,  to  lay  some  liquid  balsam  upon  the  parts  through  which  the 
section  is  to  pass,  and  then  to  place  the  specimen  before  a  fire  or  in  an 
oven  for  some  little  time,  so  as  first  to  cause  the  balsam  to  run-in,  and 
then  to  harden  it;  by  this  means  the  specimen  will  be  rendered  much 
more  fit  for  the  processes  it  has  afterwards  to  undergo. — It  not  unfre- 
quently  happens  that  the  small  size,  awkard  shape,  or  extreme  hardness 
of  the  body,  occasions  a  difficulty  in  holding  it  either  for  cutting  or  grind- 
ing; in  such  a  case,  it  is  much  better  to  attach  it  to  the  glass  in  the  first 
instance  by  any  si'de  that  happens  to  be  flattest,  and  then  to  rub  it  down 
by  means  of  the  'hold'  of  the  glass  upon  it,  until  the  projecting  portion 
has  been  brought  to  a  plane,  and  has  been  prepared  for  permanent  attach- 
ment to  the  glass.  This  is  the  method  which  it  is  generally  most  conve- 
nient to  pursue  with  regard  to  small  bodies;  and  there  are  many  which 
can  scarcely  be  treated  in  any  other  way  than  by  attaching  a  number  of 


^  The  following  directions  do  not  apply  to  Siliceous  substances;  as  sections  of 
these  can  only  be  prepared  by  those  who  possess  a  regular  Lapidary's  apparatus, 
and  have  been  specially  instructed  in  the  use  of  it. 

2    Journ.  Quekett  Microsc.  Club,"  Vol.  vi.  (1880),  p.  83. 


198 


THE  MICROSCOPE  AND  ITS  REVELATIONS. 


them  to  the  glass  at  once,  in  such  a  manner  as  to  make  them  mutually 
support  one  another/ 

193.  The  mode  in  which  the  operation  is  then  to  be  proceeded  with, 
depends  upon  whether  the  section  is  to  be  ultimately  set  up  in  Canada 
balsam  (§  210),  or  is  to  be  mounted  '  dry'  (§  169),  or  in  fluid  (§  211). 
In  the  former  case,  the  following  is  the  plan  to  be  pursued: — The  flat- 
tened surface  is  to  be  polished  by  rubbing  it  with  water  on  a  ^  Water-of- 
Ayr '  stone,  or  on  a  hone  or  '  Turkey  '-stone,  or  on  an  '  Arkansas  '-stone; 
the  first  of  the  three  is  the  best  for  all  ordinary  purposes,  but  the  two  lat- 
ter, being  much  harder,  may  be  employed  for  substances  which  resist  it.^ 
When  this  has  been  sufficiently  accomplished,  the  section  is  to  be  attached 
with  hard  Canada  balsam  to  a  slip  of  thick  well-annealed  glass;  and  as 
the  success  of  the  final  result  will  often  depend  upon  the  completeness  of 
its  adhesion  to  this,  the  means  of  most  effectually  securing  that  adhesion 
will  now  be  described  in  detail.  The  slide  having  been  placed  on  the 
cover  of  the  Water-bath,  and  the  previously-hardened  balsam  having 
been  softened  by  the  immersion  of  the  jar  containing  it  in  the  bath 
itself,  a  sufficient  quantity  of  this  should  be  laid  on  the  slide  to  form, 
when  spread  out  by  liquefaction,  a  thick  drop  somewhat  larger  than  the 
surface  of  the  object  to  be  attached.  The  slide  should  then  be  allowed 
to  cool,  in  order  that  the  hardness  of  the  balsam  should  be  tested.  If  too 
soft,  as  indicated  by  its  ready  yielding  to  the  thumb-nail,  it  should  be 
heated  a  little  more,  care  being  taken  not  to  make  it  boil  so  as  to  form 
bubbles;  if  too  hard,  which  will  be  shown  by  its  chipping,  it  should  be 
re-melted  and  diluted  with  more  fluid  balsam,  and  then  set  aside  to  cool 
as  before.  When  it  is  found  to  be  of  the  right  consistence,  the  section 
should  be  laid  upon  its  surface  with  the  polished  side  downwards;  the 
slip  of  glass  is  next  to  be  gradually  warmed  until  the  balsam  is  softened, 
special  care  being  taken  to  avoid  the  formation  of  bubbles;  and  the  sec- 
tion is  then  to  be  gently  pressed  down  upon  the  liquefied  balsam,  the 
pressure  being  at  first  applied  rather  on  one  side  than  over  its  whole  area, 
so  as  to  drive  the  superfluous  balsam  in  a  sort  of  wave  towards  the  other 
side,  and  an  equable  pressure  being  finally  made  over  the  whole.  If  this 
be  carefully  done,  even  a  very  large  section  may  be  attached  to  glass  with- 


1  Thus,  in  making  horizontal  and  vertical  sections  of  Foraminifera^  as  it  would 
be  impossible  to  slice  them  through,  they  must  be  laid  close  together  in  a  bed  of 
hardened  Canada  Balsam  on  a  slip  of  glass,  in  such  positions,  that  when  rubbed 
down,  the  plane  of  section  shall  traverse  them  in  the  desired  directions;  and  one 
flat  surface  having  been  thus  obtained  for  each,  this  must  be  turned  downwards, 
and  the  other  side  ground  away.  The  following  ingenious  plan  was  suggested  by 
Dr.  Wallich  (**Ann.  of  Nat.  Hist.,  July,  1861,  p.  58),  for  turning  a  number  of 
minute  objects  together,  and  thus  avoiding  the  tediousness  and  difficulty  of  turn- 
ing each  one  separately; — The  specimens  are  cemented  with  Canada  Balsam,  in 
the  first  instance,  to  a  thin  film  of  mica,  which  is  then  attached  to  a  glass  slide  by 
the  same  means;  when  they  have  been  ground-down  as  far  as  may  be  desired,  the 
slide  is  gradually  heated  just  sufficiently  to  allow  of  the  detachment  of  the  mica- 
film  and  the  specimens  it  carries;  and  a  clean  slide  with  a  thin  layer  of  hardened 
balsam  having  been  prepared,  the  mica-film  is  transferred  to  it  with  the  ground 
surface  downwards.  When  its  adhesion  is  complete,  the  grinding  may  be  pro- 
ceeded with;  and  as  the  mica-film  will  yield  to  the  stone  without  the  least  diffi- 
culty, the  specimens,  now  reversed  in  position,  may  be  reduced  to  requisite 
thinness. 

2  As  the  flatness  of  the  polished  surface  is  a  matter  of  the  first  importance,  that 
of  the  Stones  themselves  should  be  tested  from  time  to  time;  and  whenever  they 
are  found  to  have  been  rubbed  down  on  any  part  more  than  on  another,  they 
should  be  flattened  on  a  paving-stone  with  fine  sand,  or  on  the  lead-plate  with 
emery. 


PREPARATION,  MOUNTING,  AND  COLLECTION  OF  OBJECTS.  199 

out  the  intervention  of  any  air-bubbles;  if,  however,  they  should  present 
themselves,  and  they  cannot  be  expelled  by  increasing  the  pressure  over 
the  part  beneath  which  they  are,  or  by  slightly  shifting  the  section  from 
side  to  side,  it  is  better  to  take  the  section  entirely  olf,  to  melt  a  little 
fresh  balsam  upon  the  glass,  and  then  to  lay  the  section  upon  it  as 
before. 

194.  When  the  section  has  been  thus  secured  to  the  glass,  and  the 
attached  part  thoroughly  saturated  (if  it  be  porous)  with  hard  Canada 
balsam,  it  may  be  readily  reduced  in  thickness,  either  by  grinding  or 
filing,  as  before,  or,  if  the  thickness  be  excessive,  by  taking  oif  the  chief 
part  of  it  at  once  by  the  slitting  wheel.  So  soon,  however,  as  it 
approaches  the  thinness  of  a  piece  of  ordinary  card,  it  should  be  rubbed 
down  with  water  on  one  of  the  smooth  stones  previously  named,  the 
glass  slip  being  held  beneath  the  fingers  with  its  face  downwards,  and  the 
pressure  being  applied  with  such  equality  that  the  thickness  of  the  sec- 
tion shall  be  (as  nearly  as  can  be  discerned)  equal  over  its  entire  surface. 
As  soon  as  it  begins  to  be  translucent,  it  should  be  placed  under  the 
Microscope  (particular  regard  being  liad  to  the  precaution  specified  in 
§  143),  and  note  taken  of  any  inequality;  and  then,  when  it  is  again  laid 
upon  the  stone,  such  inequality  may  be  brought  down  by  making  special 
pressure  with  the  forefinger  upon  the  part  of  the  slide  above  it.  When  the 
thinness  of  the  section  is  such  as  to  cause  the  water  to  spread  around 
it  between  the  glass  and  the  stone,  an  excess  of  thickness  on  either  side 
may  often  be  detected  by  noticing  the  smaller  distance  to  which  the 
liquid  extends.  In  proportion  as  the  substance  attached  to  the  glass  is 
ground  away,  the  superfluous  balsam  which  may  have  exuded  around  it 
will  be  brought  into  contact  with  the  stone;  and  this  should  be  removed 
with  a  knife,  care  being  taken,  however,  that  a  margin  be  still  left  round 
the  edge  of  the  section.  As  the  section  approaches  the  degree  of  thin- 
ness which  is  most  suitable  for  the  display  of  its  organization,  great  care 
must  be  taken  that  the  grinding  process  be  not  carried  too  far;  and  fre- 
quent recourse  should  be  had  to  the  Microscope,  which  it  is  convenient  to 
have  always  at  hand  when  work  of  this  kind  is  being  carried  on.  There 
are  many  substances  whose  intimate  structure  can  only  be  displayed  in 
its  highest  perfection,  when  a  very  little  more  reduction  would  destroy  the 
section  altogether;  and  every  Microscopist  who  has  occupied  himself  in 
making  such  preparations,  can  tell  of  the  number  which  he  has  sacrificed 
in  order  to  attain  this  perfection.  Hence,  if  the  amount  of  material  be 
limited,  it  is  advisable  to  stop  short  as  soon  as  a  good  section  has  been 
made,  and  to  lay  it  aside — letting  well  alone' — whilst  the  attempt  is 
being  made  to  procure  a  better  one;  if  this  should  fail,  another  attempt 
may  be  made,  and  so  on,  until  either  success  has  been  attained,  or  the 
whole  of  the  material  has  been  consumed — the  first  section,  however,  still 
remaining:  whereas,  if  the  first,  like  every  subsequent  section,  be  sacri- 
ficed in  the  attempt  to  obtain  perfection,  no  trace  will  be  left  ^^to  show 
what  once  has  been."  In  judging  of  the  appearance  of  a  section  in  this 
stage  under  the  Microscope,  it  is  to  be  remembered  that  its  transparence 
will  subsequently  be  considerably  increased  by  mounting  in  Canada 
balsam:  this  is  particularly  the  case  with  Fossils  to  which  a  deep  hue  has 
been  given  by  the  infiltration  of  some  coloring  matter,  and  with  any  sub- 
stances whose  particles  have  a  molecular  aggregation  that  is  rather 
amorphous  than  crystalline.  When  a  sufficient  thinness  has  been  at- 
tained, the  section  may  generally  be  mounted  in  Canada  balsam;  and  the 
mode  in  which  this  must  be  managed  will  be  detailed  hereafter  (§  210). 


200 


THE  J^lICROSCOl-E  ANV  ITS  REVELATIONS. 


195.  By  a  slight  variation  in  the  foregoing  process,  sections  may  be 
made  of  structures,  in  which  (as  in  Corals)  hard  and  soft  parts  are  com- 
bined, so  as  to  show  both  to  advantage.  Small  pieces  of  the  substance 
are  first  to  be  stained  thoroughly  (§  202),  and  are  then  to  be  '  dehydrated ' 
by  alcohol  (§  190).  A  thin  solution  of  copal  in  chloroform  is  to  be  pre- 
l^ared,  in  which  the  pieces  are  to  be  immersed;  and  this  solution  is  to  be 
concentrated  by  slow  evaporation,  until  it  can  be  drawn  out  in  threads 
which  become  brittle  on  cooling.  The  pieces  are  then  to  be  taken  out, 
and  laid  aside  to  harden;  and  when  the  copal  has  become  so  firm  that 
the  edge  of  the  finger-nail  makes  no  impression,  they  are  to  be  cut  into 
slices,  and  ground  down  attached  to  glass,  in  the  manner  already 
described,  the  sections  being  finally  mounted  in  Canada  balsam. — The 
sections  (attached  to  glass)  may  be  partially  or  completely  decalcified,  the 
soft  parts  remaining  in  situ,  by  first  dissolving  out  the  copal  with  chloro- 
form; when,  after  being  well  washed  in  water,  they  should  be  again 
stained,  and  mounted  either  in  weak  spirit,  or  (after  having  been  dehy- 
drated) in  Canada  balsam.* 

196.  A  different  mode  of  procedure,  however,  must  be  adopted  when 
it  is  desired  to  obtain  sections  of  Bone,  Tooth  or  other  finely  tubular 
structures,  i^/^penetrated  by  Canada  balsam.  If  tolerably  thin  sections 
of  them  can  be  cut  in  the  first  instance,  or  if  they  are  of  a  size  and  shape 
to  be  held  in  the  hand  whilst  they  are  being  roughly  ground  down,  there 
will  be  no  occasion  to  attach  them  to  glass  at  all:  it  is  frequently  conveni- 
ent to  do  this  at  first,  however,  for  the  purpose  of  obtaining  a  '  hold  ^  upon 
the  specimen;  but  the  surface  which  has  been  thus  attached  must  after- 
wards be  completely  rubbed  away,  in  order  to  bring  into  view  a  stratum 
which  the  Canada  balsam  shall  not  have  penetrated.  As  none  but  sub- 
stances possessing  considerable  toughness,  such  as  Bones  and  Teeth,  can 
be  treated  in  this  manner,  and  as  these  are  the  substances  which  are  most 
quickly  reduced  by  a  coarse  file,  and  are  least  liable  to  be  injured  by  its 
action,  it  will  be  generally  found  possible  to  reduce  the  sections  nearly  to 
the  required  thinness,  by  laying  them  upon  a  piece  of  soft  cork  or  wood 
held  in  a  vice,  and  operating  upon  them  first  with  a  coarser  and  then  with  a 
finer  file.  When  this  cannot  safely  be  carried  farther,  the  section  must  be 
rubbed  down  upon  that  one  of  the  fine  stones  already  mentioned  (§  193) 
which  is  found  best  to  suit  it:  as  long  as  the  section  is  tolerably  thick, 
the  finger  may  be  used  to  press  and  move  it;  but  as  soon  as  the  finger 
itself  begins  to  come  into  contact  with  the  stone,  it  must  be  guarded 
by  a  flat  slice  of  cork,  or  by  a  piece  of  gutta-percha,  a  little  larger  than 
the  object.  Under  either  of  these,  the  section  may  be  rubbed  down  to 
the  desired  thinness;  but  even  the  most  careful  working  on  the  finest- 
grained  stone  will  leave  its  surface  covered  with  scratches,  which  not 
only  detract  from  its  appearance,  but  prevent  the  details  of  its  internal 
structure  from  being  as  readily  made  out  as  they  can  be  in  a  polished 
section.  This  polish  may  be  imparted  by  rubbing  the  section  with 
putty-powder  (peroxide  of  tin)  and  water  upon  a  leather  strap,  made  by 
covering  the  surface  of  a  board  with  buff-leather,  having  three  or  four 
thicknesses  of  cloth,  flannel,  or  soft  leather  beneath  it:  this  operation 
must  be  performed  on  both  sidefs  of  the  section,  until  all  the  marks  of 
the  scratches  left  by  the  stone  shall  have  been  rubbed  out;  when  the 


^  See  Koch  in  Zoologischer  Anzeig.,"  Bd.  i.,  p.  36.— The  Author,  having  seen 
(by  the  kindness  of  Mr.  H.  N.  Mosely)  some  sections  of  Corals  prepared  by  this 
process,  can  testify  to  its  complete  success. 


PREPARATION,  MOUNTING,  AND  COLLECTION  OF  OBJECTS.  201 

specimen  will  be  fit  for  mounting  ^dry'  after  having  been  carefully 
cleansed  from  any  adhering  particles  of  putty-powder. 

197.  Decalcification. — When  it  is  desired  to  examine  the  structure  of 
the  Organic  matrix,  in  which  the  Calcareous  salts  are  deposited  that  give 
hardness  to  many  Animal  and  to  a  few  Vegetable  structures  (such  as  the 
true  Corallines),  these  salts  must  be  dissolved  away  by  the  action  of  some 
Mineral  Acid,  which  may  be  either  Nitric  or  Hydrochloric.  This  should 
be  employed  in  a  very  dilute  state,  in  order  that  it  may  make  as  little 
change  as  possible  in  the  soft  tissue  it  leaves  behind.  When  the  Lime  is 
in  the  state  of  Carbonate  (as,  for  example,  in  the  skeletons  of  Echino- 
^?er??ZcS  Chap,  xiy.),  the  body  to  be  decalcified  should  be  placed  in  a 
glass  jar  or  wide-mouthed  bottle  holding  from  4  to  6  oz.  of  water,  and 
the  acid  should  be  added  drop  by  drop,  until  the  disengagement  of  air- 
bubbles  shows  that  it  is  taking  effect;  and  the  solvent  process  should  be 
allowed  to  take  place  very  gradually,  more  acid  being  added  as  required. 
When,  on  the  other  hand,  much  of  the  lime  is  in  the  state  of  Phosphate, 
as  in  Bones  and  Teeth,  the  strength  of  the  acid  solvent  must  be  increased; 
and  for  the  hardening  of  the  softer  parts  of  the  organic  matrix,  it  is 
desirable  that  Chromic  acid  should  be  used.  In  the  case  of  small  bones, 
or  delicate  portions  of  large  (such  as  the  cochlea  of  the  ear),  a  half  per 
cent  solution  of  chromic  acid  will  itself  serve  as  the  solvent;  but  larger 
masses  require  either  Nitric  or  Hydrochloric  acid  in  addition,  to  the 
extent  of  2  per  cent  of  the  former  or  5  per  cent  of  the  latter.  By  some 
the  chromic  and  the  nitric  or  muriatic  acid  are  mixed  in  the  first 
instance;  while  by  others  it  is  recommended  that  the  bone  should  lie  first 
in  the  chromic  acid  solution  for  a  week  or  ten  days,  and  that  the  second 
acid  should  be  then  added.  If  the  softening  is  not  completed  in  a 
month,  more  acid  must  be  added.  When  thoroughly  decalcified,  the 
bone  should  be  transferred  to  rectified  spirit;  and  it  may  then  be  either 
sliced  in  the  Microtome,  or  torn  into  shreds  for  the  demonstration  of  its 
lamellae. — Acid  solvents  may  also  be  employed  in  removing  the  outer 
parts  of  Calcareous  skeletons,  for  the  display  of  their  internal  cavities  (a 
plan  which  the  Author  has  often  found  very  useful  in  the  study  of  Fora- 
minifera);  or  for  getting  rid  of  them  entirely,  so  as  to  bring  into  com- 
plete view  any  ^internal  cast^  which  may  have  been  formed  by  the 
silicification  of  its  originally  soft  contents  (Figs.  332,  337).^  It  has  been 
in  this  mode,  even  more  than  by  the  cutting  of  thin  sections,  that  the 
structure  of  Eozo'on  Canadense  (Plate  xvii. )  has  been  elucidated  by  Pro- 
fessor Dawson  and  the  Author.  For  the  first  of  these  purposes,  strong 
acid  should  be  applied  (under  the  Dissecting  Microscope)  with  a  fine 
camel's  hair  pencil;  and  another  such  pencil  charged  with  water  should 
be  at  hand,  to  enable  the  observer  to  stop  the  solvent  action  wheneve;*  he 
thinks  it  has  been  carried  far  enough.  For  the  second,  it  is  better  that 
the  acid  should  only  be  strong  enough  for  the  slow  solution  of  the  shelly 
substance;  as  the  too  rapid  disengagement  of  bubbles  often  produces 
displacement  of  delicate  parts  of  the  substituted  mineral,  whilst,  if 
the  acid  be  too  strong,  the  '  internal  cast '  may  be  altogether  dissolved 
away. 

198.  Preparation  of  Vegetable  Substances.— Little  preparation  is 
required,  beyond  steeping  for  a  short  time  in  distilled  water  to  get  rid  of 
saline  or  other  impurities,  for  mounting  in  preservative  media  specimens 
of  the  minuter  forms  of  Vegetable  life,  or  portions  of  the  larger  kinds  of 
AlgcB,  Fungi,  or  other  succulent  Cryptogams.  But  the  Woody  struc- 
tures of  Phanerogams  are  often  so  consolidated  by  gummy,  resinous,  cr 


202 


THE  MICROSCOPE  AND  ITS  REVELATIONS. 


other  deposits,  that  sections  of  them  should  not  be  cut  until  they  have 
been  softened  by  being  partially  or  wholly  freed  from  these.  Accordingly, 
pieces  of  stems  or  roots  should  be  soaked  for  some  days  in  water,  with 
the  aid  of  a  gentle  heat  if  they  are  very  dense,  and  should  then  be  steeped 
for  some  days  in  methylated  spirit,  after  which  they  should  again  be 
transferred  to  water.  The  same  treatment  may  be  applied  to  hard-coated 
seeds,  the  ^stones'  of  fruit,  ^vegetable  ivory,'  and  other  like  substances. 
— Some  Vegetable  substances,  on  the  other  hand,  are  too  soft  to  be  cut 
sufficiently  thin  without  previous  hardeniiig,  either  by  allowing  them  to 
lose  some  of  their  moisture  by  evaporation,  or  by  drawing  it  out  by  steep- 
ing them  in  spirit.  Either  treatment  answers  very  well  with  such  sub- 
stances as  that  which  forms  the  tuber  of  the  Potato;  sections  of  which 
display  the  starch-grains  in  situ.  Where,  on  the  other  hand,  it  is  desired 
to  preserve  color,  spirit  must  not  be  used;  and  recourse  may  be- had  to 
Gum-imbedding  (§  191),  which  is  particularly  serviceable  where  the 
substance  is  penetrated  by  air-cavities,  as  is  the  case  with  the  Stem  of 
the  Rush,  the  thick  leaves  of  the  Water-lily ,  etc.  But  where  the  stain- 
ing process  is  to  be  employed  (§  200),  the  substance  should  be  previously 
bleached  by  the  action  of  chlorine  (preferably  by  Labarraque's  chlorinated 
soda),  and  then  treated  with  Alcohol  for  a  few  hours. 

199.  Hardening  of  Animal  Substances. — Save  in  the  case  already 
treated  of  (§  192),  in  which  the  tissues  are  consolidated  by  Calcareous 
deposit,  the  preparatory  treatment  of  Animal  substances  consists  in 
hardening  them.  The  very  soft  tissues  of  which  most  of  the  lower 
Animals  are  composed,  contain  so  large  a  proportion  of  Water,  that  the 
withdrawal  of  this  by  immersion  in  strong  spirit  causes  them  to  shrink  so 
much  as  completely  to  obscure  their  structure.  Nothing  has  yet  been 
found  so  serviceable  in  preserving  them  as  Osmic  acid ;  the  poisonous 
action  of  which  at  once  kills  living  Infusoria,  etc.,  Echinoderm  or 
Annelid  larvae,  and  the  like;  and  hardens  their  delicate  organisms,  so  as 
to  allow  them  to  be  afterwards  stained  and  preserved  with  very  little 
change;  and  thus  many  points  of  their  structure  can  be  better  made  out 
in  their  ^mounted'  than  in  their  living  state.  The  special  procedures 
which  have  been  successfully  worked  out  by  M.  Oertes  for  Infusoria,  and 
by  Mr.  Percy  Sladen  for  Echinoderm  larvce,  will  be  described  under  those 
heads. — The  hardening  of  the  general  body-substance  of  the  larger 
Invertehrata  is  for  the  most  part  sufficiently  effected  by  the  action  of  the 
Alcoholic  spirit  in  which  they  are  usually  preserved;  and  this  may  be 
carried  farther,  if  required,  by  steeping  them  for  a  time  in  absolute 
Alcohol.  For  hardening  particular  tissues,  however,  such  as  Nerves, 
recourse  must  be  had  to  some  of  those  hardening  agents,  used  in  the 
preparation  of  the  Tissues  of  the  higher  Animals,  which  will  be  now 
specified: — 

a.  Alcohol. — For  hardening  purposes,  Rectified  spirit  should  be  used  in  pref- 
erence to  methylated;  and  its  action  is  (as  a  rule)  most  beneficial  after  some  of 
the  other  hardening  agents  have  been  employed.  The  substance  to  be  hardened 
should  be  first  placed  for  a  day  or  two  in  a  mixture  of  equal  parts  of  rectified 
spirit  and  water,  then  transferred  for  about  48  hours  to  rectified  spirit,  and 
from  this  to  absolute  alcohol. — One  injurious  effect  of  this  treatment  is,  that  by 
the  coagulation  of  their  albuminous  components  many  textures  are  rendered 
opaque:  but,  as  Dr.  Beale  pointed  out,  this  may  be  corrected  by  the  addition 
of  a  little  caustic  Soda,  which  must  be  made,  however,  with  great  caution. — 
When  the  Alcoholic  treatment  is  used  merely  for  so  dehydrating  sections  pre- 
viously immersed  in  watery  solutions,  that  they  may  be  mounted  in  Canada 
balsam  or  Dammar,  they  may  be  transferred  at  once  from  rectified  spirit  to  oil 
of  turpentine,  without  treating  them  with  absolute  alcohol. 


PREPARATION,  MOUNTING,  AND  COLLECTION  OF  OBJECTS. 


203 


&.  Chromic  Acid,  which  is  one  of  the  most  generally  useful  of  hardening 
agents,  is  most  conveniently  kept  in  a  1  per  cent  solution,  which  may  be  diluted 
with  several  times  its  volume  of  water,  with  or  without  the  addition  of  spirit. 
Although  its  hardening  action  may  be  effected  by  a  strong  solution  in  two  or 
three  days,  it  is  far  better  to  prolong  the  process  by  using  the  menstruum  weak, 
especially  when  the  substance  is  in  mass;  since,  if  its  exterior  be  so  hardened  as 
to  prevent  the  penetration  of  the  fluid,  its  interior  will  soften  and  decay.  The 
following  is  the  mode  of  procedure  most  generally  approved: — The  menstruum 
having  been  prepared  by  mixing  two  parts  of  a  l-6th  per  cent  solution  of 
chromic  acid  and  one  part  of  methylated  spirit,  the  material  must  be  cut  into 
small  pieces  about  half  an  inch  square,  and  put  into  a  wide-mouthed  stoppered 
bottle  holding  from  6  to  10  ozs.  of  the  fluid;  this  fluid  should  be  changed  at  the 
end  of  24  hours,  and  then  every  third  day;  and  the  material  will  be  probably 
found  sufficiently  hardened  (which  must  be  ascertained  by  trying  whether  a  tol- 
erably thin  hand-section  can  be  made  with  a  razor)  in  the  course  of  from  8  to  13 
days.  If  not,  the  process  must  be  continued,  care  being  taken  that  it  be  not  so 
prolonged  as  to  render  the  substance  brittle.  The  hardening  may  afterwards  be 
completed  by  transferring  the  substance  first  into  dilute  and  then  into  stronger 
spirit;  and  this  will  get  rid  of  the  color  given  by  the  chromic  acid,  as  well  as  of 
other  flocculent  matter.  The  spirit  must  be  changed  as  often  as  it  becomes  foul 
and  discolored;  and  when  it  remains  bright  and  clear,  the  specimens  will  be  ready 
for  cutting. 

c.  Bichromate  of  Potass,  in  a  3  per  cent  watery  solution,  may  be  used  where 
very  slow  and  prolonged  hardening  is  required.  With  the  addition  of  1  per  cent 
of  sulphate  of  soda,  it  constitutes  Muller's  Fluid,  which  may  be  conveniently 
used  to  harden  large  pieces  that  may  be  left  in  it  for  several  weeks;  no  change 
of  the  fluid  being  necessary  after  the  first  week. — The  hardened  substance,  after 
being  well  washed,  is  to  be  treated  with  spirit,  as  in  the  preceding  case. 

d.  Picric  or  Carbazotic  Acid  is  used  for  the  same  purposes  as  Chromic  acid; 
its  hardening  power  is  not  so  great,  but  it  does  not  shrivel  the  tissues  as  much, 
its  action  is  more  rapid,  and  it  may  be  advantageously  used  where  *  decalcifica- 
tion '  is  necessary  (§  197).  As  it  is  but  slightly  soluble  in  water,  a  cold-water  so- 
lution must  be  saturated;  and  the  quantity  of  liquid  should  be  large  in  propor- 
tion to  that  of  the  substance  to  be  acted-on. — Picric  acid  is  used,  in  combination 
with  Carmine  or  Aniline-blue,  as  a  staining  material  (§  203,  b). 

e.  Kleinenberg's  Fluid. — The  following  method  of  preparing  delicate  and  per- 
ishable tissues  is  strongly  recommended  by  Kleinenberg,  who  has  had  much 
experience  of  it  in  his  investigations  on  the  anatomy  of  the  lower  Invertebrata: 
— To  a  saturated  solution  of  picric  acid  in  distilled  water,  add  2  per  cent  of  con- 
centrated sulphuric  acid;  all  the  picric  acid  which  is  precipitated  must  be  re- 
moved by  filtration.  One  part  of  the  filtrate  is  to  be  diluted  with  3  parts  of  water; 
and,  finally,  as  much  pure  kreosote  must  be  added  as  will  mix.  The  object  to 
be  preserved  must  remain  in  this  liquid  for  3,  4,  or  more  hours;  and  is  then  to 
be  transferred  for  5  or  6  hours  into  70  per  cent  alcohol,  and  thence  removed  into 
90  per  cent  alcohol,  which  should  be  changed  until  it  ceases  to  acquire  a  yellow 
tint. 

/.  Osmic  Acid  — This  agent  is  one  of  peculiar  value  to  the  Microscopist  whose 
studies  lie  among  the  lower  forms  of  Animal  and  Vegetable  life;  as  its  applica- 
tion immediately  kills  them,  without  producing  any  retraction  or  shrinking  of 
their  parts,  and  only  not  preserves  their  tissues,  but  brings  out  differences  in  those 
which  might  otherwise  escape  observation.  It  is  sold  in  the  solid  state  in  sealed* 
tubes;  and  is  most  conveniently  kept  as  a  1  per  cent  solution  in  distilled  water. 
The  solution  should  be  preserved  in  well-stoppered  bottles  secluded  from  the 
light;  and  should  be  used  with  great  caution,  as  it  gives  forth  a  pungent  vapor 
which  is  very  irritating  to  the  eyes  and  nostrils.  It  is  recommended  by 
Dr.  Pelletan,!  M.  Certes,^  and  M,  Raphael  Blanchard,^  for  fixing  and  preserving 
Animalcules  {Infusoria  and  Rotifera),  Desmidiece,  Diatomacece,  Bacteria,  and  Vi- 
briones,  etc.;  by  Dr.  Vignal^  for  Noctiluca ;  by  Mr.  T.  Jeffrey  Parker^  for  Ento- 
mostraca  and  other  small  Crustacea ;  and  it  has  been  successfully  used  also  in 
the  preparation  of  Insect  structures.    To  the  Histologist  its  special  value  lies  m 


1  Journ.  of  Rov.  Microsc.  Soc,"  Vol.  i.  (1878),  p.  189. 

2  Ibid.,  Vol.  ii.  (1879),  p.  331,  and  *Comptes  Rendus,'  1879,  p.  433. 

3  Ibid  ,  Vol.  ii.  (1879),  p.  463. 

4  Robin's    Archives  de  Physiologic,"  Tom.  xiv.  (1878),  p.  586. 
6    Journ.  of  Roy.  Microsc.  Soc,"  Vol.  ii.  (1879),  p.  381. 


204: 


THE  MICROSCOPE  AND  ITS  REVELATIONS. 


its  blackening  of  fatty  matters  and  the  medullary  substance  of  nerve-fibres. 
And  the  Embryologist  finds  it  of  peculiar  value  in  giving  firmness  and  distinct- 
ness to  the  delicate  textures  with  which  he  has  to  deal.  Various  degrees  of  dilu- 
tion of  the  1  per  cent  solution  will  be  needed  for  these  different  purposes. 
Mr.  Parker  further  states  (Zoc.  cit.)  that  he  has  found  this  agent  very  service- 
able in  the  preparation  of  delicate  Vegetable  structures.  The  acid  seems  to  be 
taken  up  by  each  granule  of  the  protoplasm,  and  these  to  be  decomposed,  giv- 
ing to  the  granule  the  characteristic  gray  color,  thus  at  the  same  time  both 
hardening  and  staining." — A  mixture  of  9  parts  of  a  l-4th  per  cent,  solution  of 
Chromic  acid,  with  one  part  of  a  1  per  cent  solution  of  Osmic  acid,  answers  for 
many  purposes  better  than  osmic  acid  alone,  the  brittleness  produced  by  its  use 
being  completely  avoided. — After  being  subjected  to  this  agent,  the  specimens 
should  be  treated  with  30  per  cent  alcohol,  gradually  increased  in  strength  to 
absolute. 

200.  Staining  Processes, — Much  attention  has  been  given  of  late 
years  to  the  use  of  agents,  which,  either  by  simply  dyeing,  or  by  chemi- 
cally acting  on  Organic  substances,  in  different  modes  and  degrees,  serve 
to  differentiate  the  different  parts  of  organs  or  tissues  of  complex  struc- 
ture, and  to  render  more  distinct  such  delicate  features  in  preparations 
mounted  in  transparent  media,  as  might  otherwise  escaps  notice.  The 
agents  which  merely  dye  the  tissues  are  for  the  most  part  Coloring  mat- 
ters of  Vegetable  or  Animal  origin;  those  which  act  upon  them  chemically 
are  Mineral  substances.  The  dyes  need  generally  to  be  ^  fixed  ^  by  some 
^mordant;'  but  the  effects  of  chemical  agents  are  usually  permanent. 
The  staining-processes  may  be  used  either  before  or  after  section -cutting, 
according  to  circumstances.  Where  the  substance  is  in  mass,  and  is  not 
readily  penetrable  by  the  staining  fluid  (which  is  especially  liable  to  be 
the  case  where  it  has  been  hardened  in  chromic  acid),  it  is  generally  bet- 
ter to  stain  the  sections  after  cutting,  if  they  hold  sufficiently  well 
together  to  bear  being  transferred  from  one  fluid  to  another.  And  if  the 
substance  is  to  be  imbedded  in  gum,  and  cut  with  the  freezing  Micro- 
tome, it  is  generally  preferable  to  stain  the  sections  after  they  have  been 
cut;  as  the  processes  necessary  for  the  removal  of  the  gum  would  be  likely 
also  to  remove  the  dye.  But  where  the  substance  to  be  cut  has  to  be 
penetrated  by  wax  or  paraffine,  it  is  better  that  the  staining  should  be 
effected  in  the  first  instance.  As  a  general  rule,  it  is  better  that  where 
the  substance  is  to  be  stained  en  masse,  the  staining  fluid  should  be  weak 
and  its  action  slow;  because  in  that  mode  the  stain  is  more  equably  dif- 
fused. When,  on  the  other  hand,  the  process  is  made  use  of  with  thin 
sections,  it  is  convenient  that  the  action  should  be  more  rapid,  and  the 
staining  fluid  may  therefore  be  stronger;  but  unless  its  operation  be  care- 
fully watched  so  as  to  be  stopped  at  the  right  stage,  the  whole  tissue  may 
.be  deeply  dyed,  and  the  value  of  the  selective  staining  altogether  lost. 

201.  It  will  generally  be  found  convenient  to  carry-on  the  staining  of 
thin  sections  either  in  watch-glasses,  or  in  small  cups  of  white  porcelain; 
but  care  must  be  taken  not  to  place  many  sections  together  so  as  to  lie 
one  upon  the  other,  as  this  will  prevent  the  staining  from  being  uniform. 
Small  delicate  sections  may  often  be  advantageously  stained  upon  the 
glass  slides  upon  which  they  are  to  be  mounted;  a  pool  of  the  staining 
fluid  being  made  upon  the  slide,  to  be  removed,  when  the  staining  has 
proceeded  far  enough,  by  the  small  glass  Syringe  (§  127).  It  is  even 
possible  to  stain  a  section  after  it  has  been  covered  with  thin  glass,  by 
depositing  the  fluid  in  contact  with  one  edge  of  the  glass  cover,  and 
drawing  it  through  by  applying  a  bit  of  blotting-paper  to  the  opposite 
margin;  and  the  process  may  thus  be  performed  while  the  section  is  actu- 
ally under  observation  on  the  stage  of  the  Microscope,  the  staining  liquid 


PREPARATION,  MOUNTING,  AND  COLLECTION  OF  OBJECTS.  205 


being  withdrawn  in  the  same  manner  when  the  desired  effect  has  been 
produced,  and  being  replaced  by  the  preservative  medium. — For  taking- 
up  sections  without  injury  to  them,  and  transferring  them  from  one  ves- 
sel to  another,  recourse  may  be  advantageously  had  to  the  ^lifter'  of  Dr. 
Sylvester  Marsh  '  (Fig.  13?);  which  is  a  strip  of  German  silver  or  copper 
of  the  thickness  of  stout  cardboard,  7  inches 
long  and  5-8ths  inches  broad,  each  end  of 
which,  carefully  smoothed  and  rounded,  is  to 
be  turned  at  the  distance  of  5-8ths  inch  to  an 
angle  of  about  35  °.  One  end  is  to  be  left  plain, 
for  lifting  the  section  with  some  of  its  fluid, 
when  it  is  to  be  deposited  on  a  slide;  while 
-the  other  is  perforated  for  letting  the  fluid 
escape,  when  the  section  is  to  be  floated-off 
into  a  vessel  filled  with  some  different  fluid. 

202.  The  relative  value  of  different  Stain- 
ing Agents,  the  best  modes  of  applying  them, 
and  the  benefits  derivable  from  their  use  in 
the  study  of  the  minute  structure  of  Man  and 
the  higher  Animals/  have  now  been  pretty 

fully  determined  by  Histologists;  and  consid-  Marsh's  Section-Lifter. 
erable  progress  has  also  been  made  in  the  ap- 
plication of  the  differential  straining  process  to  the  various  parts  of  the 
higher  Vegetable  fabrics.'  But  there  is  still  a  wide  field  which  has  been 
as  yet  but  little  cultivated,  in  the  application  of  the  staining  process  to 
the  study  of  the  lower  Organisms  of  both  Kingdoms;  and  every  one  who 
is  engaged  in  the  minute  investigations  of  any  particular  group,  must 
work  out  for  himself  the  modifications  which  the  ordinary  methods  may 
require.  All  that  can  be  here  attempted  is  to  give  such  directions  as  to 
the  agents  to  be  employed,  and  the  best  modes  of  using  them,  as  are  likely 
to  be  most  generally  useful. 

a.  Carmine, — This  was  one  of  the  first  Dyes  employed  for  staining  purposes  ; 
>,nd  its  value  was  speciahy  insisted  on  by  Dr.  Beale,  as  enabling  living  Pro- 
toplasm (by  him  designated  *  germinal  matter,'  or  *  bioplasm ')  to  be  distinguished 
;rom  any  kind  of  '  formed  material.'  It  has  a  special  affinity  for  cell-nuclei  (pro- 
toplasts) and  the  axial  cylinders  of  white  nerve-fibres;  and  thus,  if  the  substance 
lo  be  stained  be  only  left  in  the  carmine  fluid  long  enough  for  it  to  dye  these 
«jubstances,  they  are  strikingly  differentiated  from  all  others.  It  is  essential  that 
ihe  fluid  should  have  a  sHght  alkaline  reaction,  especially  where  the  substance 
has  been  hardened  with  chromic  acid.  The  presence  of  too  much  alkali  is  inju- 
rious ;  the  want  of  it,  on  the  other  hand,  causes  the  dye  to  act  on  the  tissues 
generally,  and  thus  negatives  its  differentiating  effect.  Dr.  Beale  directs  it  to  be 
prepared  as  follows:— Ten  grains  of  Carmine  in  small  fragments  are  to  be  placed 
in  a  test-tube,  and  half  a  drachm  of  strong  Liquor  Ammonias  added ;  by  agitation 
and  the  heat  of  a  spirit-lamp  the  carmine  is  soon  dissolved,  and  the  liquid,  after 
boiling  for  a  few  seconds,  is  to  be  ahowed  to  cool.  After  the  lapse  of  an  hour, 
much  of  the  excess  of  ammonia  wiU  have  escaped;  and  the  solution  is  then  to  be 
mixed  with  3  oz.  of  DistUled  Water,  2  oz.  of  pure  Glycerine,  and  \  oz.  of  Alcohol. 
The  whole  may  be  passed  through  a  filter,  or,  after  being  allowed  to  stand  for 


1  See  his  useful  little  Treatise  on    Section-Cutting."  ^    ,  ^ 

«See  the  Treatises  on  Practical  Histology"  by  Prof.  Putherford,  Prof. 
Schafer,  Dr.  Heneage  Gibbes,  Prof.  Ranvier,  Prof.  Frey,  and  others;  How  to 
Work  with  the  Microscope  "  by  Dr.  Beale;  and  Davies's  Preparation  and  Mount- 
ing of  Microscopic  Objects  "  (2d  Edition,  edited  by  Dr.  Matthews). 

3  This  has  been  chiefly  carried  out  in  the  United  States  by  Dr.  Beatty,  Mr. 
Walmsley,  and  Mr.  Merriman,  whose  processes  are  described  in  the  successive 
volumes  of  the    American  Journal  of  Microscopy." 


206 


THE  MICROSCOPE  AND  ITS  REVELATIONS. 


some  time,  the  perfectly  clear  supernatant  fluid  may  be  poured  off  and  kept  for 
use.  If,  after  long  keeping,  a  little  of  the  carmine  should  be  deposited  through 
the  escape  of  the  Ammonia,  the  addition  of  a  drop  or  two  of  Liquor  Ammonias 
will  redissolve  it.  Prof.  Rutherford  recommends  that,  for  slow  but  more  certain 
staining,  the  liquid  should  at  once  be^put  into  a  stoppered  bottle,  so  as  not  to 
allow  the  ammonia  to  evaporate,  and  should  be  diluted  by  the  addition  of  from 
two  to  seven  volumes  of  water.  Carmine  is  used  as  a  general  stain  in  *  double 
staining '  (§  203);  and  a  suitable  fluid  for  this  purpose  is  made  by  mixing  30  grains 
of  carmine  with  2  drachms  of  borax,  and  4  fl.  oz.  of  water,  and  pouring  off  the 
clear  supernatant  fluid.  To  fix  the  stain  of  carmine,  the  section  should  be 
immersed  for  a  few  minutes  in  a  mixture  of  five  drops  of  glacial  Acetic  acid  and 
1  oz.  of  water. 

b.  Picro-Carminate  of  Ammonia,  known  as  Picro-Carmine,  is  a  very  excellent 
staining  material,  which  is  applicable  to  a  great  variety  of  purposes.  Being 
somewhat  difficult  to  prepare,  it  is  best  purchased  ready  for  use  (from  Martin- 
dale,  New  Cavendish  Street).  About  ten  drops  should  be  filtered  into  a  watch- 
glass,  and  diluted  with  distilled  water;  the  sections  should  remain  in  the  solution 
for  from  20  to  30  minutes;  and  if  at  the  end  of  that  time  they  should  not  be  suf- 
ficiently stained,  a  little  more  picro-carmine  should  be  added.  This  dye,  used 
alone,  produces  a  double  staining;  nuclei  fixing  upon  the  carmine,  while  other 
tissues  are  colored  yellow  by  the  picric  acid.  If  the  sections  be  placed  in  methy- 
lated spirit,  they  may  be  kept  without  loss  of  color,  and  may  be  afterwards 
subjected  to  other  processes.  If  placed  in  water,  the  picric  acid  stain  is  removed, 
while  the  carmine  is  left. 

c.  Hcematoxylin,  or  Extract  of  Logwood,  is  now  employed  more  generally 
than  carmine  (which  it  much  resembles  in  action),  its  violet  color  being  more 
pleasant  to  the  eye.  The  following  is  given  by  Kleinenberg  as  the  best  mode  of 
preparing  it: — Make  a  saturated  solution  of  crystallized  chloride  of  calcium  in  70 
per  cent  alcohol;  mix  one  volume  of  this  solution  with  from  6  to  8  volumes  of  a 
saturated  solution  of  alum  in  70  per  cent  alcohol;  and  having  half-filled  a  watch- 
glass  with  this  mixture,  pour  into  it  as  many  drops  of  a  concentrated  solution  of 
Haematoxylin  in  absolute  alcohol  as  will  serve  to  give  the  required  intensity  of 
color.  The  object  must  remain  in  the  dye  for  a  period  varying  from  a  few 
minutes  to  six  hours,  according  to  its  size  and  the  nature  of  the  tissues  composing 
it,  and  is  then  to  be  washed  in  water.  If  it  should  be  stained  throughout,  and  it 
be  desired  that  only  tissues  to  be  specially  distinguished  should  retain  their  color, 
the  diffused  stain  may  be  removed  by  immersion  in  rectified  or  methylated  spirit, 
or  in  a  1-half  per  cent  solution  of  alum. — The  following  is  another  formula  given 
by  Dr.  Gibbes: — Mix  6  grammes  of  Extract  of  Logwood  (as  obtainable  from  Martin- 
dale,  New  Cavendish  Street)  with  18  grammes  of  alum,  and  add  28  cub.  centim. 
of  distilled  water.  Filter,  and  add  to  the  filtrate  1  drachm  of  spirit.  Keep  in  a 
stoppered  bottle  a  week  before  using.  If  what  remains  on  the  filter  be  mixed 
with  14  cub.  centim.  of  distilled  water,  and,  after  soaking  for  an  hour  or  two,  be 
filtered,  and  ^  drachm  of  spirit  be  then  added,  a  second  solution  will  be  made  aa 
strong  as  the  first.  From  7  to  10  drops  of  this  solution  are  to  be  diluted  with  a 
watch-glassful  of  distilled  water;  the  best  degree  of  dilution  being  only  to  be 
found  by  trial.  All  staining  fluids  of  this  kind  are  liable  to  change  by  keeping; 
a  portion  of  the  coloring  matter  passing  out  of  solution,  and  being  deposited  on 
the  sides  and  bottom  of  the  vessel  containing  it.  A  deposit  of  the  same  kind  is 
liable  to  occur  on  the  specimen  during  the  staining,  especially  if  the  process  be 
prolonged;  and  it  is  better  in  such  cases  at  once  to  transfer  the  specimen  to  a 
fresh  solution.  When  sufficiently  stained,  the  specimens  may  be  treated  with 
methylated  spirit,  which  will  fix  the  color;  whilst,  if  the  staining  has  been 
carried  too  far,  the  excess  of  color  may  be  removed  by  the  Acetic  acid  mixture 
which  is  used  to  flx  carmine. — If  the  substance  to  be  stained  with  Logwood 
should  have  been  previously  hardened  with  chromic  acid,  it  should  be  previously 
steeped  in  a  weak  solution  of  bicarbonate  of  soda. 

d.  Magenta  has  nearly  the  same  selective  staining  property  as  carmine;  and  is 
useful  in  the  examination  of  specimens  for  which  rapid  action  and  sharp  definition 
are  required.  But,  like  other  Aniline  dyes,  it  is  liable  to  fade;  and  should,  there- 
fore, not  be  employed  for  permanent  preparations.  Ordinary  magenta  fluid  may 
be  prepared  by  dissolving  1 J  grains  of  magenta  crystals  in  7  fl.  oz.  of  distilled 
water,  and  adding  ^  fl.  oz.  of  rectified  spirit.  The  color  of  a  section  stained  with 
this  may  be  preserved  for  some  time,  by  immersing  it  in  a  l-3d  per  cent  watery 
solution  of  corrosive  sublimate. 

e.  Eosin,  which  dyes  the  tissues  generally  of  a  beautiful  garnet-red  color, 


PREPARATION,   MOUNTING,   AND   COLLECTION   OF   OBJECTS.  207 


should  be  used  in  a  strong  watery  solution;  and  the  sections  must  be  well  washed 
in  water  after  staining.    Its  chief  use  is  in  '  double  staining'  (§  203). 

/.  For  blue  and  green  staining,  the  various  Aniline  dyes  are  principally  used. 
They  are,  for  the  most  part,  however,  rather  fugitive  in  tlieir  effects;  not  forming 
durable  combinations  with  the  tissues  they  stain.  Some  of  them  are  soluble  in 
water,  others  only  in  spirit;  and  the  selection  between  the  dyes  of  these  two  classes 
will  have  to  be  guided  by  the  mode  in  which  the  preparations  are  treated.  These 
dyes  are  for  the  most  part  best  fixed  by  benzole;  and  as  the  sections  treated  with 
this  fluid  may  be  at  once  mounted  in  Canada  balsam,  there  is  greater  probability 
of  their  colors  being  preserved.  Besides  blue  and  green,  the  Aniline  series  fur- 
nishes a  deep  rich  brown,  known  as  Bismarck's  Brown;  and  a  blue-blacky  which 
has  been  recommended  for  staining  nerve-cells. 

g.  A  good  blue  stain  (tending  to  purple)  is  also  given  by  the  substance  termed 
Indigo-Carmine;  which  is  particularly  recommended  for  sections  of  the  brain  and 
spinal  cord  that  have  been  hardened  in  chromic  acid.  A  saturated  solution  of 
the  powder  in  distilled  water  having  been  prepared,  this  may  either  be  used  with 
the  addition  of  about  4  per  cent  of  oxalic  acid ;  or,  if  an  alcoholic  fluid  be  pre- 
ferred, methylated  spirit  may  be  added  to  the  aqueous  solution,  the  mixture  being 
filtered  to  remove  any  coloring  matter  that  may  have  been  precipitated.  If  sec- 
tions thus  stained  have  an  excess  of  color,  this  may  be  removed  by  the  action  of 
a  saturated  solution  of  oxalic  acid  in  alcohol. 

h.  A  beautiful  green  hue  is  given  by  treating  with  a  saturated  solution  of 
Picric  acid  in  water,  sections  previously  stained  with  Aniline  blue;  or  the  two 
agents  may  be  used  together,  4  or  5  parts  of  a  saturated  solution  of  the  latter  be- 
ing added  to  a  saturated  aqueous  solution  of  the  former.  This  picro-aniline,  it  is 
believed,  may  be  relied  on  for  permanence;  and  it  acts  well  in  double  staining 
with  picro-carmine. 

i.  Two  chemical  agents,  Nitrate  of  Silver  and  Chloride  of  Gold,  are  much  used 
by  Histologists  for  bringing-out  particular  tissues;  the  former  being  especially 
valuable  for  the  staining  of  Epithelium-cells;  the  latter  for  staining  Nerve-cells, 
Connective-tissue  corpuscles,  Tendon-cells,  and  Cartilage-cells.  The  most  advan- 
tageous use  of  these  can  only  be  made  by  the  careful  observance  of  the  directions 
which  will  be  found  in  treatises  on  Practical  Histology. 

k,  Molyhdate  of  Ammonia  is  recommended  as  affording  a  cool  blue-gray  or 
neutral-tint  general  stain,  which  affords  a  pleasant  '  ground '  to  pares  strongly 
colored  by  bright  selective  stains. 

203.  Double  and  Triple  Staining, — Very  instructive  as  well  as  beauti- 
ful effects  are  produced  by  the  simultaneous  or  successiye  action  of  two 
or  three  staining  fluids;  which  will  respectively  pick  out  (so  to  speak)  the 
parts  of  a  section  for  which  they  have  special  affinities.  Thus,  if  a  section 
through  the  base  of  the  tongue  of  a  cat  or  dog,  be  stained  with  picro- 
carmine,  rosein,  and  iodine-green,  the  muscles-fibres  will  take  the  first, 
the  connective  tissue  and  protoplasm  of  cells  will  be  colored  by  the  second, 
while  the  third  will  lay  hold  of  the  nuclei  in  the  superficial  epithelium, 
serous  glands,  and  non-striated  muscle  in  the  vessels;  and,  further  the 
mucous  glands  will  show  a  purple  formed  by  the  combined  action  of  the 
red  and  green  (Gibbes).'  A  very  striking  contrast  of  the  like  kind  is 
shown  in  the  double  staining  of  the  frond  of  a  Pern  with  log- wood  and 
aniline  blue;  the  80ri  taking  the  latter,  and  standing  out  brilliantly  on 
the  general  surface  tinged  by  the  former.— The  effects  produced  by  using 
one  stain  after  the  other,  are  generally  much  better  than  those  obtained 
by  simultaneous  staining.  The  selective  action  of  a  second  stain  is  not 
prevented  by  a  previous  general  staining;  for  the  dye  which  gives  the  lat- 
ter seems  to  be  more  weakly  held  by  the  parts  which  take  the  former,  so 
as  to  be  (as  it  were)  displaced  by  it.  Thus,  if  a  section  of  a  Stem  be 
stained  throughout  by  a  solution  of  Eosin  (2  grains  to  1  oz.),  and  be  then 
placed,  after  washing  in  strong  alcohol,  in  a  half -grain  solution  of  Nichol- 


1  See  his  Practical  Histology,"  Chap,  v.,  and  his  Paper  in  Journ.  of  Hoy. 
Microsc.  Soc,"  Yol.  iii.  (1880),  p.  390. 


208 


THE  MICROSCOPE   AND  ITS  REVELATIONS. 


son's  blue  made  neutral,  the  blue  will  in  no  long  time  entirely  drive  out 
the  red;  but  by  carefully  watching  the  process,  it  will  be  seen  that  the 
different  tissues  will  change  color  in  different  times,  the  softer  cells  giving 
up  their  red  and  taking-in  the  blue  more  quickly  than  the  harder;  so  that 
by  stopping  the  process  at  the  right  point  (which  must  be  determined  by 
taking-out  a  section,  dipping  it  in  alcohol,  and  examining  it  under  the 
microscope),  the  two  kinds  of  cells  are  beautifully  differentiated  by  their 
coloring/  The  best  effects  are  usually  produced  by  Carmine  and  Indigo- 
carmine,  Logwood  and  Picro-carmine,  Carmine  or  Logwood  and  Aniline- 
blue  or  Aniline-green.  But  very  much  has  yet  to  be  learned  on  this  sub- 
ject; and  the  further  investigation  of  it  will  be  likely  to  produce  results 
that  will  amply  repay  the  time  and  labor  bestowed. 

204.  Chemical  Testing, — It  is  often  requisite,  alike  in  Biological  and 
in  Mineralogical  investigations,  to  apply  Chemical  Tests  in  minute  quan- 
tity to  objects  under  Microscopic  examination.  Various  contrivances  have 
been  devised  for  this  purpose;  but  the  Author  would  recommend,  from 
his  own  experience,  the  small  glass  Syringe  already  described  (Fig.  106), 
with  a  fine-pointed  nozzle,  as  the  most  convenient  instrument.  One  of 
its  advantages  is  the  very  precise  regulation  of  the  quantity  of  the  test  to 
be  deposited,  which  can  be  obtained  by  the  dextrous  use  of  it;  whilst 
another  consists  in  the  power  of  withdrawing  any  excess.  Care  must  be 
taken  in  using  it,  to  avoid  the  contact  of  the  test-liquid  with  the  packing 
of  the  piston. — Whatever  method  is  employed,  great  care  should  be  taken 
to  avoid  carrying  away  from  the  slide  to  which  the  test-liquid  is  applied, 
any  loose  particles  which  may  lie  upon  it,  and  which  may  be  thus  trans- 
ferred to  some  other  object,  to  the  great  perplexity  of  the  Microscopist. 
For  testing  Inorganic  substances,  the  ordinary  Chemical  Eeagents  are  of 
course  to  be  employed;  but  certain  special  Tests  are  required  in  Biologi- 
cal investigation,  the  following  being  those  most  frequently  required  : 

a.  Solution  of  Iodine  in  water  (1  gr.  of  iodine,  3  grs.  of  iodide  of  potassium,  1 
oz.  of  distilled  water)  turns  Starch  blue  and  Cellulose  brown;  it  also  gives  an  in- 
tense brown  to  Albuminous  substances. 

h.  Dilute  Sulphuric  Acid  (one  of  acid  to  two  or  three  parts  of  water),  gives  to 
Cellulose  thsit  has  been  previously  dyed  with  iodine  a  blue  or  purple  hue;  also, 
when  mixed  with  a  solution  of  sugar,  it  gives  a  rose-red  hue,  more  or  less  deep, 
with  Nitrogenous  substances  and  with  bile  (Pettenkofer's  test). 

c.  What  is  known  as  Schulze's  Test  is  a  solution  of  Chloride  of  Zinc,  Iodine, 
and  Iodide  of  Potassium,  made  in  the  following  way : — Zinc  is  dissolved  in 
Hydrochloric  acid,  and  the  solution  is  permitted  to  evaporate  in  contact  with 
metallic  zinc,  until  it  attains  the  thickness  of  a  syrup;  this  syrup  is  then  saturated 
with  iodide  of  potassium,  and  iodine  is  last  added.  This  solution  serves .  like  the 
preceding,  to  detect  the  presence  of  Cellulose ;  and  has  the  advantage  over  sul- 
phuric acid  of  being  less  destructive  to  the  tissues.  Each  will  sometimes  succeed 
where  the  other  fails;  consequently,  in  doubtful  cases,  both  should  be  employed, 

d.  Concentrated  Nitric  Acid  gives  to  Albuminous  substances  an  intense  yel- 
low. 

e.  Acid  Nitrate  of  Mercury  (Millon's  Test)  colors  Albuminous  substances 
red. 

/.  Acetic  Acid^  which  should  be  kept  both  concentrated  and  diluted  with  from 
3  to  5  parts  of  water,  is  very  useful  to  the  Animal  Histologist  from  its  power  of 
dissolving,  or  at  least  of  reducing  to  such  a  state  of  transparence  that  they  can  no 
longer  be  distinguished,  certain  kinds  of  membranous  and  fibrous  tissues,  so  that 
other  parts  (especially  nuclei)  are  brought  more  strongly  into  view. 

gr.  Solution  of  Caustic  Potass  or  Soda  (the  latter  being  generally  preferable) 
has  a  remarkable  solvent  effect  upon  many  Organic  substances,  both  Animal  and 
Vegetable;  and  is  extremely  useful  in  rendering  some  structures  transparent, 


» See    Journ.  of  Roy.  Microsc.  Soc,"  Vol.  iii.  (1880),  p.  694. 


PREPARATION,  MOUNTING,   AND  COLLECTION  OF  OBJECTS. 


209 


while  others  are  brought  into  view,— its  special  action  being  upon  Tiorn?/ textures, 
whose  component  cells  are  thus  rendered  more  clearly  distinguishable. 

h  Ether  dissolves  Resins,  Fats,  and  Oils;  but  it  will  not  act  on  these  through 
membranes  penetrated  with  watery  fluid. 

i.  Alcohol  dissolves  Resins  and  some  Volatile  Oils;  but  it  does  not  act  on  ordi- 
nary Oils  and  Fats.  It  coagulates  Albuminous  matters,  and  consequently  renders 
more  opaque  such. textures  as  contain  them.  The  opacity,  however,  may  be  re- 
moved by  the  addition  of  a  small  quantity  of  Soda. 

205.  Preservative  Media, — We  have  now  to  consider  the  various  modes 
of  preserving  the  preparations  that  have  been  made  by  the  several  methods 
now  indicated;  and  shall  first  treat  of  such  as  are  applicable  to  those 
minute  Animal  and  Vegetable  organisms,  and  to  tliose  Sections  or  Dis- 
sections of  large  structures,  which  are  suitable  for  being  mounted  as 
transparent  objects.  A  broad  distinction  may  be  in  the  first  place  laid 
down  between  resinous  and  aqueous  preservative  media;  to  the  former 
belong  only  Canada  Balsam  and  Dammar;  whilst  tlie  latter  include  all 
the  mixtures  of  Avhich  Water  is  component. — The  choice  between  the 
two  kinds  of  media  will  partly  depend  upon  the  nature  of  the  processes 
to  which  the  object  may  have  been  previously  subjected,  and  partly  upon 
the  degree  of  transparence  which  may  be  advantageously  imparted  to  it. 
Sections  of  substances  which  have  been  not  only  imbedded  in,  but  pene- 
trated by  paraffine,  wax,  or  cacao-butter,  and  have  been  stained  (if 
desired)  previously  to  cutting,  are,  as  a  rule,  most  conveniently  mounted 
in  Canada  balsam  or  Dammar;  since  they  can  bo  at  once  transferred  to 
either  of  these  from  the  menstruum  by  which  the  imbedding  material  has 
been  dissolved-out.  The  durability  of  this  method  of  mounting  makes  it 
preferable  in  all  cases  to  which  it  is  suitable;  the  exception  being  where 
it  renders  a  very  thin  section  too  transparent,  which  is  specially  liable  to 
happen  with  Dammar. — When  it  is  desired  to  mount  in  either  of  these 
media  Sections  of  structures  that  have  been  imbedded  in  gum  or  gelatine, 
these  substances  must  first  be  completely  dissolved-out  by  steeping  in 
water;  the  sections  must  then  be  dehydrated  ^  by  subjecting  them  to 
mixtures  of  spirit  and  water  progressively  increased  in  strength  to  absolute 
alcohol;  and  after  this  has  been  effected,  they  are  to  be  transferred  to  tur- 
pentine, and  thence  to  benzole.  In  this  process  much  of  the  staining  is 
apt  to  be  lost;  so  that  stained  sections  are  often  more  advantageously 
mounted  in  some  of  those  aqueous  preparations  of  Glycerine,  which 
approach  the  resinous  media  in  transparance  and  permanence. — When 
Canada  balsam  was  first  employed  for  mounting  preparations,  it  was  em- 
ployed ia  its  natural  semifluid  state,  in  which  it  consists  of  a  solution  of 
resin  in  volatile  oil  of  turpentine;  and  unless  a  large  proportion  of  the 
latter  constituent  was  driven  off  by  heat  in  the  process  of  mounting 
(bubbles  being  thus  formed  of  which  it  was  often  difficult  to  get  rid),  or 
the  mounted  slide  was  afterwards  subjected  to  a  more  moderate  heat  of 
long  continuance,  the  balsam  would  remain  soft,  and  the  cover  liable  to 
displacement.  This  is  avoided  by  the  method  now  generally  adopted,  of 
previously  getting  rid  of  the  turpentine  by  protracted  exposure  of  the 
balsam  to  a  heat  not  sufficient  to  boil  it,  and  dissolving  the  resin  tlius 
obtained  either  in  benzole  or  chloroform,  the  solution  being  made  (with 
the  aid  of  gentle  heat)  of  such  viscidity  as  will  allow  it  to  'run 'freely 
when  slightly  warmed.  Either  of  these  sol  vents  evaporates  so  much  more 
quickly  than  turpentine,  that  the  balsam  left  behind  hardens  in  a  com- 
paratively short  time. — The  natural  Balsam,  however,  may  be  preferably 
used  (with  care  to  avoid  the  liberation  of  bubbles  by  overheating)  in 
mounting  sections  already  cemented  to  the  slides  by  hardened  balsam 
14 


210 


THE  MICROSCOPE  AND  ITS  REVELATIONS. 


(§  193);  and  also  for  mounting  the  chitinous  textures  of  Insects,  which  it 
has  a  peculiar  power  of  rendering  transparent,  and  which  seem  to  be 
penetrated  by  it  more  thoroughly  than  they  are  by  the  artificially-prepared 
solution  (§  210). — The  solution  of  Dammar  in  benzole  is  very  convenient 
to  work  with,  and  hardens  quickly. 

206,  The  following  are  the  principal  aqueous  media  whose  value  has 
been  best  tested  by  general  and  protracted  experience: — 

a.  Fresh  specimens  of  minute  Protophytes  can  often  be  very  well  preserved  in 
in  Distilled  Water  saturated  with  Camphor;  the  complete  exclusion  of  air  serving 
botli  to  check  their  living  actions  and  to  prevent  decomposing  changes.  When 
the  preservation  of  color  is  not  a  special  object,  about  a  tenth  part  of  Alcohol  may 
be  added,  and  this  will  be  found  a  suitable  medium  for  the  preservation  of  many 
delicate  Animal  textures. 

b.  Aqueous  Solution  of  Carbolio  Acid. — Even  the  very  small  quantity  of  this 
agent  which  cold  water  will  take  up,  has  a  powerful  preservative  effect;  and  the 
solution  may  be  advantageously  employed  for  mounting  preparations  of  many 
delicate  structures,  both  Animal  and  Vegetable. 

c.  The  same  may  be  said  of  Salicylic  Acid,  which  has  been  very  successfully 
employed  for  delicate  preparations  in  th3  small  proportion  that  will  dissolve  in 
cold  water.  For  coarser  structures  a  stronger  solution  is  preferable;  and  this  may 
be  made  by  combining  with  the  acid  a  small  quantity  either  of  borax  dissolved  in 
glycerine  or  of  acetate  of  potass. 

d.  Where  the  preservation  of  minute  histological  detail  is  not  so  much  desired, 
as  the  exhibition  of  larger  structural  features  of  objects  to  be  viewed  by  reflected 
light,  nothing  is  better  than  Dilute  Spirit;  the  proportion  most  generally  service- 
able being  1  of  Alcohol  to  4  or  5  of  water;  and  an  even  weaker  mixture  serving  to 
prevent  further  change  in  tissues  already  hardened  by  strong  Alcohol.  The 
Author  has  a  series  of  the  beautiful  Pentacrinoid  larvae  of  Comatula  (Plate  xxi.) 
thus  preserved  in  cells  twenty  years  ago;  which  are  as  perfect  as  when  f  ist 
mounted.    These  weaker  mixtures  have  no  action  on  Gold-Size. 

Of  late  years,  Glycerine  has  been  largely  used  as  a  preservative;  either 
alone,  according  to  the  method  of  Dr.  Beale  (§  208),  or  diluted  with 
water,  or  mixed  with  gelatinous  substances. — It  is  much  more  favorable 
to  the  preservation  of  color  than  most  other  media;  and  is  therefore  spe- 
cially useful  as  a  constituent  of  fluids  used  for  mounting  Vegetable  objects 
in  their  natural  aspects.  It  has  also  the  property  of  increasing  the  trans- 
parence of  Animal  structures,  though  in  a  less  degree  than  resinous  sub- 
stances; and  may  thus  be  advantageously  employed  as  a  component  of 
media  for  mounting  objects  that  are  rendered  too  transparent  by  Balsam 
or  Dammar. — Two  cautions  should  be  given  in  regard  to  the  employment 
of  Glycerine;  -first,  that,  as  it  has  a  solvent  power  for  Carbonate  of  Lime, 
it  should  not  be  used  for  mounting  any  object  having  a  calcareous  skeleton; 
and  second,  that  in  proportion  as  it  increases  the  transparence  of  organic 
substances,  it  diminishes  the  reflecting  power  of  their  surfaces,  and  should 
never  be  employed,  therefore,  in  the  mounting  of  objects  to  be  viewed  by 
reflected  light,  although  many  objects  mounted  in  the  media  to  be  pres- 
ently specified  are  beautifully  shown  by  ^black-ground'  illumination. 

e.  A  mixture  of  one  part  of  Glycerine  and  two  parts  of  Camphor- water  may  be 
used  for  the  preservation  of  many  Vegetable  structures. 

/.  For  preserving  soft  and  delicate  Marine  Animals  which  are  shrivelled-up, 
so  to  speak,  by  stronger  agents,  the  Author  has  found  a  mixture  of  1  part  of  Gly- 
cerine and  1  of  Spirit  with  8  or  10  parts  of  Sea  Water,  the  most  suitable  preser- 
vative. 

g.  For  preserving  minute  Vegetable  preparations,  the  following  method, 
devised  by  Hantzsch,  is  said  to  be  peculiarly  efficient: — A  mixture  is  made  of  3 
parts  of  pure  Alcohol,  2  parts  of  Distilled  Water,  and  1  part  of  Glycerine;  and  the 
object,  laid  in  a  cement-cell,  is  to  be  covered  with  a  drop  of  this  liquid,  and  then 
put  aside  under  a  bell-glass.  The  Alcohol  and  Water  soon  evaporate,  so  that  the 
Glycerine  alone  is  left;  and  another  drop  of  the  liquid  is  then  be  added,  and  a 


PREPARATION,  MOUNTING,  AND  COLLECTION  OF  OBJECTS.  211 


second  evaporation  permitted;  the  process  being  repeated,  if  necessary,  until 
enough  Glycerine  is  left  to  fill  the  cell,  which  is  then  to  be  covered  and  closed  in 
the  usual  mode.  ^ 

h.  The  Glycerine  Jelly  prepared  after  the  manner  of  Mr.  Lawrence  may  be 
strongly  recomaiended  as  suitable  for  a  great  variety  of  objects.  Animal  as  well  as 
Vegetable,  subject  to  the  cautions  already  given: — Take  any  quantity  of  Nel- 
son's Gelatine,  and  let  it  soak  for  two  or  three  hours  in  cold  water,  pour  off  the 
superfluous  water,  and  heat  the  soaked  gelatine  until  melted.  To  each  fluid 
ounce  of  the  Gelatine  add  one  drachm  of  Alcohol,  and  mix  well;  then  add  a  fluid 
drachm  of  the  white  of  an  egg.  Mix  well  while  the  Gelatine  is  fluid,  but  cool. 
Now  boil  until  the  albumen  coagulates,  and  the  Gelatine  is  quite  clear.  Filter 
through  fine  flannel,  and  to  each  fluid  ounce  of  the  clarified  Gelatine  add  six  fluid 
drachms  of  Price's  pure  Glycerine,  and  mix  well.  For  the  six  fiuid  drachms  of 
Glycerine,  a  mixture  of  two  parts  of  Glycerine  to  four  of  Camphor-water  may 
be  substituted.  The  objects  intended  to  be  mounted  in  this  medium  are  best 
prepared  by  being  immersed  for  some  time  in  a  mixture  of  one  part  of  Glycerine 
with  one  part  of  diluted  Alcohol  (1  of  alcohol  to  6  of  water)."  ^  A  small  quan- 
tity of  Carbolic  acid  may  be  added  to  it  with  advantage.  When  used,  the  jelly 
must  be  liquefied  by  gentle  warmth,  and  it  is  useful  to  warm  both  the  slide  and 
the  cover-glass  previously  to  mounting  — This  takes  the  place  of  what  was  for- 
merly known  as  Deane's  Medium,  in  which  honey  was  used  to  prevent  the  har- 
dening of  the  gelatine. 

i.  For  objects  which  would  be  injured  by  the  small  amount  of  heat  required 
to  liquefy  the  last-mentioned  medium,  the  Glycerine  and  Gum  Medium  of  Mr. 
Farrants  will  be  found  very  useful.  This  is  made  by  dissolving  4  parts  (by 
weight)  of  picked  Gum  Arabic  in  4  parts  of  cold  Distilled  Water,  and  then  add- 
ing 2  parts  of  Glycerine.  The  solution  must  be  made  without  the  aid  of  heat, 
the  mixture  being  occasionally  stirred,  but  not  shaken,  whilst  it  is  proceeding: 
after  it  has  been  completed,  the  liquid  should  be  strained  (if  not  perfectly  free 
from  impurity)  through  fine  cambric  previously  well  washed  out  by  a  current  of 
clean  cold  water;  and  it  should  be  kept  in  a  bottle  closed  with  a  glass  stopper  or 
cap  (not  with  cork),  containing  a  small  piece  of  Camphor. — The  great  advantage 
of  this  Medium  is  that  it  can  be  used  cold,  and  yet  soon  viscifies  without  crack- 
ing; it  is  well  suited  to  preserve  delicate  Animals  as  well  as  Vegetable  tissues, 
and  in  most  cases  increases  their  transparence. 

It  often  is  quite  impossible  to  predicate  beforehand  what  preservative 
medium  will  answer  best  for  a  particular  kind  of  preparation;  and  it  is 
consequently  desirable,  where  there  is  no  lack  of  material,  to  mount  simi- 
lar objects  in  two  or  three  different  ways,  marking  on  each  slide  the 
method  employed,  and  comparing  the  specimens  from  time  to  time,  so  as 
to  judge  the  condition  of  each. 

207.  In  dealing  with  the  small  quantities  of  fluid  media  required  in 
mounting  Microscopic  objects,  it  is  essential  for  the  operator  to  be  pro- 
vided with  the  means  of  transferring  very  small  quantities  from  the  ves- 
sels containing  them  to  the  slide,  as  well  as  of  taking  up  from  the  slide 
what  may  be  lying  superfluous  upon  it.  Where  some  one  fluid,  such  as 
Diluted  Alcohol  or  the  Carbolic  acid  solution,  is  in  continual  use,  it  will 
be  found  very  convenient  to  keep  it  in  the  small  Dropping-bottle  repre- 
sented in  Fig.  138.  The  stopper  is  perforated,  and  is  elongated  below  into 
a  fine  tube,  whilst  it  expands  above  into  a  bulbous  funnel,  the  mouth  of 
which  is  covered  with  a  piece  of  thin  Vulcanized  India-rubber  tied  firmly 
round  its  lip.  If  pressure  be  made  on  this  cover  with  the  point  of  the 
finger,  and  the  end  of  the  tube  be  immersed  in  the  liquid,  in  the  bottle, 


'  See  the  Rev.  W.  W.  Spicer's  Handy-book  to  the  Collection  and  Preparation 
of  Freshwater  and  Marine  Algae,  etc.,"  pp.  57-59.  ''Nothing,"  says  Mr.  Spicer, 
"can  exceed  the  beauty  of  the  preparations  of  Desmidiacece  prepared  after  Herr 
Hantzsch's  method;  the  form  of  the  plant  and  the  coloring  of  the  endochrome 
having  undergone  no  change  whatever." 

2  A  very  pure  Glycerine  jelly,  of  which  the  Author  has  made  considerable 
use,  is  prepared  by  Mr.  Rimmington,  chemist,  Bradford,  Yorkshire. 


212 


THE  MICROSCOPE  AND  ITS  KEVELATIONS. 


this  will  rise  into  it  on  the  removal  of  the  finger;  if,  then,  the  funnel  be 
inverted,  and  the  pressure  be  re-applied,  some  of  the  residual  air  will  be 
forced  out,  so  that  by  again  immersing  the  end  of  the  tube,  and  remov- 
ing the  pressure,  more  fluid  will  enter.  This  operation  may  be  repeat- 
ed as  often  as  may  be  necessary,  until  the  bulb  is  entirely  filled;  and 
when  it  is  thus  charged  with  fluid,  as  much  or  as  little  as  may  be  needed 
is  then  readily  expelled  from  it  by  the  pressure  of  the  finger  on  the  cover, 
the  bulb  being  always  refilled  if  care  be  taken  to  immerse  the  lower  end 
of  the  tube  before  the  pressure  is  withdrawn.  The 
Author  can  speak  from  large  experience  of  the  value 
of  this  little  implement;  as  he  can  also  of  the  utility 
of  the  small  Glass  Syringe  (§  127)  for  the  same  pur- 
pose, and  this  not  only  for  fine  Aqueous  liquids,  but 
also  for  Glycerine  jelly,  and  Canada  balsam.  For 
these  media  having  been  poured,  when  liquefied  by 
warmth,  each  into  its  own  syringe  (its  piston  having 
been  previously  drawn  out),  can  be  forced  out  as  oc- 
casion requires,  by  pressure  on  the  replaced  piston, 
which  may  be  graduated  with  great  nicety,  when  the 
syringe  has  been  gently  warmed  by  lying  for  a  short 
time  on  the  AVater-bath  cover  (§  177).  Farrants's 
medium  may  be  conveniently  used  in  the  same  manner. 
But  the  solutions  of  Canada  Balsam  and  Gum  Dammar  in  volatile  fluids 
will  not  be  sufficiently  secure  from  change  by  evaporation  through  the 
point  of  the  syringe;  and  are  better  kept  in  wide-mouthed  capped  jsiYS,  the 
liquid  being  taken-out  on  a  pointed  glass  rod,  or  ^ stirrer^  cut  to  such  a 
length  as  will  enable  it  to  stand  in  the  jar  when  its  cap  is  in  place. — Great 
care  should  be  taken  to  keep  the  inside  of  the  cap  and  the  part  of 
the  neck  of  the  jar  on  which  it  fits,  quite  dean^  so  as  to  prevent  the  fix- 
ation of  the  neck  by  the  adhesion  between  these  two  surfaces.  Should 
such  adhesion  take  place,  the  cautious  application  of  the  heat  of  a  spirit- 
lamp  will  usually  make  the  cap  removable.  In  taking  out  the  liquid, 
care  should  be  taken  not  to  drop  it  prematurely  from  the  rod, — a  mis- 
chance which  may  be  avoided  by  not  taking  up  more  than  it  will  properly 
carry,  and  by  holding  it  in  a  horizontal  position,  after  drawing  it  out  of 
of  the  bottle,  until  its  point  is  just  over  the  slip  or  cover  on  which  the 
liquid  is  to  be  deposited. 

208.  Mountijig  Thin  Sectio?is. — The  thin  sections  cut  by  the  Micro- 
tome, or  membranes  obtained  by  Dissection,  do  not  require  to  be  placed 
in  cells  when  mounted  in  any  viscid  medium;  since  its  tenacity  will  serve 
to  keep  off  injurious  pressure  by  the  cover-glass.  When  the  preparation 
has  been  previously  immersed  in  Aqueous  liquids,  and  is  to  be  mounted  in 
glycerine,  glycerine  jelly,  or  Farrants's  medium,  the  best  mode  of  placing 
it  on  the  slide  is  to  float  it  in  a  saucer  or  shallow  capsule  of  water,  to  place 
the  slide  beneath  it,  and,  when  the  object  lies  in  a  suitable  position  above 
it,  to  raise  the  slide  cautiously,  holding  the  object  in  place  by  a  needle, 
until  it  is  entirely  out  of  water.  The  slide  is  then  to  be  wiped  by  an 
absorbent  cloth,  taking  care  not  to  touch  the  object  with  it;  and  the 
small  quantity  of  liquid  still  surrounding  the  object  is  to  be  carefully 
drawn  off  by  a  bit  of  blotting-paper,  care  being  taken  not  to  touch  the 
object  with  it  (as  its  fibres  are  apt  to  adhere),  or  to  leave  any  loose  fibres 
on  the  side.  Before  the  object  is  covered,  it  should  be  looked  at  under  a 
Dissecting  or  Mounting  Microscope,  for  the  purpose  of  improving  (if 
desirable)  its  disposition  on  the  slide,  and  of  removing  any  foreign  particles 


PREPARATION,  MOUNTING,  AND  COLLECTION  OF  OBJECTS.  213 


that  may  be  accidentally  present.  A  drop  of  the  medium  (liquefied,  if 
necessary  by  a  gentle  warmth)  is  then  to  be  placed  upon  it,  and  another 
drop  placed  on  the  cover  and  allowed  to  spread  out.  The  cover  being 
then  taken  up  with  a  pair  of  forceps,  must  be  inverted  over  the  object, 
and  brought  to  touch  the  slide  at  one  part  of  its  margin;  the  slide  being 
itself  inclined  in  the  direction  of  the  place  of  contact,  so  that  the  medium 
accumulates  there  in  a  little  pool.  By  gently  letting  down  the  cover,  a 
little  wave  of  the  medium  is  pressed  before  it;  and,  if  enough  of  the 
medium  has  been  deposited,  the  whole  space  beneath  the  cover  will  be 
filled,  and  the  object  completely  saturated.  If  air-bubbles  should  unfor- 
tunately show  themselves,  the  cover  must  be  raised  at  one  margin,  and  a 
further  quantity  of  the  medium  deposited.  If,  again,  there  are  no  air- 
bubbles,  but  the  medium  does  not  extend  itself  to  the  edge  of  the  cover, 
the  cover  need  not  be  raised,  but  a  little  may  be  deposited  at  its  edge, 
whence  it  will  soon  be  drawn  in  by  capillary  attraction,  especially  if  a 
gentle  warmth  be  applied  to  the  slide.  It  will  then  be  advantageous 
again  to  examine  the  preparation  under  the  Dissecting  Microscope,  for  it 
will  often  happen  that  an  opportunity  may  thus  be  found  of  spreading  it 
better,  by  the  application  of  gentle  pressure  to  one  part  or  another  of  the 
covering-glass,  which  may  be  done  without  injurious  effect  either  with  a 
stiff  needle  or  by  a  pointed  stick,  a  method  whose  peculiar  value,  when 
viscid  media  are  employed,  was  first  pointed  out  by  Dr.  Beale. — The  slide 
should  then  be  set  aside  for  a  few  days,  after  which  its  mounting  may  be 
completed.  Any  excess  of  the  medium  must  first  be  removed.  If  Gly- 
cerine has  been  employed,  much  of  it  may  be  drawn  off  by  blotting-paper 
(taking  care  not  to  touch  the  edge  of  the  cover,  as  it  will  be  very  easily 
displaced);  and  the  remainder  may  be  washed  away  with  a  camel-hair 
brush  dipped  in  water,  which  may  be  thus  carried  to  the  edge  of  the  cover. 
The  water  having  been  drawn  off,  a  narrow  ring  of  liquefied  glycerine- 
jelly  may  be  made  around—not  on — the  margin  of  the  cover  (according 
to  the  suggestion  of  Dr.  S.  Marsh)  for  the  purpose  of  fixing  it  before  the 
cement  is  applied;  and  when  this  has  set,  the  slide  may  be  placed  on  the 
Turn-table  (§  176),  and  the  preparation  '  sealed  ^  by  a  ring  either  of  Dam- 
mar or  of  Bell's  cement,  which  should  be  carried  a  little  over  the  edge  of 
the  cover,  and  outside  the  margin  of  the  ring  of  glycerine- jelly.  This 
'ringing'  should  be  repeated  two  or  three  times;  and  if  the  preparation 
is  to  be  viewed  with  ^oil-immerson^  lenses,  it  should  be  finished  off  with 
a  coat  of  Hollis's  glue,  which  is  not  attacked  by  cedar-oil.  Until  the 
cover  has  been  perfectly  secured,  a  slide  carrying  a  glycerine  preparation 
should  never  be  placed  in  an  inclined  position,  as  its  cover  will  be  almost 
sure  to  slide  by  its  own  weight. — If  Glycerine-jelly  or  Farrants's  medium 
have  been  employed,  less  caution  need  be  used,  as  the  cover-glass,  after  a 
few  days'  setting,  will  adhere  with  sufficient  firmness  to  resist  displace- 
ment. The  superfluous  medium  having  been  removed  by  the  cautious 
use  of  a  knife,  the  slide  and  the  margin  of  the  cover  may  be  completely 
cleansed  by  a  camel-hair  brush  dipped  in  warm  water;  and  when  quite 
dried,  the  slide,  placed  on  the  Turn-table,  may  be  sealed  with  Gold-size, 
— any  other  Cement  being  afterwards  added  either  for  additional  security 
or  for  ^appearance.' 

209.  When,  on  the  other  hand,  the  Section  or  other  preparation  is  to 
be  mounted  in  a  Kesinous  medium,  it  must  have  been  previously  prepared 
for  this  in  the  modes  already  described  (§§  190,  191),  which  will  present 
it  to  the  mounter  either  in  Turpentine  or  some  other  essential  oil,  or  in 
Alcohol.    From  either  of  these  it  may  be  transferred  to  the  slide  by  the 


214: 


THE  MICROSCOPE  AND  ITS  REVELATIONS. 


^  lifter^  (§  201);  its  unpevforeited  end  being  employed,  so  as  to  carry  with 
the  object  a  small  pool  of  the  fluid  from  which  it  has  been  taken. — This 
will  greatly  facilitate  the  transfer  of  the  object  from  the  lifter  to  the  slide; 
as  it  may  be  readily  floated  off  with  the  aid  of  a  slight  touch  of  a  needle. 
The  fluid  thus  deposited  with  it  having  been  drained  away  by  blotting- 
paper,  the  object  may  be  treated  (if  desirable  for  thoroughly  clearing  it) 
with  a  drop  of  Clove-oil,  which  should  be  deposited,  not  on  the  object, 
but  near  it,  and  made  to  run  to  it  by  inclining  the  slide,  so  as,  by  running 
lender  it,  to  rise  through  it  and  saturate  it  thoroughly.  After  about  two 
minutes,  the  clove-oil  is  to  be  drained  away,  and  the  Balsam  or  Dammar 
solution  applied  by  the  glass  rod;  one  drop  being  placed  on  the  object, 
and  another  on  the  cover,  which  is  then  to  be  turned  and  lowered-down 
on  the  object  in  the  manner  already  described.  The  presence  of  a  few 
air-bubbles  may  be  here  disregarded,  as  they  will  ultimately  disappear; 
but  care  must  be  taken  that  the  resinous  solution  not  only  fills  the  space 
between  the  cover  and  the  slide,  but  extends  beyond  its  entire  margin,  as 
much  shrinkage  will  be  produced  by  the  evaporation  of  the  solvent.  If 
this  precaution  be  attended-to,  and  '  appearance  ^  is  not  a  serious  consid- 
eration, nothing  more  is  requisite  for  the  protection  of  the  preparation; 
since  the  margin  of  resin  left  by  the  evaporation  of  its  solvent  forms  an 
adequate  cement,  especially  if  the  cover  be  secured  by  gummed-paper 
from  being  loosened  by  a  ^  jar.'  But  if  it  be  desired  to  replace  this  by  a 
black  or  colored  cement,^  the  resin  must  first  be  scraped  away  with  the 
edge  of  an  awP  carried  alo7ig  (not  towards)  the  margin  of  the  cover;  and 
the  slide,  being  then  cleaned  with  benzole,  and  finally  wiped  with  methy- 
lated spirit,  may  finally  be  ^  ringed '  on  the  Turn-table. 

210.  Mounting  Objects  in  Canada  Balsam, — Although  it  is  prefer- 
able for  Histological  purposes  to  employ  a  solution  of  hardened  Balsam, 
yet  as  there  are  many  objects  for  mounting  for  which  the  use  of  the 
'  natural '  Balsam  is  preferable,  it  will  be  well  to  give  some  directions  for 
its  use. — When  Sections  of  hard  substances  have  been  ground  down  on 
the  slides  to  which  they  have  been  cemented  (§  194),  it  is  much  better 
that  they  should  be  mounted  without  being  detached,  unless  they  have 
become  clogged  with  the  abraded  particles,  and  require  to  be  cleansed 
out — as  is  sometimes  the  case  with  sections  of  the  shells,  spines,  etc.,  of 
Echinoderms,  when  the  balsam  by  which  they  have  been  cemented  is  too 
soft.  If  the  detachment  of  a  specimen  be  desirable,  it  may  be  loosened 
by  heat,  and  lifted  off  with  a  camel-hair  brush  dipped  in  Oil  of  Turpen- 
tine. But,  where  time  is  not  an  object,  it  is  far  better  to  place  the  slide 
to  steep  in  Ether  or  Chloroform  in  a  capped  jar,  until  the  object  then 
falls  off  of  itself  the  solution  of  its  cement.  It  may  be  thoroughly 
cleansed  by  boiling  it  in  methylated  spirit,  and  afterwards  laid  upon  a 
piece  of  blotting-paper  to  dry;  after  which  it  may  be  mounted  in  fresh 
balsam  on  a  slide,  just  as  if  it  had  remained  attached.  The  slide  having 
been  warmed  on  the  water-bath  lid,  a  sufficient  quantity  of  balsam  should 
be  pressed  out  from  the  syringe  on  the  object;  and  care  should  be  taken 
that  this,  if  previously  loosened,  should  be  thoroughly  penetrated  by  it. 
If  any  air-bubbles  arise,  they  should  be  broken  with  the  needle-point. 


^  The  great  Scientific  investigators  of  Germany,  who  cut  an  entire  Worm  into 
thin  transverse  sections,  carefully  mounted  in  their  order,  would  scorn  to  spend 
time  in  such  a  mere  *  finish,'  which  they  would  consider  only  worthy  of  Amateurs. 

2  The  Author  has  found  this  implement,  mounted  in  a  small  handle,  far  less 
liable  to  disturb  the  cover,  than  the  '  old  penknife,'  the  slipping  of  whose  point  in 
chip  ping-away  hard  resin  has  oftened  occasioned  him  much  mischief. 


PREPARATION,  MOUNTING,  AND  COLLECTION  OF  OBJECTS.  215 

The  cover  having  been  similarly  warmed,  a  drop  of  balsam  should  be 
placed  on  it,  and  made  to  spread  over  its  surface;  and  the  cover  should 
then  be  turned  over  and  let  down  on  the  object  in  the  manner  already 
described.  If  this  operation  be  performed  over  the  water-bath,  instead 
of  over  the  spirit-lamp,  there  will  be  little  risk  of  the  formation  of  air- 
bubbles.  However  large  the  section  may  be,  care  should  be  taken  that 
the  Balsam  is  well-spread  both  over  its  surface  and  that  of  its  cover;  and 
by  attending  to  the  precaution  of  making  it  accumulate  on  one  side  by 
sloping  the  slide,  and  letting  down  the  cover  so  as  to  drive  a  wave  before 
it  to  the  opposite  side,  very  large  sections  may  thus  be  mounted  without 
a  single  air-bubble.  (The  author  has  thus  mounted  sections  of  Eozoon 
three  inches  square.) — In  mounting  minute  Balsam-objects,  such  as 
Diatoms,  Polycystina,  Sponge-spicules,  and  the  beautiful  minute  spines 
of  Opliiurida,  great  advantage  will  be  obtained  from  following  the  plan 
suggested  by  Mr.  James  Smith,  for  which  his  Mounting  Instrument 
(Fig.  130)  is  specially  adapted.  The  slide  being  j^laced  upon  its  slide- 
plate,  and  the  object  having  been  laid  upon  the  glass  in  the  desired 
jDOsition,  the  covering  glass  is  laid  upon  this,  and  the  ivory  knob  is  to  be 
screwed  down,  so  as,  by  a  very  slight  pressure  on  the  cover,  to  keep  in  its 
place.  The  slide  is  then  to  be  very  gently  warmed,  and  the  Balsam  to  be 
applied  at  the  edge  of  the  cover,  from  which  it  will  be  drawn  in  by 
capillary  attraction,  penetrating  the  objects,  and  leaving  no  bubbles  if 
too  much  heat  be  not  applied.  In  this  manner  the  objects  are  kept 
exactly  in  the  places  in  which  they  were  at  first  laid;  and  scarcely  a 
particle  of  superfluous  balsam,  if  due  care  has  been  employed,  remains  on 
the  slide. — When  the  chitinous  textures  of  Insects  are  to  be  thus 
mounted,  they  must  be  first  softened  by  steeping  in  Oil  of  Turpentine; 
and  a  large  drop  of  Balsam  being  placed  on  a  warmed  slide,  the  object, 
taken  up  in  the  forceps,  is  to  be  plunged  in  it,  and  the  cover  (balsamed 
as  before)  let  down  upon  it.  It  is  with  objects  of  this  class,  that  the 
Spring- Clip  (Fig.  128)  and  the  Spring-Press  (Fig.  129)  prove  most 
useful  in  holding  down  the  cover  until  the  balsam  has  hardened  suffi- 
ciently to  prevent  its  being  lifted  by  the  elasticity  of  the  object. — Various 
objects  (such  as  the  palates  of  Gasteropods),  which  have  been  prepared 
by  dissection  in  water  or  weak  spirit,  may  be  advantageously  mounted  in 
Balsam;  for  which  purpose  they  must  be  first  dehydrated,  and  then 
transferred  from  rectified  Spirit  into  Turpentine.  Carholio  Acid  lique- 
fied by  heat  has  been  lately  recommended  by  Dr.  Ealph^  as  most  efficient 
in  drawing  out  water  from  specimens  to  be  mounted  in  Balsam  or 
Dammar,  which  afterwards  readily  take  its  place. — Sections  of  Horns, 
Hoofs,  etc.,  which  afford  most  beautiful  objects  for  the  Polariscope,  are 
best  mounted  in  natural  Balsam,  which  has  a  remarkable  power  of 
increasing  their  transparence. — It  is  better  to  set  aside  in  a  warm  place 
the  slides  which  have  been  thus  mounted,  before  attempting  to  clean  off 
the  superfluous  Balsam;  in  order  that  the  covers  may  be  fixed  by  the  grad- 
ual hardening  of  what  lies  beneath  them. 

211.  Mou7iting  Objects  in  Aqueous  Liquids. — By  far  the  greater 
number  of  preparations  which  are  to  be  preserved  in  liquid,  however, 
should  be  mounted  in  a  Cell  of  some  kind,  which  forms  a  of  suitable 
depth,  wherein  the  preservative  liquid  may  be  retained.  This  is  absolutely 
necessary  in  the  case  of  all  objects  whose  thickness  is  such  as  to  prevent 


1  See  the  accout  of  Dr.  Ralph's  method  in  Journ.  of  Roy.  Microsc.  Soc,"  VoL 
iii.  (1880),  p.  858. 


2J6 


THE  MICROSCOPE  AND  ITS  REVELATIONS. 


the  glass-cover  from  coming  into  close  approximation  with  the  slide;  and 
it  is  desirable  whenever  that  approximation  is  not  such  as  to  cause  the 
cover  to  be  drawn  to  the  glass-slide  by  capillary  attraction,  or  whenever 
the  cover  is  sensibly  kept  apart  from  the  slide  by  the  thickness  of  any 
portion  of  the  object.  Hence  it  is  only  in  the  case  of  objects  of  the  most 
extreme  tenuity,  that  the  Cell  can  be  advantageously  dispensed  with;  the 
danger  of  not  employing  it,  in  many  cases  in  which  there  is  no  difficulty 
in  mounting  the  object  without  it,  being  that  after  a  time  the  cement  is 
apt  to  run-in  beneath  the  cover,  which  process  is  pretty  sure  to  continue 
when  it  may  have  once  commenced.  When  Cement-cells  (§  170)  are  em- 
ployed for  this  purpose,  care  must  be  taken  that  the  surface  of  the  ring  is 
perfectly  flat,  so  that  when  the  cover- glass  is  laid-on,  no  tilting  is  pro- 
duced by  pressure  on  any  part  of  its  margin.  As  a  general  rule  it  is 
desirable  that  the  object  to  be  mounted  should  be  steeped  for  a  little  time 
previously  in  the  preservative  fluid  employed. — A  sufficient  quantity  of 
this  fluid  being  deposited  from  the  Syringe  or  Dropping-bottle  to  over-fill 
the  cell,  the  object  is  to  be  introduced  into  it  either  with  the  Forceps  or 
the  Dipping-tube  (§  126);  and  the  slide  should  then  be  examined  on  the 
Dissecting  Microscope,  that  its  entire  freedom  from  foreign  particles  and 
from  air  bubbles  may  be  assured,  and  that  its  disposition  may  be  corrected 
if  necessary.  The  cover  should  then  be  laid  on  very  cautiously,  so  as  not 
to  displace  the  object;  which  in  this  case  is  best  done  by  keeping  the  drop 
highest  in  the  centre,  and  keeping  the  cover  parallel  to  the  slide  whilst 
it  is  being  lowered,  so  as  to  expel  the  superfluous  fluid  all  around.  This 
being  taken  up  by  the  syringe,  the  cement  ring  and  the  margin  of  the 
cover  are  to  be  dried  with  blotting-paper,  especial  care  being  taken  to 
avoid  drawing-ofl  too  much  liquid,  which  will  cause  the  gold-size  to  run- 
in.  It  is  generally  best  to  apply  the  first  coat  of  Gold-size  tlmi,  with  a 
very  small  and  flexible  brush  worked  with  the  hand;  this  will  dry  suffi- 
ciently in  an  hour  or  two,  to  hold  the  cover  whilst  being  '  ringed  ^  on  the 
Turn-table.  And  it  is  safer  to  apply  a  third  coat  a  day  or  two  afterwards: 
old  Gold-size,  which  lies  thickly,  being  then  applied  so  as  to  raise  the 
the  ring  to  the  level  of  the  surface  of  the  cover.  As  experience  shows 
that  preparations  thus  mounted,  which  have  remained  in  perfectly  good 
order  for  many  years,  may  be  afterwards  spoiled  by  leakage,  the  Author 
strongly  recommends  that  to  prevent  the  loss  of  valuable  specimens,  an 
additional  coating  of  gold-size  be  laid-on  from  time  to  time. 

212.  Mou7iting  of  Objects  in  Deep  Cells.— objects  which  require 
deep  cells  are,  as  a  rule,  such  are  as  to  be  viewed  by  reflected  light;  and 
are  usually  of  sufficient  size  and  substance  to  allow  of  air  being  entangled 
in  their  tissues.  This  is  especially  liable  to  occur  where  they  have  under- 
gone the  process  of  decalcification  (§  198);  which  will  very  probably  leave 
behind  it  bubbles  of  Carbonic  acid.  For  the  extraction  of  such  bubbles, 
the  use  of  an  Air-pump  is  commonly  recommended;  but  the  Author  has 
seldom  found  this  answer  the  purpose  satisfactorily,  and  is  much  disposed 
to  place  confidence  in  a  method  lately  recommended— steeping  the  speci- 
men in  a  stoppered  jar  filled  with  freshly  boiled  water,  which  has  great 
power  of  drawing  into  itself  either  Air  or  Carbonic  acid.  Where  the 
structure  is  one  which  is  not  injured  by  Alcohol,  prolonged  steeping  in 
this  will  often  have  the  same  effect. — The  next  point  of  importance  is  to 
select  a  cover  of  a  size  exactly  suitable  to  that  of  the  ring,  of  whose 
breadth  it  should  cover  about  two-thirds,  leaving  an  adequate  margin 
uncovered  for  the  attachment  of  the  cement.  And  the  perfect  flatness 
<if  that  ring  should  then  be  carefully  tested,  since  on  this  mainly  depends 


PREPARATION,  MOUNTING,  AND  COLLECTION  OF  OBJECTS.  217 

tlie  security  of  the  mounting.  It  is  to  secure  this,  that  the  Author  pre- 
fers rings  of  tin  (§  171)  to  those  of  glass,  for  cells  of  moderate  depth;  for 
their  surface  can  be  easily  made  perfectly  Hat  by  grinding  with  water, 
first  on  a  piece  of  grit,  and  then  on  a  Water- of- Ayr  stone — these  stones 
having  been  previously  reduced  to  a  plane  surface  (§  193).  If  glass  rings 
are  not  found  to  be  '  true  ^  they  must  be  ground-down  with  fine  emery 
on  a  plate  of  lead.  When  the  cell  has  been  thus  finished-off,  it  must  be 
carefully  cleaned-out  by  syringing  into  it  some  of  the  mounting-fluid; 
and  should  be  then  examined  under  the  Dissecting  Microscope  for  minute 
air-bubbles,  which  often  cling  to  the  bottom  or  sides.  These  having 
been  got  rid  of  by  the  needle,  the  cell  should  be  finally  filled  with  the 
preservative  liquid,  and  the  object  immersed  in  it,  care  being  taken  that 
no  air-bubbles  are  carried-down  beneath  it.  The  cell  being  completely 
filled  so  that  the  liquid  is  running  over  its  side,  the  cover  may  then  be 
lowered  down  upon  it  as  in  the  preceding  case;  or,  if  the  cell  be  quad- 
rangular, the  cover  may  be  sloped  so  as  to  rest  one  margin  on  its  wall  and 
fresh  liquids  may  be  thrown  in  by  the  Syringe,  while  the  other  edge  is 
lowered.  When  the  cover  is  in  place,  and  the  liquid  expelled  from  it 
has  been  taken  up  by  the  syringe,  it  should  again  be  examined  under  a 
lens  for  air-bubbles;  and  if  any  of  these  troublesome  intruders  should 
present  themselves  beneath  the  cover,  the  slide  should  be  inclined,  so  as 
to  cause  them  to  rise  towards  the  highest  part  of  its  circumference,  and 
the  cover  slipped  away  from  that  part,  so  as  to  admit  of  the  introduction 
of  a  little  additional  fluid  by  the  pipette  or  syringe;  and  when  this 
has  taken  the  place  of  the  air-bubbles,  the  cover  may  be  slipped  back  into 
its  place.  The  surface  of  the  ring  and  the  edge  of  the  cover  must  then 
be  thoroughly  dried  Avith  blotting-paper,  care  being  taken  that  the  fluid  be 
not  drawn  away  from  between  the  cover  and  the  edge  of  the  cell  on  which 
it  rests.  These  minutiae  having  been  attended  to,  the  closure  of  the  cell 
may  be  at  once  effected  by  carrying  a  thin  layer  of  Gold -size  or  Dammar 
around  and  upon  the  edge  of  the  glass-cover,  taking  care  that  it  touches 
every  point  of  it,  and  fills  the  angular  channel  which  is  left  along  its 
margin.  The  Author  has  found  it  advantageous,  however,  to  delay 
closing  the  cell  for  some  little  time  after  the  superfluous  fluid  has  been 
drawn  off;  for  as  soon  as  evaporation  from  beneath  the  edge  of  the  cover 
begins  to  diminish  the  quantity  of  fluid  in  the  cell,  air-bubbles  often  begin 
to  make  their  appearance,  which  were  previously  hidden  in  the  recesses 
of  the  object;  and  in  the  course  of  half  an  hour,  a  considerable  number 
are  often  collected.  The  cover  should  then  be  slipped  aside,  fresh  fluid 
introduced,  the  air-bubbles  removed,  and  the  cover  put  on  again;  and  this 
operation  should  be  repeated  until  it  fails  to  draw  forth  any  more  air- 
bubbles.  It  will  of  course  be  observed,  that  if  the  evaporation  of  fluid 
should  proceed  far,  air-bubbles  will  enter  beneath  the  cover;  but  these 
will  show  themselves  on  the  surface  of  the  fluid;  whereas  those  which 
arise  from  the  object  itself  are  found  in  the  deeper  parts  of  the  cell. 
When  all  these  have  been  successfully  disposed  of,  the  cell  may  be  '  sealed  ' 
and  '  ringed  ^  in  the  manner  already  described. 

213.  Importance  of  Cleanliness, — The  success  of  the  result  of  any  of 
the  foregoing  operations  is  greatly  detracted-from,  if,  in  consequence  of 
the  adhesion  of  foreign  substances  to  the  glasses  whereon  the  objects  are 
mounted,  or  to  the  implements  used  in  the  manipulations,  any  extraneous 
particles  are  brought  into  view  with  the  object  itself.  Some  such  will 
occasionally  present  themselves,  even  under  careful  management;  espe- 
cially fibres  of  silk,  wool,  cotton,  or  linen,  from  the  handkerchiefs,  etc., 


218 


THE  MICROSCOPE  AND  ITS  REVELATIONS. 


with  which  the  glass-slides  may  have  been  wiped;  fibres  of  the  blotting- 
paper  employed  to  absorb  superfluous  fluid;  and  grains  of  starch,  whicii 
often  remain  obstinately  adherent  to  the  thin  glass-covers  kept  in  it. 
But  a  careless  and  uncleanly  manipulator  will  allow  his  objects  to  con- 
tract many  other  impurities  than  these;  and  esj  ecially  to  be  contaminated 
by  particles  of  dust  floating  through  the  air,  the  access  of  which  may  be 
readily  prevented  by  proper  precautions.  It  is  desirable  to  have  at  hand  a 
well-closed  cupboard  furnished  with  shelves,  or  a  cabinet  of  well-fitted 
drawers,  or  a  number  of  bell-glasses  upon  a  flat  table,  for  the  purpose  of 
securing  glasses,  objects,  etc.,  from  this  contamination  in  the  intervals  of 
the  work  of  preparation;  and  the  more  readily  accessible  these  receptacles 
are,  the  more  use  will  the  Microscopist  be  likely  to  make  of  them.  Great 
care,  ought,  of  course,  to  betaken  that  the  Media  employed  for  mounting 
should  be  freed  by  effectual  filtration  from  all  floating  particles,  and  that 
they  should  be  kept  in  well-closed  bottles. 

214.  Labelling  and  Preserving  of  Objects. — Whenever  the  mounting 
of  an  object  has  been  completed,  its  name  ought  to  be  at  once  marked  on 
it,  and  the  slide  should  be  put  away  in  its  appropriate  place.  Some  inscribe 
the  name  on  the  glass  itself  with  a  writing  diamond;  whilst  others  prefer 
to  gum  a  labels^  on  the  slide;  and  others,  again,  cover  one  or  both  surfaces 
of  the  slide  with  colored  paper,  and  attach  the  label  to  it.  In  the  case 
of  objects  mounted  dry  or  in  balsam,  the  latter  method  has  the  advantage 
of  rendering  the  glass-cover  more  secure  from  displacement  by  a  slight 
blow  or  ^  jar  ^  when  the  varnish  or  balsam  may  have  become  brittle  by  the 
lapse  of  years.  Instead,  however,  of  attaching  the  white  label  on  which 
the  name  of  the  object  is  written,  to  the  outside  of  the  colored  paper 
with  which  the  slide  is  covered,  it  is  better  to  attach  the  label  to  the  glass, 
and  to  punch  a  hole  out  of  the  colored  paper,  sufficiently  large  enough 
to  show  the  name,  in  the  part  corresponding  to  it:  in  this  manner  the 
label  is  prevented  from  falling-off,  which  it  frequently  does  when  attached 
to  the  glass  without  protection,  or  to  the  outside  of  the  paper  cover. 
When  objects  are  mounted  in  fluid,  either  with  or  without  cells,  paper 
coverings  to  the  slides  had  better  be  dispensed  with;  and  besides  the  name 
of  the  object,  it  is  desirable  to  inscribe  on  the  label  that  of  the  fluid  in 
it  is  mounted. — For  the  preservation  of  objects,  the  pasteboard  boxes  now 
made  at  a  very  reasonable  cost,  with  wooden  racks,  to  contain  G,  12,  or 
24  slides,  will  be  found  extremely  useful.  In  these,  however,  the  slides 
must  always  stand  upon  their  edges;  a  position  which,  besides  interfering 
with  that  ready  view  of  them  which  is  required  for  the  immediate  selec- 
tion of  any  particular  specimen,  is  unfavorable  to  the  continued  soundness 
of  preparations  mounted  in  fluid.  Although  such  boxes  are  most  useful, 
indeed  almost  indispensable,  to  the  Microscopist,  for  holding  slides  which 
he  desires  (for  whatever  purpose)  to  keep  for  awhile  constantly  at  hand, 
yet  his  regularly-classified  series  is  much  more  conveniently  stored  either 
in  a  Cabinet  containing  numerous  very  shallow  drawers,  in  which  they 
lie  flat  and  exposed  to  view,  or  (which  the  Author  finds  much  preferable) 
in  a  series  of  smaller  cases,  each  holding  a  dozen  trays,  everyone  of  which 
is  divided  into  twelve  compartments  for  as  many  slides.  These  have  the 
advantage,  not  only  of  cheapness  (their  outside  case  being  made  of  pol- 
ished pine,  while  the  trays  are  made  of  thin  pasteboard  glued  to  a  wooden 

*  Very  neat  gummed  labels,  of  the  various  sizes  and  patterns  suitable  to  the 
wants  of  the  Microscopist,  may  be  obtained  from  the  Drapers'  Stationers"  in 
the  City;  and  covering  slips  of  various  patterns  are  supply  by  many  of  the  dealers 
in  Microscopic  Apparatus. 


PREPARATION,  MOUNTING,  AND  COLLECTION  OF  OBJECTS.  219 

framing)  but  also  of  facilitating  the  classification  of  Objects  in  groups, 
and  of  enabling  any  particular  series  to  be  transported  without  risk  of 
injury,  every  slide  being  lodged  in  its  own  receptacle.  Further,  when  pro- 
vision has  to  be  made  for  slides  requiring  greater  depth  than  usual  (such, 
for  instance,  as  extra-thick  wooden  slides,  or  glasses  bearing  deep  cells), 
trays  can  be  made  can  be  made  either  of  double  the  usual  depth,  or  in  the 
proportion  of  3  to  2  (two  such  trays  equalling  three  ordinary  ones  in  thick- 
ness), so  as  still,  by  keeping  the  case  filled,  to  prevent  shake  to  its 
content  when  it  is  carried.  Smaller  Slide-cases  of  the  same  kind,  con- 
taining from  two  to  six  trays,  each  of  which  holds  six  slides,  are  made  for 
the  pocket. 

Section  3.    Collectio7i  of  Objects. 

215.  A  large  proportion  of  the  objects  with  which  the  Microscopist  is 
concerned,  are  derived  from  the  minute  parts  of  those  larger  organisms, 
whether  Vegetable  or  Animal,  the  collection  of  which  does  not  require  any 
other  methods  than  those  pursued  by  the  ordinary  Naturalist.  With 
regard  to  such,  therefore,  no  special  directions  are  required.  But  there  are 
several  most  interesting  and  important  groups  both  of  Plants  and  Ani- 
mals, which  are  themselves,  on  account  of  their  minuteness,  essentially 
microscopic;  and  the  collection  of  these  requires  peculiar  methods  and 
implements,  which  are,  however,  very  simple, — the  chief  element  of  suc- 
cess lying  in  the  knowledge  where  to  look  and  wliat  to  look  for.  In  the 
present  place,  general  directions  only  will  be  given;  particular  details 
relating  to  the  several  groups,  being  reserved  for  the  account  to  be 
hereafter  given  of  each. 

216.  Of  the  Microscopic  organisms  in  question,  those  which  inhabit 
fresh  water  must  be  sought  for  in  pools,  ditches,  or  streams,  through 
which  some  of  them  freely  move;  whilst  others  attach  themselves  to  the 
stems  and  leaves  of  aquatic  Plants,  or  even  to  pieces  of  stick  or  decaying 
leaves,  etc.,  that  may  be  floating  on  the  surface  or  submerged  beneath  it; 
while  others,  again,  are  to  be  sought  for  in  the  muddy  sediments  at  the 
bottom.  Of  those  which  have  the  power  of  free  motion,  some  keep  near 
the  surface,  whilst  others  swim  in  the  deeper  waters;  but  the  situation  of 
many  depends  entirely  upon  the  light,  since  they  rise  to  the  surface  in 
sunshine,  and  subside  again  afterwards.  The  Collector  will,  therefore, 
require  a  means  of  obtaining  samples  of  water  at  different  depths,  and  of 
drawing  to  himself  portions  of  the  larger  bodies  to  which  the  microscopic 
organisms  may  be  attached.  For  these  purposes  nothing  is  so  con- 
venient as  the  Pond'Stich  (sold  by  Mr.  Baker),  which  is  made  in  two 
lengths,  one  of  them  sliding  within  the  other,  so  as  when  closed  to  serve 
as  a  walking-stick.  Into  the  extremity  of  this  may  be  fitted,  by  means 
of  a  screw  socket,  (1)  a  cutting-hook  or  curved  knife,  for  bringing  up  por- 
tions of  larger  Plants  in  order  to  obtain  the  minute  forms  of  Vegetable  or 
Animal  life  that  may  be  parasitic  upon  them;  (2)  a  broad  collar,  with  a 
screw  in  its  interior,  into  which  is  fitted  one  of  the  screw-topped  Bottles 
made  by  the  York  Glass  Company;  (3)  a  ring  or  hoop  for  a  muslin  King- 
Net.  When  the  Bottle  is  used  for  collecting  at  the  surface,  it  should  be 
moved  sideways  with  its  mouth  partly  below  the  water;  but  if  it  be 
desired  to  bring  up  a  sample  of  the  liquid  from  below,  or  to  draw  into 
the  bottle  any  bodies  that  may  be  loosely  attached  to  the  submerged 
plants,  the  bottle  is  to  be  plunged  into  the  water  with  its  mouth  down- 
wards, carried  into  the  situation  in  which  it  is  desired  that  it  should  be 
tilled,  and  then  suddenly  turned  with  its  mouth  upwards.    By  unscrew- 


220 


THE  MICKOSCOPE  AND  ITS  KEVELATIONS. 


ing  the  bottom  from  the  collar,  and  screwing  on  its  cover,  the  contents 
may  be  securely  preserved.  The  Net  should  be  a  bag  of  fine  muslin, 
which  may  be  simply  sewn  to  a  ring  of  stout  wire.  But  it  is  desirable 
for  many  purposes  that  the  muslin  should  be  made  removable;  and  this 
may  be  provided  for  ("as  suggested  in  the  Micrographic  Dictionary," 
Introduction,  p.  xxiv.)  by  the  substitution  of  a  wooden  hoop  grooved 
on  its  outside,  for  the  wire  ring;  the  muslin  being  strained  upon  it  by  a 
ring  of  vulcanized  India-rubber,  which  lies  in  the  groove,  and  which  may 
be  readily  slipped  off  and  on,  so  as  to  allow  a  fresh  piece  of  muslin  to  be 
put  in  the  place  of  that  which  has  been  last  used.  The  collector  should 
also  be  furnished  with  a  number  of  Bottles,  into  which  he  may  transfer 
the  samples  thus  obtained,  and  none  are  so  convenient  as  the  screw- 
topped  bottles  made  in  all  sizes  by  the  York  Glass  Company.  It  is  well 
that  the  bottles  should  be  fitted  into  cases,  to  avoid  the  risk  of  breakage. 
When  Animacules  are  being  collected,  the  bottles  should  not  be  above 
two-thirds  filled,  so  that  adequate  air-space  may  be  left. — Whilst  engaged 
in  the  search  for  Microscopic  objects,  it  is  desirable  for  the  Collector  to 
possess  a  means  of  at  once  recognizing  the  forms  which  he  may  gather, 
where  this  is  possible,  in  order  that  he  may  decide  whether  the  ^gather- 
ing Ms  or  is  not  worth  preserving;  for  this  purpose  either  a  powerful 
'  Coddington  ^  or  '  Stanhope '  lens  (§  24),  a  Beale's  Pocket  Microscope 
(§  76),  or  the  Travelling  Microscope  of  Messrs.  Baker  or  other  opticians 
(§  78),  will  be  found  most  useful,  according  to  the  class  of  objects  of 
which  the  Collector  is  in  search.  The  former  will  answer  very  well  for 
Zoophytes  and  the  larger  Diatomaceae;  but  the  latter  will  be  needed  for 
Desmidiaceae,  the  smaller  Diatomaceae,  and  Animalcules. 

217.  The  same  general  method  is  to  be  followed  in  the  collection  of 
such  marine  forms  of  Vegetable  and  Animal  life  as  inhabit  the  neighbor- 
hood of  the  shore,  and  can  be  reached  by  the  Pond-stick.  But  there  are 
many  which  need  to  be  brought  up  from  the  bottom  by  means  of  the 
Dredge;  and  many  others  which  swim  freely  through  the  waters  of  the 
Ocean,  and  are  only  to  be  captured  by  the  Toio-net,  As  the  former  is 
part  of  the  ordinary  equipment  of  every  Marine  Naturalist,  whether  he 
concern  himself  with  the  Microscope  or  not,  the  mode  of  using  it  need 
not  be  here  described;  but  the  use  of  the  latter  for  the  purposes  of  the 
Microscopist  requires  special  management.  The  net  should  be  of  fine 
muslin,  firmly  sewn  to  a  ring  of  strong  w^ire  about  10  or  12  inches  in 
diameter.  This  may  be  either  fastened  by  a  pair  of  strings  to  the  stern 
of  a  boat,  so  as  to  tow  behind  it,  or  it  may  be  fixed  to  a  stick  so  held  in 
the  hand  as  to  project  from  the  side  of  the  boat.  In  either  case  the  net 
should  be  taken  in  from  time  to  time,  and  held  up  to  allow  the  water  it 
contains  to  drain  through  it;  and  should  then  be  turned  inside  out  and 
moved  about  in  a  bucket  of  water  carried  in  the  boat,  so  that  any  minute 
organisms  adhering  to  it  may  be  washed  off  before  it  is  again  immersed. 
It  is  by  this  simple  method  that  Marine  Animalcules,  the  living  forms  of 
Radiolaria,  the  smaller  Medusoids  (with  their  allies,  Beroe^wdi  Cydippe), 
Noctilucay  the  free-swimming  larvae  of  Echinodermaia,  some  of  the 
most  curious  of  the  Tunicata,  the  larvae  of  Mollusca,  Turbellaria,  and 
Armelida,  some  curious  adult  forms  of  these  classes,  Entomostracay  and  the 
larvae  of  higher  Crustacea,  are  obtained  by  the  Naturalist;  and  the  great 
increase  in  our  knowledge  of  these  forms  which  has  been  gained  within 
recent  years,  is  mainly  due  to  the  assiduous  use  which  has  been  made  of 
it  by  qualified  observers. — It  is  important  to  bear  in  mind,  that,  for  the 
collection  of  all  the  more  delicate  of  the  organisms  just  named  (such,  for 


PREPARATION,  MOUNTING,   AND  COLLECTION  OF  OBJECTS.  221 


instance,  as  Ecliinoderm  larvce),  it  is  essential  that  the  boat  should  be 
rowed  so  slowly  that  the  net  may  move  gently  through  the  water,  so  as  to 
avoid  crushing  its  soft  contents  against  its  sides.  Those  of  firmer  struc- 
ture (such  as  the  Entomostraca)  on  the  other  hand,  may  be  obtained  by 
the  use  of  a  Tow-net  attached  to  the  stern  of  a  sailing  vessel,  or  even  of 
a  steamer,  in  much  more  rapid  motion.^  When  this  metliod  is  employed, 
it  will  be  found  advantageous  to  make  the  net  of  conical  form,  and  to 
attach  to  its  deepest  part  a  wide-mouthed  bottle,  which  may  be  prevented 
from  sinking  too  deeply  by  suspending  it  from  a  cork  float:  into  this 
bottle  many  of  the  minute  Animals  caught  by  the  net  will  be  carried  by 
the  current  produced  by  the  motion  of  the  vessel  through  the  water,  and 
they  will  be  thus  removed  from  liability  to  injury.  It  will  also  be  useful 
to  attach  to  the  ring  an  inner  net,  the  cone  of  which,  more  obtuse  than 
that  of  the  outer,  is  cut  off  at  some  little  distance  from  the  apex;  this 
serves  as  a  kind  of  valve,  to  prevent  objects  once  caught  from  being 
washed  out  again.  The  net  is  to  be  drawn  in  from  time  to  time,  and  the 
bottle  to  be  thrust  up  through  the  hole  in  the  inner  cone;  and  its  contents 
being  transferred  to  a  screw-capped  bottle  for  examination,  the  net  may 
be  again  immersed.  This  form  of  net,  however,  is  less  suitable  for  the 
most  delicate  objects,  than  the  simple  Stich-net  used  in  the  manner  just 
described. — The  Microscopist  on  a  visit  to  the  seaside,  who  prefers  a 
quiet  row  in  tranquil  waters  to  the  trouble  (and  occasional  7nalaise)  of 
dredging,  will  find  in  the  collection  of  floating  Animals  by  the  careful 
use  of  the  Stick-net  or  Tow-net  a  never-ending  source  of  interesting 
occupation. 


'  In  the  *  Challenger'  Expedition,  Tow-nets  were  almost  constantly  kept  in  use, 
not  only  at  the  surfaee,  but  at  various  depths  beneath  it;  being  attached  to  a 
line  which  was  made  to  hang  vertically  in  the  water  by  the  attachment  of  heavy 
weights  at  its  extremity.  The  collections  thus  made  showed  the  enormous 
amount  of  minute  Animal  life  pervading  the  upper  waters  of  the  Ocean. 


222 


THE  MICEOSCOPE  AND  ITS  REVELATIONS. 


CHAPTER  VI. 

MICROSCOPIC  FORMS  OF  VEGETABLE  LIFE.— SIMPLER  ALG^. 

218.  Those  who  desire  to  make  themselves  familiar  with  Microscopic 
appearances,  and  to  acquire  dexterity  in  Microscopic  manipulation,  can- 
not do  better  than  educate  themselves  for  more  difficult  inquiries  by  the 
study  of  those  humblest  types  of  Vegetation,  which  present  Organic 
Structure  under  its  most  elementary  aspect.  And  such  as  desire  to 
search  out  the  nature  and  conditions  of  Living  Action,  will  find  in  the 
study  of  its  simplest  manifestations  the  best  clue  to  the  analysis  of  those 
intricate  and  diversified  combinations,  under  which  it  presents  itself  in 
the  highest  Animal  Organisms,    For  it  has  now  been  put  beyond  ques- 

,  tion,  tliat  the  fundamental  phenomena  of  Life  are  identical  in  Plants  and 
in  Animals;  and  that  the  living  substance  which  exhibits  them  is  of  a 
nature  essentially  the  same  throughout  both  Kingdoms.  The  determi- 
nation of  this  general  fact,  which  forms  the  basis  of  the  Science  of 
Biology,  is  the  most  important  result  of  modern  Microscopic  inquiry; 
and  the  illustration  of  it  will  be  kept  constantly  in  view,  in  the  exposi- 
tion now  to  be  given  of  the  chief  applications  of  the  Microscope  to  the 
study  of  those  minute  Protopliytes  (or  simplest  forms  of  Plant-life),  with 
whose  form  and  structure,  and  with  whose  very  existence  in  many  cases, 
we  can  only  acquaint  ourselves  by  its  aid. 

219.  It  was  formerly  supposed  that  living  action  could  only  be 
exhibited  by  organized  structure.  But  we  now  know  that  all  the  func- 
tions of  Life  may  be  carried  on  by  minute  ^jelly-specks,'  in  whose  appar- 
ently homogeneous  semi-fluid  substance  nothing  like  '  organization  ^  can 
be  detected;  and  further,  that  even  in  the  very  highest  organisms,  which 
present  us  with  the  greatest  yariety  of  ^  differentiated '  structures,  the 
essential  part  of  the  Life-work  is  done  by  the  same  material — these 
structures  merely  furnishing  the  mechanism  (so  to  speak)  through  which 
its  wonderful  properties  exert  themselves.  Hence  this  substance,^  known 
in  Vegetable  Physiology  as  protoplasm^  but  often  referred  to  by  Zoolo- 
gists as  sarcode,  has  been  appropriately  designated  by  Prof.  Huxley  ^^the 


^  Attention  was  drawn  in  1885  by  Dujardin  (the  French  Zoologist  to  whom  we 
owe  the  transfer  of  the  Foraminifera  from  the  highest  to  the  lowest  place  among 
Invertebrate  Animals),  to  the  fact  that  the  bodies  of  some  of  the  lowest  members 
of  the  Animal  kingdom  consist  of  a  structureless,  semi-fluid,  contractile  sub- 
stance, to  which  he  gave  the  name  sarcode  (rudimentary  flesh).  In  1851,  the 
eminent  botanist  Von  Mohl  showed  that  a  similar  substance  forms  the  essential 
constituent  of  the  cells  of  Plants,  and  termed  it  protoplasm  (primitive  plastic  or 
organizable  material).  And  in  1863  it  was  pointed  out  by  Prof.  Max  Schultze, 
who  had  made  a  special  study  of  the  Rhizopod  group,  that  the  *  sarcode '  of  Ani- 
mals and  the  *  protoplasm'  of  Plants  are  identicaL — See  his  Memoir  *'Ueber  das 
Protoplasma  der  Rhizopoden  und  Pflanzenzellen." 


MICROSCOPIC  FORMS  OF  VEGETABLE  LIFE. 


223 


Physical  Basis  of  Life.''  In  its  typical  state  (such  as  it  presents  among 
Rldzopods,  §  396)  it  is  a  semi-fluid,  tenacious,  gl^i^T  substance,  resem- 
bling— alike  in  aspect  and  in  composition — the  albumen  (or  uncoagulated 
^ white')  of  an  unboiled  egg.  But  it  is  fundamentally  distinguished 
from  that  or  any  other  form  of  dead  matter,  by  two  attributes,  which  (as 
being  peculiar  to  living  substances)  are  designated  vital : — 1,  its  power  of 
increase,  by  assimilating  (that  is,  converting  into  the  likeness  of  itself,  and 
endowing  with  its  own  properties)  nutrient  material  obtained  from  with- 
out; 2,  its  power  of  sponta^ieoiis  movement,  which  shows  itself  in  an 
extraordinary  variety  of  actions,  sometimes  slow  and  progressive,  some- 
times rapid,  sometimes  wave-like  and  continuous,  and  sometimes  rhyth- 
mical with  regular  intervals  of  rest.  When  examined  under  a  sufficiently 
high  magnifying  power,  multitudes  of  minute  granules  are  usually  seen 
to  be  diffused  through  it;  but  these  do  not  apjiear  to  belong  to  it,  their 
presence  being  (so  to  speak)  accidental,  depending  upon  the  nature  of 
the  material  which  is  undergoing  assimilation. — Protoplasm,  whether 
living  or  dead,  has  a  great  power  of  absorbing  water;  but  the  distinction 
between  these  two  states  is  singularly  marked  by  its  behavior  in  regard  to 
any  coloring  matter  which  the  water  may  contain.  Thus,  if  living 
protoplasm  be  treated  with  a  solution  of  carmine,  it  will  remain  un- 
stained so  long  as  it  retains  its  vitality.  But  if  the  protoplasm  be  dead, 
the  carmine  will  at  once  pervade  its  whole  substance,  and  stain  it 
throughout  with  a  color  even  more  intense  than  that  of  the  solution; 
thus  furnishing  (as  was  first  pointed  out  by  Dr.  Beale)  a  ready  means  of 
distinguishing  the  ^germinal  matter'  or  protoplasmic  component  of  the 
Tissues  of  higher  Animals,  from  the  ^formed  material'  which  is  the  most 
conspicuous  part  of  their  structure  (Chap,  xx.) 

220.  All  those  minute  and  simple  forms  of  Life  with  which  the 
Microscope  brings  us  into  acquaintance,  essentially  consist  of  particles  of 
protoplasm;  each  kind  having  usually  a  tolerably  definite  size  and  shape, 
and  showing  (at  least  in  some  stage  of  its  existence)  something  distinctive 
in  its  habit  of  life.  And  it  is  rather  according  to  the  manner  in  which 
they  respectively  live,  grow,  and  multiply,  than  on  account  of  any  struc- 
tural peculiarities,  that  they  are  assigned  to  the  Vegetable  or  to  the 
Animal  kingdom  respectively.  It  is  impossible,  in  the  present  state  of 
our  knowledge,  to  lay  down  any  definite  line  of  demarcation  between  the 
two  Kingdoms;  since  there  is  no  single  character  by  which  the  Animal 
or  Vegetable  nature  of  any  organism  can  bo  tested.  Probably  the  one 
which  is  most  generally  applicable  among  those  that  most  closely  approx- 
imate to  one  another,  is  not,  as  formerly  supposed,  the  presence  or 
absence  of  spontaneous  motion;  but,  on  the  one  hand,  the  dependence  of 
the  organism  for  nutriment  upon  organic  coinpoimds  already  formed, 
which  it  takes  (in  some  way  or  other)  into  the  interior  of  its  body;  or, 
on  the  other,  its  possession  of  the  power  of  producing  the  organic  com- 
pounds which  it  applies  to  the  increase  of  its  fabric,  at  the  expense  of 
the  inorganic  elements  with  which  it  is  supplied  by  Air  and  Water.  The 
former,  though  perhaps  not  an  absolute  is  a  general  characteristic  of  the 
Animal  kingdom;  the  latter,  but  for  the  existence  of  which  Animal  life 
would  be  impossible,  is  certainly  the  prominent  attribute  of  the  Vege- 
table.  We  shall  find  that  the  Protozoa  (or  simplest  Animals,  Chaps,  x., 
XI.)  are  supported  as  exclusively  either  upon  other  Protozoa  or  upon 
Protophytes,  as  are  the  highest  Animals  upon  the  flesh  of  other  Animals 
or  upon  the  products  of  the  Vegetable  kingdom;  whilst  Protophytes,  in 
common  with  the  highest  Plants,  draw  their  nourishment  from  the 


224 


THE  MICROSCOPE  AND  ITS  REVELATIONS. 


atmosphere  or  the  water  in  which  they  live;  and,  like  them,  are  distin- 
guished by  their  power  of  decomposing  Carbonic  acid  (CO^)  under  the 
influence  of  Light — setting  free  its  Oxygen,  and  combining  its  Carbon 
with  the  elements  of  Water  to  form  the  Carbo-hydrogen  compounds 
(Starch,  Cellulose,  etc.),  and  with  those  of  atmospheric  Ammonia  to 
form  Nitrogenous  (albuminoid)  compounds.  And  we  shall  find,  more- 
over, that  even  such  Protozoa  as  have  neither  stomach  nor  mouth, 
receive  their  alimentary  matter  direct  into  the  very  substance  of  their 
bodies,  in  which  it  undergoes  a  kind  of  digestion;  whilst  Protophyta 
absorb  through  their  external  surface  only,  and  take  in  no  solid  particles 
of  any  description.  With  regard  to  motion,  which  was  formerly  consid- 
ered the  distinctive  attribute  of  Animality,  we  now  know  not  merely  that 
many  Protophytes  (perhaps  all,  at  some  period  or  other  of  their  lives) 
possess  a  power  of  spontaneous  movement,  but  also  that  the  instruments 
of  motion  (when  these  can  be  discovered)  are  of  the  very  same  character 
in  the  Pla»t  as  in  the  Animal;  being  little  hair-like  filaments,  termed 
cilia  (from  the  Latin  ciliuni,  an  eye-lash),  or  longer  whip-like^(i^^eZ/a,  by 
whose  rhythmical  vibrations  the  body  of  which  they  form  part  is  pro- 
pelled in  definite  directions.  The  peculiar  contractility  of  these  organs 
seems  to  be  an  intensification  of  that  of  the  general  protoplasmic  sub- 
stance, of  which  they  are  special  extensions. 

221.  There  are  certain  Plants,  however,  which  resemble  Animals  in 
their  dependence  upon  Organic  compounds  prepared  by  other  organisms; 
being  themselves  unable  to  effect  that  fixation  of  Carbon  by  the  decom- 
position of  the  CO^  of  the  Atmosphere,  which  is  the  first  stage  in  their 
production.  Such  is  the  case,  among  Phanerogams  (flowering  plants), 
with  the  leafless  parasites'  which  draw  their  support  from  the  juices  of 
their  '  hosts.'  And  it  is  the  case  also,  among  the  lower  Cryptogams, 
with  the  entire  group  of  Fungi;  which,  however,  seem  generally  to 
depend  rather,  for  their  nutritive  materials,  upon  organic  matter  in  a 
state  of  decomposition,  many  of  them  having  the  power  of  promoting 
that  process  by  their  zymotic  (fermentative)  action  (Chap.  vii.). — Among 
Animals,  again,  there  are  several  in  whose  tissues  are  found  organic  com- 
pounds, such  as  Chlorophjdl,  Starch,  and  Cellulose,  which  are  charac- 
teristically Vegetable;  but  it  has  not  yet  been  proved  that  they  generate 
these  compounds  for  themselves,  by  the  decomposition  of  C0^ 

222.  The  plan  of  Organization  recognizable  throughout  the  Veg'eta- 
ble  kingdom  presents  this  remarkable  feature  of  uniformity, — that  the 
fabric,  alike  in  the  highest  and  most  complicated  Plants,  and  in  the  low- 
est and  simplest  forms  of  Vegetation,  consists  of  nothing  else  than  an 
aggregation  of  the  bodies  termed  Cells;  every  one  of  which  (save  in  the 
forms  that  lie  near  the  border-ground  between  Animal  and  Vegetable 
life)  has  its  little  particle  of  protoplasm  inclosed  by  a  casing  of  the  sub- 
stance termed  cellulose — a  non-nitrogenous  substance  nearly  allied  in 
chemical  composition  to  starch.  The  entire  mass  of  cells  of  which  any 
Vegetable  organism  is  composed,  has  been  generated  from  one  primordial 
cell  by  processes  of  self-multiplication  to  be  presently  described:  and  the 
difference  between  the  fabrics  of  the  lowest  and  of  the  highest  Plants 
essentially  consist  in  this, — that  whilst  the  cells  produced  by  the  self- 
multiplication  of  the  primordial  cell  of  the  Protophyte  are  all  mere 
repetitions  of  it  and  of  one  another,  each  living  ly  and  for  itself — those 
produced  by  the  like  self -multiplication  of  the  primordial  cell  in  the 
Oak  or  Palm,  not  only  remain  in  mutual  connection,  but  undergo  a  pro- 
gressive ^differentiation,'  the  ordinary  type  of  the  Cell  undergoing  vari' 


MICROSCOPIC  FORMS  OF   VEGETABLE  LIFE. 


225 


ous  modifications  to  be  described  in  their  proper  place  (Chap.  viii.). 
A  composite  structure  is  thus  developed,  which  is  made  up  of  a  number 
of  distinct  ^organs'  (stem,  leaves,  roots,  flowers,  etc.);  each  of  them 
characterized  by  specialties  not  merely  of  external  form,  but  of  intimate 
structure;  and  each  performing  actions  peculiar  to  itself,  which  contri- 
bute to  the  life  of  the  Plant  as  a  tuhole.  Hence,  as  was  first  definitely 
stated  by  Schleiden,  it  is  in  the  life  history  of  the  individual  cell  that  we 
find  the  true  basis  of  the  study  of  Vegetable  Life  in  general 

223.  We  have  now  to  consider  in  more  detail  the  structure  and  life- 
history  of  the  typical  Plant-cell;  and  shall  begin  by  treating  of  the  Cell- 
walh — This  consists  of  two  layers,  differing  entirely  in  composition  and 
properties.  It  is  the  inner,  termed  the  ^  primordial  utricle/  that  is  first 
formed,  and  is  most  essential  to  the  existence  of  the  cell;  it  is  extremely 
thin  and  delicate,  so  that  it  escapes  attention  so  long  as  it  remains  in  con- 
tact with  the  external  layer;  and  it  is  only  brought  into  view  when  sep- 
arated from  this,  either  by  developmental  changes  (Fig.  141,  a),  or  by 
the  influence  of  reagents  which  cause  it  to  contract  by  dravving-forth 
part  of  its  contents  (Pig.  139,  c).  It  is  not  sharply  defined  on  its  inter- 
nal face,  but  passes  gradationally  into  the  protoplasmic  substance  it  in- 
closes, from  which  it  is  chiefly  distinguishable  by  the  absence  of  granules. 
And  it  is  shown  by  the  effects  of  re-agents  to  have  the  albuminous  com- 
position of  protoplasm.  It  may  thus  be  regarded  as  the  slightly  con- 
densed external  film  of  the  protoplasmic  layer  with  which  its  inner  sur- 
face is  in  contact;  and  as  it  essentially  corresponds  with  the  ^  ectosarc ' 
of  Amoeba  or  any  other  Khizopod  (§  396),  it  may  be  termed  the  ecto- 
plasm,— The  outer  layer,  on  the  other  hand,  entirely  consists  of  cellulose, 
which  seems  to  be  excreted  from  the  surface  of  the  ^  ectoplasm '  for  the 
protection  of  its  contents;  it  is  usually  thick  and  strong,  and  can  often 
be  seen  to  consist  of  several  layers.  The  '  ectoplasm '  and  '  cellulose  wall ' 
can  be  readily  distinguished  from  one  another  by  Chemical  tests  (§  204); 
and  also  by  the  action  of  Carmine,  which  stains  the  protoplasmic  sub- 
stance (when  dead)  without  affecting  the  cellulose-wall. 

224.  The  contents  of  the  Plant-cell,  which  may  be  collectively  termed 
the  endoplasm  (answering  to  the  ^endosarc'  of  Ehizopods),  or,  when 
strongly  colored  throughout  (as  in  many  Algce)  the  endochrome,  consists 
in  the  first  place  of  the  layer  of  protoplasmic  substance  which  lines  the 
^ ectoplasm;^  secondly,  of  a  watery  fluid,  called  ^ cell-sap,^  which  holds 
in  solution  sugar,  vegetable  acids,  saline  matters,  etc. ;  thirdly,  of  the  pe- 
culiar body  termed  the  ^nucleus;'  and  fourthly,  of  chlorophyll-cor- 
puscles (inclosing  starch-granules),  oil-particles,  etc. — In  the  young  state 
of  the  cell,  the  whole  cavity  is  occupied  by  the  protoplasmic  substance, 
which  is,  however,  viscid  and  granular  near  the  cell-wall,  but  more  watery 
towards  the  interior.  With  the  enlargement  of  the  cell  and  the  imbibi- 
tion of  water,  clear  spaces  termed  vacuoles,  filled  with  watery  cell  sap, 
are  seen  in  the  protoplasmic  substance  ;  and  these  progressively  increase 
in  size  and  number,  until  they  come  to  occupy  a  considerable  proportion 
of  the  cavity,  the  protoplasm  stretching  across  it  as  an  irregular  network 
of  bands.  Where,  as  usually  happens,  the  ^  nucleus '  lies  imbedded  in 
the  outer  protoplasmic  layers,  these  bands  are  gradually  withdrawn  into 
it,  so  that  the  separate  vacuoles  unite  into  one  large  general  vacuole  which 
is  filled  with  watery  cell-sap.  But  where  the  ^  nucleus  ^  occupies  the  cen- 
tre of  the  cell,  part  of  the  protoplasm  collects  around  it,  and  bands  or 
threads  of  protoplasm  stretch  thence  to  various  parts  of  the  parietal  layer. 
It  is  by  the  contractility  of  the  protoplasmic  layer,  that  the  curious  '  cy- 

15 


228 


THE  MICROSCOPE  AKD  ITS  REVELATIONS. 


closis '  hereafter  to  be  described  (§  258)  is  carried-on  within  the  Plant-cell, 
which  is  the  most  interesting  to  the  Microscopist  of  all  its  manifestations 
of  yital  activity. — The  nucleus  is  a  small  body,  usually  of  lenticular  or 
sub-globose  form  (Fig.  139,  A,  a)^  and  of  albuminous  composition,  that 
lies  imbeded  in  protoplasmic  substance,  either  on  the  cell-wall  or  in  the 
central  cavity.  It  is  not,  however,  constantly  present  even  in  the  higher  | 
forms  of  cell-structure  ;  for  in  those  cells  whose  active  life  has  been  com- 
pleted, the  nucleus  is  usually  absent,  having  probably  been  resolved  again 
into  the  protoplasm  from  which  it  was  originally  formed.  And  in  the 
cells  of  many  of  the  lower  Cryptogams,  ifcannot  be  distinguished  at  any 
stage  of  their  existence.  Within  the  nucleus  are  often  seen  one  or  more 
small  distinct  particles  termed  nucleoli  (Fig.  139,  A,  h),  which  can  be 
best  distinguished  by  the  strong  coloration  they  receive  from  a  24  hours' 
immersion  in  carmine,  and  subsequent  washing  in  water  slightly  acidu- 
lated with  acetic  acid.  Though  the  precise  function  of  the  nucleus  is 
still  unknown,  there  can  be  no  reasonable  doubt  of  its  peculiar  relation 
to  the  vital  activity  of  the  cell:  for  in  the  nucleated  cells  which  exhibit 
^  cyclosis,'  it  may  be  observed  that  if  the  nucleus  remains  attached  to  the 
cell-wall,  it  constitutes  a  centre  from  which  the  protoplasmic  streams 
diverge,  and  to  which  they  return;  whilst  if  it  retains  its  freedom  to  wan- 
der about,  the  course  of  the  streams  alters  in  conformity  with  its  position. 
But  it  is  in  the  multiplication  of  cells  by  binary  subdivision  which 
will  be  presently  described  (§  226),  that  the  speciality  of  the  nucleus  as 
the  centre  of  the  vital  activity  of  the  cell  is  most  strongly  manifested. 
— The  chlorophyll  corpuscles,  which  are  limited  to  the  cells  of  the  parts  of 
plants  acted-on  by  light,  are  specialized  particles  of  protoplasm  through 
which  a  green  coloring  matter  is  difEused  :  and  it  is  by  them  that  the 
work  of  decomposing  C0%  and  of  ^fixing'  its  carbon,  by  union  with  the 
oxygen  and  hydrogen  of  water,  into  starch  (which  seems  to  be  the  basis 
of  all  other  vegetable  compounds),  is  effected.  The  characteristic  green 
of  chlorophyll  often  gives  place  to  other  colors,  which  seem  to  be  pro- 
duced from  it  by  chemical  action. — Starch-grains  are  always  formed  in 
the  first  mtance  in  the  interior  of  the  chlorophyll-corpuscles,  and  gradu- 
ally increase  in  size  until  they  take  the  places  of  the  corpuscles  that  pro- 
duced them.  So  long  as  they  continue  to  grow,  they  are  always  imbeded  in 
the  protoplasm  of  the  cell ;  and  it  is  only  when  fully  formed,  that  they 
lie  free  within  its  cavity  (Fig.  246). 

225.  But  although  these  component  parts  may  be  made-out  without 
any  difficulty  in  a  large  proportion  of  Vegetable  Cells,  yet  they  cannot  be 
distinguished  in  some  of  those  humble  organisms  which  are  nearest  to 
the  border-line  between  the  two  Kingdoms.  For  in  them  we  find  tho 
^  cell-wall '  very  imperfectly  differentiated  from  the  '  cell-contents;'  the 
former  not  having  by  any  means  the  firmness  of  a  perfect  membrane,  and 
the  latter  not  possessing  the  liquidity  which  elsewhere  characterizes 
them.  And  in  some  instances  the  cell  appears  to  be  represented  only  by 
a  mass  of  endochrome,  so  viscid  as  to  retain  its  external  form  without 
any  limitary  membrane,  though  the  superficial  layer  seems  to  have  a 
firmer  consistence  than  the  interior  substance;  and  this  may  or  may  not 
be  surrounded  by  a  gelatinous-looking  envelope,  which  is  equally  far 
from  possessing  a  membranous  firmness,  and  yet  is  the  only  representative 
of  the  cellulose-wall.  This  viscid  endochrome  consists,  as  elsewhere,  of 
a  colorless  protoplasm,  through  which  minute  coloring  particles  are 
diffused,  sometimes  uniformly,  sometimes  in  local  aggregations,  leaving 
parts  of  the  protoplasm  uncolored.    The  superficial  layer,  in  particular, 


MICROSCOPIC  FORMS  OF  VEGETABLE  LIFE. 


227 


is  frequently  destitute  of  color;  and  the  partial  solidification  of  its  surface 
gives  it  the  character  of  an  ^ectoplasm/  The  nucleus,  as  already  men- 
tioned, is  frequently  absent  from  the  cells  of  the  lower  Protophytes. — It 
is  an  extremely  curious  feature  in  the  cell-life  of  certain  Frotophi/tes,  that 
they  not  only  move  like  Animalcules  by  cilia  or  flagella,  but  that  they, 
exhibit  the  rhythmically-contracting  vacuoles  which  are  specially  charac-i 
teristic  of  Protozoic  organisms.  ^ 
226.  So  far  as  we  yet  know,  every  Vegetable  Cell  derives  its  existence 
from  a  pre-existing  cell;  and  this  derivation  may  take  place  (in  the  ordi- 
nary process  of  growth  and  extension,  as  distinguished  from  '  sexual 
generation')  in  one  of  two  modes: — either  (1)  binary  subdivision  of  the 
parent-cell,  or  (2)  free  cell-formation  within  the  parent-cell. — The  first 
stage  of  the  former  process  consists  in  the  elongation  and  transverse 
constriction  of  the  nucleus;  and  this  constriction  becomes  deeper  and 
deeper,  until  the  nucleus  divides  itself  into  two  halves  (Fig.  139,  b,  a,  a'). 
These  then  separating  from  each 
other,  the  endoplasm  of  the  pa- 
rent-cell collects  round  the  two 
new  centres,  so  as  to  divide  itself 
into  two  distinct  masses  (c,  a,  a')) 
and  by  the  investment  of  these 
two  secondary  '  endoplasms,'  first 
with  ^ectoplasms,' and  afterwards 
with  cellulose-walls,  a  complete 
pair  of  new  cells  (d,  a,  a')  is 
formed  within  the  cavity  of  the 
parent-cell. — The  latter  process, 
which  is  very  common  among 
Protophytes  (being  that  by  which  ^| 
^zoospores,'  or  ^swarm-spores,'  ^( 
are  commonly  produced,  §  245),  ^  - 
is  chiefly  seen  among  Phanero- 
gams in  the  production  of  a  num- 
ber of  cells  at  once  within  the 
cavity  of  the  '  embryo-sac '  (§  349), 
which  may  itself  be  considered 
as  a  distended  parent-cell.  The 

endoplasm,  in  the  former  of  these  Duplicative  subdivision  of  CelU  in  End^^sperm 
cases,  instead  of  dividing  itself  of  seed  of  Scarlet-runner:-A  ordinary  cell,  with 
.    ,     I       11  nil  nucleus  a,  and  nucleolus  6,  imbedded  in  its  proto- 

mtO  two  halves,  usually  breaks  up  plasm;-B,  cell  showing  subdivision  of  nucleus  into 
infn  nnmprnnQ  QPo-TnPTif<2  pnrvp  two  halves,  a  and  a' ;—c,  cell  in  same  stage,  showing 
into  numeiOUS  segments  COire-  contraction  of  endoplasm  (produced  by  addition  of 
spending  with  one  another  m  size  water),  into  two  separate  masses  round  thetwoseg- 
^t.A  4?^^  /T7^^  1  /<n\  ^^^1,  ^4?  ments  of  original  nucleus;— d,  two  complete  cells 
and    lorm    (J^lg.     149),    each   Ot  within  mother-cell,  divided  by  partition. 

which  escaping  from  the  parent 

cavity  becomes  an  independent  cell,  and  gives  origin  by  duplica- 
tive subdivision  to  a  new  fabric.  In  the  second  case,  the  endoplasm 
groups  itself,  more  or  less  completely,  round  several  centres,  each  of 
which  may  or  may  not  contain  a  nucleus  in  the  first  instance;  and  these 
secondary  cells,  in  various  stages  of  development,  lie  free  within  the 
cavity  of  the  parent-cell,  imbedded  in  its  residual  endoplasm,  each 
proceeding  to  complete  itself  as  a  cell  by  the  formation  of  a  limiting  wall, 
and  by  the  development  of  a  nucleus  if  none  was  previously  present 
(Fig.  140).  Now,  in  this  second  case,  as  the  new  brood  of  cells  continues 
to  form  part  of  the  fabric  in  which  it  originated,  its  production  is  clearly 


228 


THE  MICROSCOPE  AND  ITS  REVEL  ACTIONS. 


Fig; 


an  act  of  groiuth;  and  although,  in  the  first  case,  the  setting-free  of  the 
'  swarm-spores'  from  the  parent-cell  calls  into  existence  a  fresh  brood  of 
secondary  organisms,  this  is  no  more  to  be  regarded  in  strictness  as  a 
^new  generation,'  than  is  the  putting-forth  of  a  new  set  of  leaf-buds  by 
a  tree — every  one  of  them,  when  separated  from  its  stock,  deyeloping 

itself  under  favorable  conditionst 
into  the  likeness  of  that  which 
produced  it.  As  a  ^new  gen- 
eration,' in  any  Phanerogamic 
plant,  has  its  origin  in  the  fer- 
tilization of  a  highly  specialized 
^  germ-cell '  (contained  within 
the  ovule)  by  the  contents  of  a 
^sperm-cell'  (the  pollen-grain) 
so  do  we  find  among  all  save  the 
lowest  Cryptogams  a  provision 
the  union  of  the  contents  of 
highly  specialized  cells;  the 
rm-cells'  being  fertilized  by 
the  access  of  motile  filaments 
(antherozoids),  set  free  from  the 

Successive  stages  of  free  Cell-formation  in  Em-  ^ovifipa    nf     flip     ^  cr»orm  noil  a  ' 
biyo-sac  of  Seed  of  Scarlet-runner ;-a,  a,  a,  com-  CavlUCb     UI     tlie  Speim-CCllS 
pleted  cells,  each  having  its  proper  cell-wall,  nucleus,  withm  whlch  tlieV  WCrC  dcvelop- 
andendoplasm,  lying  in  a  protoplasmic  mass,  through         /o  cj^qx       -d..;    oUhr»nfT>i  flio 
which  are  dispersed  nuclei  and  cells  m  various  stages  tiu        KiOV),     jjuu  di  iiiuugn  iiie 

of  development.  sexual  proccss  Can  be  traced 

downwards  under  this  form 
into  the  group  of  Protophytes,  we  find  among  the  lower  types  of  that 
group  a  yet  simpler  mode  of  bringing  it  about;  for  there  is  strong  reason 
to  regard  the  act  of  ^conjugation,'  which  takes  place  among  the  ^unicel- 
lular' Algce  (§§  229,  235),  in  the  same  light,  and  to  look  upon  the 
^ oospore'^  which  is  its  immediate  product,  as  the  originator  (like  the 
fertilized  embryo-cell  of  the  Phanerogamic  seed)  of  a  '  new  generation.' 

226.  In  the  lowest  form  of  vegetation,  every  single  cell  is  not  only 
capable  of  living  in  a  state  of  isolation  from  the  rest,  but  even  normally 
does  so;  and  thus  the  plant  may  be  said  to  be  unicellular,  every  cell  having 
an  independent  ^individuality.'  There  are  others,  again,  in  which 
amorphous  masses  are  made  up  by  the  aggregation  of  cells,  which,  though 
quite  capable  of  living  independently,  remain  attached  to  each  other  by 
the  mutual  fusion  (so  to  speak)  of  their  gelatinous  investments.  And 
there  are  others,  moreover,  in  which  a  definite  adhesion  exists  between 
the  cells,  and  in  which  regular  plant-like  structures  are  thus  formed. 


^  The  term  spore  has  been  long  used  by  Cryptogamists  to  designate  the  minute 
reproductive  particles  (such  as  those  set  free  from  the  '  fructification '  of  Ferns, 
Mosses,  etc.),  which  were  supposed — in  the  absence  of  all  knowledge  of  their 
sexual  relations — to  be  the  equivalents  of  the  Seeds  of  Flowering  plants.  But  it  is 
now  known  that  such  *  spores'  have  (so  to  speak)  very  different  values  in  differ- 
ent cases;  being,  in  by  far  the  larger  proportion  of  Cryptogams,  but  the  remote 
descendants  of  the  fertilized  cell  which  is  the  immediate  product  of  the  sexual 
act  under  any  of  its  forms.  This  cell,  which  will  be  distinguished  throughout  the 
present  treatise  as  the  oospore,  is  the  real  representative  of  the  '  primordial  cell ' 
of  the  *  embryo'  developed  within  the  seed  of  the  Flowering  plant.  On  the  other 
hand,  the  various  kinds  of  non-sexual  spores  emitted  by  Cryptogams,  which  have 
received  a  great  variety  of  designations,  are  all  to  be  regarded  (as  will  be  pres- 
ently explained)  as  equivalents  of  the  leaf 'buds  of  Flowering  plants.  (See  the 
next  Note.) 


MICROSCOPIC  FORMS  OF  VEGETABLE  LIFE. 


22& 


notwithstanding  that  every  cell  is  but  a  repetition  of  every  other,  and  is 
capable  of  living  independently  if  detached,  so  as  still  to  answer  to  the 
designation  of  a  'unicellular^  or  single-celled  Plant,  These  different 
conditions  we  shall  find  to  arise  out  of  the  mode  in  which  each  particular 
species  multiplies  by  binary  subdivision  (§  226):  for  where  the  cells  of 
the  new  pair  that  is  produced  within  the  previous  cell  undergo  a  complete 
separation  from  one  another  they  will  henceforth  live  independently; 
but,  if,  instead  of  undergoing  this  complete  fission,  they  should  be  held 
together  by  the  intervening  gelatinous  envelope,  a  siiapeless  mass  results 
from  repeated  subdivisions  not  taking  place  on  any  determinate  plan^ 
and  if,  moreover,  the  binary  subdivision  should  always  take  place  in  a 
determinate  direction,  a  long  narrow  filament  (Fig.  145,  d),  or  a  broad 
flat  leaf-like  expansion  (g),  may  be  generated.  To  such  extended  fabrics 
the  term  'unicellular'  plants  can  scarcely  be  applied  with  propriety; 
since  they  may  be  built-up  of  many  thousands  or  millions  of  distinct 
cells,  which  have  no  disposition  to  separate  from  each  other  spontane- 
ously. Still  they  correspond  with  those  which  are  strictly  unicellular,  as 
to  the  absence  of  differentiation  either  in  structure  or  in  actions  between 
their  component  cells;  each  one  of  these  being  a  repetition  of  the  rest, 
and  no  relation  of  mutual  dependence  existing  among  them.  And  all 
such  simple  organisms,  therefore,  may  still  be  included  under  the  general 
term  of  Protophytes. 

228.  Excluding  Lichens,  for  the  reasons  to  be  stated  hereafter  (§  325), 
Botanists  now  rank  these  Protophytes  under  two  series: — AlgcB,  which 
form  chlorophyll,  and  can  support  themselves  upon  air,  water,  and  min- 
eral matters;  and  Fungi,  which,  not  forming  chlorophyll  for  themselves, 
depend  for  their  nutriment  upon  materials  drawn  from  other  organisms. 
Each  series  contains  a  large  variety  of  forms,  which,  when  traced  from 
below  upwards,  present  gradationally  increasing  complexities  of  struc- 
ture;  and  these  gradations  show  themselves  especially  in  the  provisions 
made  for  the  Generative  process.  Thus,^n  the  lowest,  a  'zygospore'  is 
produced  by  the  fusion  of  the  contents  of  two  cells,  which  neither  present 
any  sexual  difference,  the  one  from  the  other,  nor  can  be  distinguished 
in  any  way  from  the  rest  (§  229).  In  the  following  stage,  while  the  '  cour 
jugating'  cells  are  still  apparently  undifferentiated  from  the  rest  of  the 
structure,  a  sexual  difference  shows  itself  between  them;  the  contents  of 
one  cell  (male)  passing  over  into  the  cavity  of  the  other  (female),  within 
which  the  '  zygospore'  is  formed  (§  235).  The  next  stage  in  the  ascent 
is  the  resolution  of  the  contents  of  the  male  cell  into  motile  filaments 
('antheroids'),  which,  escaping  from  it,  move  freely  through  the  watej, 
and  find  their  way  to  the  female  cell,  whose  contents,  fertilized  by  mix- 
ture with  the  material  they  bring  (§  249),  form  an  'oospore.'  In  tha 
lower  forms  of  this  stage,  again,  the  generative  cells  are  not  distinguish- 
able from  the  rest,  until  the  contents  begin  to  show  their  characteristic- 
ally sexual  aspect  (§  253);  but  in  the  higher  they  are  developed  in  special 
organs,  constituting  a  true  '  fructification  '  (§  259).  This  must,  how- 
ever, be  distinguished  from  organs,  which,  though  commonly  spoken  of 
as  the  '  fructification,'  have  no  real  analogy  with  the  generative  apparatus^ 
of  Flowering-plants;  their  function  being  merely  to  give  origin  to  goni-^ 
dial  ^  cells  or  groups  of  cells,  which  simply  multiply  the  parent  stock,  in 

^  The  term  Gonidia^  originally  applied  to  certain  green  cells  in  the  Lichen- 
crusts,  that  are  capable,  when  detached,  of  reproducing  the  vegetative  portion 
of  the  Plant  (g  325),  has  latterly  come  into  use  as  a  designation  of  the  non-sexual 
spores  of  Cryptogamia  generally,  which  it  is  very  important  to  discriminate  from 


^30 


THE  MICBOSCOPBi  AND  ITS  REVELATIONS. 


the  same  manner  that  many  Flowering-plants  (such  as  the  Potato),  can 
be  propagated  by  the  artificial  separation  of  their  leaf -buds.  It  fre- 
quently happens  among  Cryptogamia,  that  this  .^omWmZ  fructification  is 
by  far  the  more  conspicuous;  the  sexual  fructification  being  often  so  ob- 
scure that  it  cannot  be  detected  at  all  without  great  difficultv.  And  we 
shall  presently  see  that  there  are  some  Protophytes  in  which  the  produc- 
tion of  gonidia  seems  to  go  on  indefinitely,  no  form  of  sexual  generation 
having  been  detected  in  them  (§  245). — These  general  statements  will 
now  be  illustrated  by  sketches  of  the  Life-history  of  some  of  those  humble 
Protophytes,  which  present  the  phenomena  of  cell-division,  conjugation, 
and  gonidial  multiplication,  under  their  simplest  and  most  instructive 
aspect, 

229.  The  first  of  these  is  the  Palmoglma  macrococca  (Kiitzing);  one 
of  those  humble  kinds  of  vegetation  which  spreads  itself  as  a  green  slime 
over  damp  stones,  walls,  etc.  When  this  slime  is  examined  with  the 
microscope,  it  is  found  to  consist  of  a  multitude  of  green  cells  (Plate 
VIII.,  fig.  1,  a),  each  surrounded  by  a  gelatinous  envelope;  the  cell, 
which  does  not  seem  to  have  any  distinct  membranous  wall,  is  filled  with 
a  granular  ^endochrome^  consisting  of  green  particles  diffused  through 
colorless  protoplasm;  and  in  the  midst  of  this  a  nucleus  may  sometimes 
be  distinguished,  but  can  always  be  brought  into  view  by  tincture 
of  iodine,  which  turns  the  ^endochrome^  cell  to  a  brownish  hue,  and 
makes  the  nucleus  (g)  dark  brown.  Other  cells  are  seen  (b),  which  are 
considerably  elongated,  some  of  them  beginning  to  present  a  sort  of 
hour-glass  contraction  across  the  middle;  and  when  cells  in  this  condition 
are  treated  with  tincture  of  iodine,  the  nucleus  is  seen  to  be  undergoing 
the  like  elongation  and  constriction  (h).  A  more  advanced  state  of  the 
process  of  subdivision  is  seen  at  c,  in  which  the  constriction  has  pro- 
ceeded to  the  extent  of  completely  cutting-off  the  two  halves  of  the  cell, 
as  well  as  of  the  nucleus  (i),  from  each  other,  though  they  still  remain  in 
mutual  contact;  but  in  a  yet  later  stage  they  are  found  detached  from 
each  other  (d),  though  still  included  within  the  same  gelatinous  envelope. 
Each  new  cell  then  begins  to  secrete  its  own  gelatinous  envelope,  so  that 
by  its  intervention,  the  two  aro  usually  soon  separated  from  one  another 
(e).  Sometimes,  however,  this  is  not  the  case;  the  process  of  subdivision 
being  quickly  repeated  before  there  is  time  for  the  production  of  the 
gelatinous  envelope,  so  that  a  series  of  cells  (f)  hanging-on  one  to  another 
is  produced.  There  appears  to  be  no  definite  limit  to  this  kind  of  mul- 
tiplication; and  extensive  areas  maybe  quickly  covered,  in  circumstances 
favorable  to  the  growth  of  the  plant,  by  the  products  of  the  binary  sub- 
division of  one  primordial  cell.  This,  as  already  shown  (§  226),  is  really 
an  act  of  growth^  which  continues  indefinitely  so  long  as  moisture  is 
abundant,  and  the  temperature  low. — But  under  the  influence  of  heat 
and  dryness,  the  process  of  cell-multiplication  gives  place  to  that  of  '  con- 


the  generative  *  oospores.'  If  possessed  of  moh'Ze  powers,  they  are  spoken  of  as 
*  zoospores,'  or  sometimes  (on  account  of  the  appearance  they  present  when  a 
number  are  set  free  at  once)  as  *  swarm-spores.'  In  contradistinction  to  *  motile ' 
gonidia  or  *  zoospores,'  those  which  show  no  movement  are  often  termed  resting 
spores  or  statospores:  but  such  may  be  either  sexual  oospores  or  non-sexual 
gonidia;  the  latter,  like  the  former,  often  *  encysting'  themselves  in  a  firm 
envelope,  and  remaining  dormant  within  it  for  long  periods  of  time.  Gonidial 
spores,  again,  are  sometimes  distinctively  named  according  to  their  size;  some  of 
them,  which  consist  of  numerous  cell-particles  clustered  together,  being  desig- 
nated macro-gonidia,  in  contrast  to  the  micro-gonidia  consisting  of  single  cell- 
particles,  which,  when  motile,  are  known  as  *  zoospores.' 


MICBOSOOPIC  FORMS  OF  VEGETABLE  LIFE. 


231 


ELATE  Vni, 


DEVELOPMENT  OP  PALMOGLJEA  AND  PROTOCOCcus  (after  Braun  and  Cohn). 

Fig.  1,  A— I.  Successive  stages  of  binary  subdivision  of  Palmoglcea  ;  k— m,  successive  stages  of 
•onjugation. 

2,  A— c.  Binary  subdivision  of  'still'  form  ot  Proctococcus ;  d—g,  multiplication  of  motile 
form;      l,  different  phases  of  *  motile '  condition. 


232 


THE  MICROSCOPE  AND  ITS  KEVELATIONS. 


jugation;'  in  which  two  cells,  apparently  similar  in  all  respects,  fuse 
together  for  the  production  of  a  ^  zygospore/ Avhich  (like  the  seed  of  a 
Flowering-plant)  can  endure  being  reduced  to  a  quiescent  state  for  an  un- 
limited time,  and  may  be  so  completely  dried  up  as  to  seem  like  a  par- 
ticle of  dust,  yet  resumes  its  vegetative  activity  whenever  placed  in  the 
conditions  favorable  to  it.  The  conjugating  process  commences  by  the 
putting-fortli  of  protrusions  from  the  boundaries  of  two  adjacent  cells, 
which  meet,  fuse  together  (thereby  showing  the  want  of  firmness  of  their 
^  ectoplasms and  form  a  connecting  bridge  between  their  cavities  (k). 
The  fusion  extends  before  long  through  a  large  part  of  the  contiguous 
sides  of  the  two  cells  (l);  and  at  last  becomes  so  complete,  that  the  com- 
bined mass  (m)  shows  no  trace  of  its  double  origin.  It  soon  forms  for 
itself  a  firm  cellulose  envelope,  which  bursts  when  the  '  zygospore '  is 
wetted;  and  the  contained  cell  begins  life  as  a  new  generation^  speedily 
multiplying,  like  the  former  ones,  by  binary  subdivision. — It  is  curious 
to  observe  that  during  this  conjugating  process  a  production  of  oil-parti- 
cles takes  place  in  the  cells;  those  at  first  are  small  and  distant,  but 
gradually  become  larger,  and  approximate  more  closely  to  each  other,  and 
at  last  coalesce  so  as  to  form  oil-drops  of  various  sizes,  the  green  granular 
matter  disappearing;  and  the  color  of  the  conjugated  body  changes, 
with  the  advance  of  this  process,  from  green  to  a  light  yellowish-brown. 
When  the  zygospore  begins  to  vegetate,  on  the  other  hand,  a  converse 
change  occurs;  the  oil-globules  disappear,  and  green  granular  matter 
takes  their  place.  This  is  precisely  what  happens  in  the  formation  of 
the  seed  among  the  higher  Plants;  for  starchy  substances  are  transformed 
into  oil,  which  is  stored  up  in  the  seed  for  the  nutrition  of  the  embryo, 
and  is  applied  during  germination  to  the  purposes  which  are  at  other 
times  answered  by  them. 

230.  If  this  (as  seems  probable)  constitutes  the  entire  life-cycle,  of  the 
Palmoglcea^  it  affords  no  example  of  that  curious  '  motile'  stage  which  is 
exhibited  by  most  Algal  protophytes  in  some  stage  of  their  existence,  and 
which  constitutes  a  large  part  of  the  life  history  of  the  minute  unicellular 
organism  now  to  be  described,  the  Protococcus  pluvialis^  (Plate  viii.,  fig. 


^  The  Author  had  under  his  own  observation,  thirty-five  years  ago,  an  extra- 
ordinary abundance  of  what  he  now  feels  satisfied  must  have  been  this  Protophyte, 
in  an  open  rain-water  cistern  which  had  been  newly  cleaned-out.  His  notice  was 
attracted  to  it  by  seeing  the  surface  of  the  water  covered  with  a  green  froth, 
whenever  the  sun  shone  upon  it  On  examining  a  portion  of  this  froth  under  the 
Microscope,  he  found  that  the  water  was  crowded  with  green  cells  in  active 
motion;  and  although  the  only  bodies  at  all  resembling  them  of  which  he  could 
find  any  description,  were  the  so-called  Animalcules  constituting  the  genus 
Chlamydomonas  of  Prof.  Ehrenberg,  and  very  little  was  known  at  that  time  of 
the  *  motile '  conditions  of  Plants  of  this  description,  yet  of  the  Vegetable  nature 
of  these  bodies  he  could  not  entertain  the  smallest  doubt.  They  appeared  in 
freshly  collected  rain-water,  and  could  not,  therefore,  be  deriving  their  support 
from  Organic  matter :  under  the  influence  of  light  they  were  obviously  decompos- 
ing Carbonic  acid  and  liberating  Oxygen;  and  this  influence  he  found  to  be  essen- 
tial to  the  continuance  of  their  growth  and  development,  which  took  place 
entirely  upon  the  Vegetative  plan.  Not  many  days  after  the  Protophyte  first 
appeared  in  the  Water,  a  few  Wheel -animalcules  presented  themselves:  these  fed 
greedily  upon  it,  and  increased  so  rapidly  (the  weather  being  very  warm)  that 
they  speedily  became  almost  as  crowded  as  the  cells  of  the  Proctococcus  had  been; 
and  it  was  probably  due  in  part  to  their  voracity,  that  the  plant  soon  became  less 
abundant,  and  before  long  disappeared  altogether.  Had  the  Author  been  then  aware 
of  its  assumption  of  the  *  still '  condition  he  might  have  found  it  at  the  bottom  of 
the  cistern,  after  it  had  ceased  to  present  itself  at  the  surface. — The  account  of 
this  Plant  given  above  is  derived  from  that  of  Dr.  Cohn,  in  the    Nova  Acta  Acad, 


MICROSCOPIC  FOKMS  OF  VEGETABLE  LIFE. 


233 


'2),  which  is  not  uncommon  in  collections  of  rain-water.  Not  only  has  this 
Protophyte^  in  its  motile-condition,  been  very  commonly  regarded  as  an 
Animalcule,  but  its  different  states  have  been  described  under  several  differ- 
ent names.  In  the  first  place,  the  color  of  its  cells  varies  considerably; 
since,  although  they  are  usually  green  at  the  period  of  their  most  actiyc 
life,  they  are  sometimes  red;  and  their  red  form  has  received  the  distin- 
guishing appellation  of  Hcematococcus.  Very  commonly  the  red-coloring 
matter  forms  only  a  central  mass  of  greater  or  less  size,  having  the  appear^ 
ance  of  a  nucleus  (as  shown  at  e);  and  sometimes  it  is  reduced  to  a  single 
granular  point,  which  has  been  erroneously  represented  by  Prof.  Bhren- 
berg  as  the  eye  of  these  so-called  Animalcules.  It  is  quite  certain  that 
the  red  coloring-substance  is  very  nearly  related  in  its  chemical  character 
to  the  green,  and  that  the  one  may  be  converted  into  the  other  :  though 
the  conditions  under  which  this  conversion  takes  place  are  not  precisely 
known.  In  the  still  form  of  the  cell,  with  which  we  may  commence  the 
history  of  its  life,  the  endoplasm  consists  of  a  colorless  protoplasm, 
through  which  red  or  green-colored  granules  are  more  or  less  uniformly 
diffused:  and  the  surface  of  the  colorless  protoplasm  is  condensed  into 
an  ectoplasm,  which  is  surrounded  by  a  tolerably  firm  layer,  consisting  of 
cellulose  or  of  some  modification  of  it.  Outside  this  (as  shown  at  a), 
when  the  '  still '  cell  is  formed  by  a  change  in  the  condition  of  a  cell  that 
has  been  previously  *  motile,^  we  find  another  envelope,  which  seems  to 
be  of  the  same  nature,  but  which  is  separated  by  the  interposition  of 
aqueous  fluid;  this,  however,  may  be  altogether  wanting.  The  multipli- 
cation of  the  ^  still  ^  cells  by  subdivision  takes  place  as  in  Palmoglcsa;  the 
endoplasm  first  undergoing  separation  into  two  halves  (as  seen  at  b),  and 
each  of  these  halves  subsequently  developing  a  cellulose  enveloj)e  around 
itself,  and  undergoing  the  same  division  in  its  turn.  Thus,  2,  4,  8,  IG 
new  cells  are  successively  produced;  and  these  are  sometimes  set-free  by 
the  complete  dissolution  of  the  envelope  of  the  original  cell;  but  they  are 
more  commonly  held  together  by  its  transformation  into  a  gelatinous  in- 
vestment, in  which  they  remain  imbedded.  Sometimes  the  endoplasm 
subdivides  at  once  into  four  segments  (as  at  d),  of  which  every  one  forth- 
with acquires  the  characters  of  an  independent  cell;  but  this,  although  an 
ordinary  method  of  multiplication  among  the  '  motile^  cells,  is  compara- 
tively rare  in  the  ^  still  ^  condition.  Sometimes,  again,  the  endoplasm  of 
the  *  still  ^  form  subdivides  at  once  into  eight  portions,  which,  being  of 
small  size,  and  endowed  with  motile  power,  may  be  considered  as  zoospores. 
It  is  not  quite  clear  what  becomes  of  these;  but  there  is  reason  to 
believe  that  some  of  them  retain  their  motile  powers,  and  develop  them- 
selves into  the  ordinary  ^  motile  ^  cells;  that  others  produce  a  firm  cellu- 
lose envelope,  and  become  ^  still  ^  cells;  and  that  others  (perhaps  the 
majority)  perish  without  any  further  change. 

231.  When  the  ordinary  self-division  of  the  ^ still'  cells  into  two  seg- 
ments has  been  repeated  four  times,  so  as  to  produce  16  cells — and  some- 
times at  an  earlier  period — the  new  cells  thus  produced  assume  the 
^ motile^  condition;  being  liberated  before  the  development  of  the  cellulose 
envelope,  and  becoming  furnished  with  two  long  y\hY2iti\Q  flagella,  which 
seem  to  be  extensions  of  the  colorless  protoplasm-layer  that  accumulates 
au  their  base  so  as  to  form  a  sort  of  transparent  beak  (h).    In  this  condi- 


Mat.  Curios."  (Bonn,  1850),  Tom.  xxii.;  of  which  an  abstract  by  Mr.  George  Busk 
ib  contained  in  the  "  Botanical  and  Physiological  Memoirs"  published  by  the  Ray 
Society  for  1853. 


234: 


THE  MICROSCOPE  AND  ITS  REVELATIONS. 


tion  it  seems  obvious  that  the  colorless  protoplasm  is  more  developed^ 
relatively  to  the  coloring  matter,  than  it  is  in  the  ^stilP  cells;  and  it 
usually  contains  ^vacuoles'  occupied^  only  by  clear  aqueous  fluid,  which 
are  sometimes  so  numerous  as  to  take-in  a  large  part  of  the  cavity  of  the 
cell,  so  that  the  colored  contents  seem  only  like  a  deposit  on  its  walls. 
Before  long,  this  ^motile'  cell  acquires  a  peculiar  saccular  investment, 
Avhich  seems  to  correspond  with  the  cellulose  envelope  of  the  ^ still'  cells, 
but  is  not  so  firm  in  its  consistence  (i,  K,  l);  and  between  this  and  the 
surface  of  the  ectoplasm  a  considerable  space  intervenes,  traversed  by 
thread-like  extensions  of  the  latter,  which  are  rendered  more  distinct  by 
iodine,  and  can  be  made  to  retract  by  means  of  re-agents.  The  flagella 
pass  through  the  cellulose  envelope,  which  invests  their  base  with  a  sort 
of  sheath;  and  in  the  portion  that  is  within  this  sheath  no  movement  is 
seen.  During  the  active  life  of  the  '  motile'  cell,  the  vibration  of  these 
flagella  is  so  rapid,  that  it  can  be  recognized  only  by  the  currents  it  pro- 
duces in  the  water  through  which  the  cells  are  quickly  propelled;  but 
when  the  motion  becomes  slacker,  the  filaments  themselves  are  readily 
distinguishable;  and  they  may  be  made  more  obvious  by  the  addition  of 
iodine. 

232.  The  multiplication  of  these  ^motile'  cells  may  take  place  in 
various  modes,  giving  rise  to  a  great  variety  of  appearances.  Sometimes 
they  undergo  a  regular  binary  subdivision  (b),  whereby  a  pair  of  motile 
cells  is  produced  (c),  each  resembling  its  single  predecessor  in  possessing 
the  cellulose  investment,  the  transparent  beak,  and  the  vibratile  filaments, 
before  the  dissolution  of  the  original  investment.  Sometimes,  again,  the 
contents  of  the  primordial  cell  undergo  a  segmentation  in  the  first  instance 
into  four  divisions  (d);  which  may  either  become  isolated  by  the  dissolu- 
tion of  their  envelope,  and  may  separate  from  each  other  in  the  condi- 
tion of  free  primordial  utricles  (h),  developing  their  cellulose  investments 
at  a  future  time;  or  may  acquire  their  cellulose  investments  (as  in  the 
preceding  case)  before  the  solution  of  that  of  the  original  cell;  while  some- 
times, even  after  the  disappearance  of  this,  and  the  formation  of  their 
own  independent  investments,  they  remain  attached  to  each  other  at  their 
beaked  extremities,  the  primordial  utricles  being  connected  with  each 
other  by  peduncular  prolongations,  and  the  whole  compound  body  having 
the  form  of  a  -h.  This  quaternary  segmentation  appears  to  be  a  more 
frequent  mode  of  multiplication  among  the  ^motile'  cells,  than  the  sub- 
division into  two;  although,  as  we  have  seen,  it  is  less  common  in  the 
'  still '  condition.  So,  also,  a  primary  segmentation  of  the  entire  endo- 
chrome  of  the  ^motile'  cells  into  8, 16,  or  even  32  parts,  may  take  place 
(e,  f),  thus  giving  rise  to  as  many  minute  gonidial  cells.  These  7nicro- 
gonidiay  when  set  free,  and  possessing  active  powers  of  movement,  rank 
as  '  zoospores '  (g)  :  they  may  either  develop  a  loose  cellulose  investment 
or  cyst,  so  as  to  attain  the  full  dimensions  of  the  ordinary  motile  cells 
(i,  k),  or  they  may  become  clothed  with  a  dense  envelope  and  lose  their 
flagella,  thus  passing  into  the  ^ still'  condition  (a);  and  this  last  trans- 
formation may  even  take  place  before  they  are  set  free  from  the  envelope 
within  which  they  were  produced,  so  that  they  constitute  a  mulberry- 
like mass,  which  fills  the  whole  cavity  of  the  original  cell,  and  is  kept  in 
motion  by  its  flagella. 

233.  These  varied  forms,  whose  relation  to  each  other  has  been  clearly 
proved  by  watching  the  successional  changes  that  make  up  the  history  of 
this  one  Plant,  have  been  described,  not  merely  as  distinct  species,  but  as 
distinct  genera  of  Animalcules,  such  as  ChlamydomonaSy  Euglena  Trache* 


MICROSCOPIC  FORMS  OF  VEGETABLE  LIFE. 


235 


lomonaSy  GygeSy  Gonium,  Pandorina,  BotryocystiSy  Uvella,  Syncrypta, 
Monas,  Astasia,  Bodo,  and  probably  many  others.  Certain  forms,  such 
as  the  '  motile '  cells  i,  k,  l,  appeal*  in  a  given  infusion,  at  first  exclu- 
sively and  then  principally;  they  gradually  diminish,  become  more  and 
more  rare, and  finally  disappear  altogether,  being  replaced  by  ^stilP  form. 
After  some  time,  the  number  of  the  ^  motile '  cells  again  increases,  and 
reaches,  as  before,  an  extraordinary  amount;  and  this  alternation  maybe 
repeated  several  times  in  the  course  of  a  few  weeks.  The  process  of  seg- 
mentation is  often  accomplished  with  great  rapidity.  If  a  number  of 
motile  cells  be  transferred  from  a  larger  glass  into  a  smaller,  it  will  be 
found,  after  the  lapse  of  a  few  hours,  that  most  of  them  have  subsided  to 
the  bottom;  in  the  course  of  the  day,  they  will  all  be  observed  to  be  upon 
the  point  of  subdivision;  on  the  following  morning,  the  divisional  brood 
will  have  become  quite  free;  and  on  the  next,  the  bottom  of  the  vessel  will 
be  found  covered  with  a  new  brood  of  self-dividing  cells,  which  again  pro- 
ceed to  the  formation  of  a  new  brood,  and  so  on. — Theactivity  of  Motion 
and  the  activity  of  Multiplication  seem  to  stand,  in  some  degree,  in  a 
relation  of  reciprocity  to  each  other;  for  the  self-dividing  process  takes 
place  with  greater  rapidity  in  the  ^ still ^  cells,  than  it  does  in  the  ^mo- 
tile/ 

234.  What  are  the  precise  conditions  which  determine  the  transition 
between  the  '  still '  and  the  '  motile '  states,  cannot  yet  be  precisely  stated; 
but  the  influence  of  certain  agencies  can  be  predicted  with  tolerable 
certainty.  Thus  it  is  only  necessary  to  pour  the  water  containing  these 
organisms  from  a  smaller  and  deeper  into  a  larger  and  shallower  vessel, 
at  once  to  determine  segmentation  in  numerous  cells — a  phenomenon 
which  is  observable  also  in  many  other  Protophytes.  The  '  motile '  cells 
geem  to  be  favorably  affected  by  Light,  for  they  collect  themselves  ait 
the  surface  of  the  water  and  at  the  edges  of  the  vessel;  but  when  they  are 
about  to  undergo  segmentation,  or  to  pass  into  the  ^ still'  condition,  they 
sink  to  the  bottom  of  the  vessel,  or  retreat  to  that  part  of  it  in  which  they 
are  least  subjected  to  light.  When  kept  in  the  dark,  the  '  motile '  cells 
undergo  a  great  diminution  of  their  chlorophyll,  which  becomes  very  pale, 
and  is  diffused,  instead  of  forming  definite  granules;  they  continue  their 
movement,  however,  uninterruptedly,  without  either  sinking  to  the  bot- 
tom, or  passing  into  the  still  form,  or  undergoing  segmentation.  A  mod- 
erate warmth,  particulai^ly  that  of  the  vernal  sun,  is  favorable  to  the 
development  of  the  ^motile'  cells;  but  a  temperature  of  excessive  eleva- 
tion prevents  it.  Eapid  evaporation  of  the  water  in  which  the  '  motile ' 
forms  may  be  contained,  kills  them  at  once;  but  a  more  gradual  loss, 
such  as  takes  place  in  deep  glasses,  causes  them  merely  to  pass  into  the 
'still 'form;  and  in  this  condition — especially  when  they  have  assumed 
a  red  hue — they  may  be  completely  dried-up,  and  may  remain  in  a  state 
of  dormant  vitality  for  many  years.  It  is  in  this  state  that  they  are 
wafted-about  in  atmospheric  currents,  and  that,  being  brought-down  by 
rain  into  pools,  cisterns,  etc.,  they  may  present  themselves  where  none 
had  been  previously  known  to  exist;  and  there,  under  favorable  circum- 
stances, they  may  undergo  a  very  rapid  multiplication,  and  may  maintain 
themselves  until  the  water  is  dried-up,  or  some  other  change  occurs  which 
is  incompatible  with  the  continuance  of  their  vital  activity.  They  then 
very  commonly  become  red  throughout,  the  red  coloring-substance  ex- 
tending itself  from  the  centre  towards  the  circumference,  and  assuming 
an  appearance  like  that  of  oil-drops;  and  these  red  cells,  acquiring  thick 
cell- walls  and  a  mucous  envelope,  float  in  flocculent  aggregations  on  the 


236 


THE  MICROSCOPE  AND  ITS  REVELATIONS. 


surface  of  the  water.  This  state  seems  to  correspond  with  the  ^  winter^ 
spores^  of  other  Protophytes;  and  it  may  continue  until  warmth,  air,  and 
moisture  cause  the  development  of  the  red  cells  into  the  ordinary  '  still ' 
cells,  green  matter  being  gradually  produced,  until  the  red  substance 
forms  only  the  central  part  of  the  endochrome.  After  this,  the  cycle  of 
changes  occurs  which  has  been  already  described;  and  the  Plant  may  pass 
through  a  long  series  of  these,  before  it  returns  to  the  state  of  the  red 
thick-walled  cell,  in  which  it  may  again  remain  dormant  for  an  unlimited 
period. — Even  this  cycle,  however,  cannot  be  regarded  as  completing  the 
history  of  the  Protococcus;  since  it  does  not  include  the  performance  of 
any  true  Generative  act.  There  can  be  little  doubt  that,  in  some  stage 
of  its  existence,  a  ^  conjugation  ^  of  two  cells  occurs,  as  in  Palmoglcea; 
and  the  attention  of  observers  should  be  directed  to  its  discovery,  as  well 
as  to  the  detection  of  other  varieties  in  the  condition  of  this  interesting 
little  Plant,  which  will  be  probably  found  to  present  themselves  before  and 
after  the  performance  of  that  act.* 

235.  Nearly  related  to  the  foregoing  in  the  independence  of  their 
individual  cells,  are  the  two  groups  Desmidiacece  and  Diatomacece,  which, 
in  a  systematic  view,  rank  as  subordinate  divisions  of  the  family  Conju- 
gatece;  their  Generative  process  being  performed  in  the  same  simple  man- 
ner as  that  of  Palmoglcea  (§  229).  But  these  two  tribes  being  of  such 
special  interest  to  the  Microscopist  as  to  require  separate  treatment  (§  260), 
only  that  higher  group,  the  Zygnemacece,  will  be  here  noticed,  in  which  the 
cells  produced  by  binary  subdivision  remain  attached  to  each  other,  end 
to  end,  so  as  to  form  long  unbranched  filaments  (Fig.  141),  whose  length 
is  continually  being  increased  by  a  repetition  of  the  same  process,  which 
may  take  place  in  any  part  of  the  filaments,  and  not  at  their  ends  alone. 
The  plants  of  this  group  are  not  found  so  much  in  running  streams,  as 
in  waters  that  are  perfectly  still,  such  as  those  of  ponds,  reservoirs,  ditches, 
or  marshy  grounds;  and  they  are  for  the  most  part  unattached,  floating 
freely  or  at  near  the  surface,  especially  when  buoyed-up  by  the  bubbles 
of  gas  which  are  liberated  from  the  midst  of  them  under  the  influence 
of  solar  light  and  heat.  In  the  early  stage  of  their  growth,  whilst  as  yet 
the  cells  are  undergoing  multiplication  by  subdivision,  the  endochrome 
is  commonly  diffused  pretty  uniformly  through  their  cavities  (Pig.  141,  a)  ; 
but  as  they  advance  towards  the  stage  of  conjugation,  it  ordinarily  ar- 
ranges itself  into  regular  spirals  (b),  though  occasionally  in  some  other 
forms.  The  act  of  conjugation  usually  occurs  between  the  cells  of  two 
distinct  filaments  that  happen  to  lie  in  proximity  to  each  other;  and  all 
the  cells  of  each  filament  generally  take  part  in  it  at  once.  The  adjacent 
cells  put  forth  little  protuberances,  which  come  into  contact  with  each 
other,  and  then  coalesce  by  the  breaking-down  of  the  intervening  parti- 
tions, so  as  to  establish  a  free  passage  between  the  cavities  of  the  conju- 
gating cells.  In  some  genera  of  this  family  (such  as  Mesocarpus),  the 
conjugating  cells  pour  their  endochromes  into  a  dilatation  of  the  passage 
that  has  been  established  between  them;  and  it  is  there  that  they  com- 
mingle so  as  to  form  the  ^  zygospore.^  But  in  the  Zygnema  (Fig.  141,  b), 
which  is  among  the  commonest  and  best-known  of  Conjugateae,  the 


^  In  the  above  sketch,  the  Author  has  presented  the  facts  described  by  Dr.  Cohn, 
under  the  relation  which  they  seemed  to  him  naturally  to  bear,  but  which  differs 
from  that  in  which  they  will  be  found  in  the  original  Memoir;  and  he  is  glad  to 
be  able  to  state,  from  personal  communication  with  its  able  Author,  that  Dr. 
Cohn's  later  observations  have  led  him  to  adopt  a  view  of  the  relationship  of  the 
*  etiir  and  *  motile'  forms,  which  is  in  essential  accordance  with  his  own. 


MICROSCOPIO  FORMS  OF  VEGETABLE  LIFE. 


237 


endochrome  of  one  cell  passes  over  entirely  into  the  cavity  of  the  other; 
and  it  is  within  the  latter  that  the  ^zygospore'  is  formed  (c),  the  two 
endochromes  coalescing  into  a  simple  mass,  around  which  a  firm  envelope 
gradually  makes  its  appearance.  Further,  it  may  be  generally  observed 
that  all  the  cells  of  one  filament  thus  empty  themselves,  whilst  all  the  cells  of 
the  other  filament  become  the  recipients:  here,  therefore,  we  seem  to 
have  a  foreshadowing  of  the  sexual  distinction  of  the  Generative  cells  into 
^sperm-cells'  and  ^germ-cells/  which  we  shall  presently  see  in  the  fila- 
mentous ConfervacecB.  Multiplication  by  *  zoospores'  has  not  been  seen 
to  take  place  among  the  Oonjugateae. 

236.  From  the  composite  '  motile  '  forms  of  Prodococcus  (§  232),  the 
transition  is  easy  to  the  group  of  Volvociiiece — an  assemblage  of  minute 
Plants  of  the  greatest  interest  to  the  Microscopist,  on  account  both  of 
the  Animalcule-like  activity  of  their  movements,  and  of  the  great  beauty 
and  regularity  of  their  forms.  The  most  remarkable  example  of  this 
group  is  the  well-known  Voloox  globator  (FR0i5"TSPiECE),  which  is  not 


Various  stages  of  the  history  of  Zygnema  quininum:—A,  three  cells  a,  b,  c,  of  a  young  filament 
of  which  b  is  undergoing  subdivision;  b.  two  filaments  in  the  first  stage  of  conjugation,  showmg 
the  spiral  disposition  of  their  endochromes,  and  the  protuberances  from  the  conjugatmg  cells;  c, 
completion  of  the  act  of  conjugation,  the  endochromes  of  the  cells  of  the  filament  a  ha vmg  entirely 
passed  over  to  those  of  filament  6,  in  which  the  zygospores  are  formed. 

uncommon  in  fresh-water  pools,  and  which,  attaining  a  diameter  of 
about  l-50th  or  even  l-30th  of  an  inch,  may  be  seen  with  the  naked  eye 
when  the  drop  containing  it  is  held  up  to  the  light,  swimming  through 
the  water  which  it  inhabits.  Its  onward  motion  is  usually  of  a  rolling 
kind;  but  it  sometimes  glides  smoothly  along,  without  turning  on  its 
axis;  whilst  sometimes,  again,  it  rotates  like  a  top,  without  changing  its 
position.  When  examined  with  a  sufficient  magnifying  power,  the 
Volvox  is  seen  to  consist  of  a  hollow  sphere,  composed  of  a  very  pellucid 
material,  which  is  studded  at  regular  intervals  with  minute  green  spots, 
and  which  is  often  (but  not  constantly)  traversed  by  green  threads  con- 
necting these  spots  together.  From  each  of  the  spots  proceed  two  long 
fiagellaj  so  that  the  entire  surface  is  beset  with  these^  lashing  filaments, 
to  whose  combined  action  its  movements  are  due.  Within  the  external 
sphere  may  generally  be  seen  from  two  to  twenty  other  globes,  of  a  darker 
color,  and  of  varying  sizes;  the  smaller  of  these  are  attached  to  the  inner 
surface  of  the  investing  sphere,  and  project  into  its  cavity;  but  the  larger 
lie  freely  within  the  cavity,  and  may  often  be  observed  to  revolve  bj  the 
agency  of  their  own  flagella.    After  a  time,  the  original  sphere  bursts. 


238 


THE  MICROSCOPE  AND  ITS  REVELATIONS. 


and  the  contained  spherules  swim  forth  and  speedily  develop  themselves 
into  the  likeness  of  that  within  which  they  have  been  evolved;  their 
colored  particles,  which  are  at  first  closely  aggregated  together,  being 
separated  from  each  other  by  the  interposition  of  the  transparent  pellicle. 
— It  was  long  supposed  that  the  Volvox  is  a  si7igle  Animal;  and  it  was 
first  shown  to  be  a  composite  fabric,  made  up  of  a  repetition  of  organisms 
in  all  respects  similar  to  each  other,  by  Prof.  Ehrenberg;  who,  however, 
considered  these  organisms  as  Monads,  and  described  them  as  each 
possessing  a  mouth,  several  stomachs,  and  an  eye  !  Our  present  knowl- 
edge of  their  nature,  however,  leaves  little  doubt  of  their  Vegetable 
character;^  and  the  peculiarity  of  their  history  renders  it  desirable  to 
describe  it  in  some  detail. 

237.  Each  of  the  so-called  ^monads'  (Plate  ix.,  figs.  9,  11)  is  a 
somewhat  flask-shaped  Plant-cell,  about  l-3000th  of  an  inch  in  diameter; 
consisting,  as  in  the  previous  instances,  of  green  chlorophyll-granules 
diffused  through  a  colorless  protoplasm,  constituting  an  '  endochrome ' 
(which  commonly  includes  also  a  red  spot  of  altered  chlorophyll);  and 
bounded  by  an  ^ectoplasm'  formed  of  the  condensed  and  colorless 
surface-layer  of  the  protoplasmic  mass.  It  is  prolonged  outwardly  (or 
towards  the  circumference  of  the  sphere)  into  a  sort  of  colorless  beak  or 
proboscis,  from  which  proceed  two flagella  (fig.  11);  audit  is  invested  by 
a  pellucid  or  hyaline  envelope  (fig.  9,  d)  of  considerable  thickness,  the 
borders  of  which  are  flattened  against  those  of  other  similar  envelopes 
(fig.  5,  c),  but  which  does  not  appear  to  have  the  tenacity  of  a  true 
membrane.  It  is  impossible  not  to  recognize  the  precise  similarity 
between  the  structure  of  this  body,  and  that  of  the  motile  '  encysted  ^ 
cell  of  Protococcus  pluvialis  (Plate  viii.,  fig.  2,  k);  there  is  not,  in  fact, 
any  perceptible  difference  between  them,  save  that  which  arises  from  the 
regular  aggregation,  in  Volvox,  of  the  cells  which  normally  detach  them- 
selves from  one  another  in  Protococcus.  The  presence  of  cellulose  in  the 
hyaline  substance  is  not  indicated,  in  the  ordinary  condition  of  Volvox 
gloiator,  by  the  iodine  and  the  sulphuric  acid  test,  though  the  use  of 
'  Schultz's  solution  ^  gives  to  it  a  faint  blue  tinge;  there  can  be  no  doubt 
of  its  existence,  however,  in  the  hyaline  envelope  of  Volvox  aureus 
(§  240).  The  flagella  and  endoplasm,  as  in  the  motile  forms  of  Protococ- 
cus, are  tinged  of  a  deep  brown  by  iodine,  with  the  exception  of  one  or 
two  starch-particles  in  each  cell,  which  are  turned  blue;  and  when  the 
contents  of  the  cell  are  liberated,  bluish  flocculi,  apparently  indicative  of 
the  presence  of  cellulose,  are  brought  into  view  by  the  action  of  sulphuric 
acid  and  iodine.  All  these  reactions  are  characteristically  Fe^^^^^^S/e  in 
their  nature. — When  the  cell  is  approaching  maturity,  its  endoplasm 
always  exhibits  one  or  more  ^  vacuoles^  (fig.  9,  a,  a),  of  a  spherical  form, 
and  usually  about  one-third  of  its  own  diameter;  and  these  ^vacuoles' 
(which  are  the  so-called  ^stomachs'  of  Prof.  Ehrenberg)  have  been 
observed  by  Mr.  Gr.  Busk  to  undergo  a  very  curious  rhythmical  contrac- 
tion and  dilatation  at  intervals  of  about  40  seconds;  the  contraction 
(which  seems  to  amount  to  complete  obliteration  of  the  cavity  of  the 
vacuole)  taking  place  rapidly  or  suddenly,  whilst  the  dilatation  is  slow 


'Prof.  Stein,  however,  in  the  last-published  part  of  his  great  work  on  the 
Infusoria  (**Organismus  der  Infusionsthiere,"  Abtheilung  iii.,  Leipzig,  1878),  still 
ranks  the  Volvocinece  among  the  Flagellate  animalcules,  to  which  they  undoubt- 
edly show  a  remarkable  parallelism  in  structure,  the  chief  evidence  of  their 
Vegetable  nature  lying  in  their  physiological  conformity  to  undoubted  Proto^ 
phytes. 


f 

MICROSCOPIC  FORMS  OF  VEGETABLE  LIFE. 


239 


DEVELOPMENT  OP  voLvox  GLOBATOR  (after  Williamsoii/. 
Fig.  1.  Young  Volvox ;  a,  primordial  cell  of  secondary  sphere  ;  b,  polygonal  masses  of  endo- 
chrome,  separated  by  hyaline  substance. 

2.  The  same  more  advanced  ;  a,  a,  polygonal  masses  of  endochrome  ;  6,  6,  their  connecting  pro- 
cesses ;  c,  primordial  cell  of  secondary  sphere. 

3.  The  same  more  advanced,  showing  an  increase  in  the  size  of  the  connecting  processes,  a,  a, 
and  duplicative  subdivision  of  the  primordial  cell. 

4.  The  same  more  advanced,  showing  the  masses  of  endochrome  more  widely  separated  by  the 
interposition  of  hyaline  substance,  and  each  furnished  with  a  pair  of  flagella;  whilst  the  primordial 
cell,/,  has  undergone  a  second  segmentation. 

5.  Portion  of  the  spherical  wall  of  a  mature  Volvox,  showing  the  wide  separation  of  the  endo- 
chrome-masses  still  connected  by  the  processes  6,  b  ;  the  lines  of  areolation,  c,  dividing  the  hyaline 
substance ;  and  the  long  flagella,  e. 

6.  7,  8.  Secondary  sphere,  or  macro-gonidium,  developed  by  the  progressive  segmentation  of  the 
primordial  cell. 

9.  Single  cell  from  the  wall  of  a  mature  Volvox,  showing  the  endochrome  mass,  6,  to  contain 
two  vacuoles  a,  a,  and  to  be  surrounded  by  a  hyaline  envelope,  d,  having  polygonal  borders. 

10.  Portion  of  the  wall  of  a  young  Volvox,  seen  edgeways,  showing  that  its  sphere  is  still  invested 
by  the  hyaline  envelope  of  the  original  cell,  which  the  flagella  penetrate  but  do  not  pierce. 

11.  Two  cells  from  a  mature  Volvox,  seen  edgeways,  showing  the  inclosure  of  the  endochrome- 
masses  in  their  own  hyaline  investment,  and  the  persistence  of  the  general  investment  (pierced  by 
the  flagella)  around  the  entire  sphere. 


240  THE  MICROSCOPE  AND  ITS  REVELATIONS, 

and  gradual.  This  curious  action  ceases,  however,  as  the  cell  arrives  at 
its  full  maturity;^  a  condition  which  seems  to  be  marked  by  the  greater 
consolidation  of  the  ^ectoplasm/  by  the  removal  or  transformation  of 
some  of  the  chlorophyll,  and  by  the  formation  of  the  red  spot  (b),  which 
obviously  consists,  as  in  Protococcus,  of  a  peculiar  modification  of  chloro- 
phyll. 

238.  Each  cell  normally  communicates  with  the  cells  in  nearest  prox- 
imity with  it,  by  extensions  of  its  own  endochrome,  which  are  some- 
times single  and  sometimes  double  (fig.  5,  b);  and  these  connecting 
processes  necessarily  cross  the  lines  of  division  between  their  respective 
hyaline  investments.  The  thickness  of  these  processes  varies  very  con- 
siderably; for  sometimes  they  are  broad  bands,  and  in  other  cases  mere 
threads;  whilst  they  are  occasionally  wanting  altogether.  This  differ- 
ence seems  partly  to  depend  upon  the  age  of  the  specimen,  and  partly 
upon  the  abundance  of  nutriment  which  it  obtains;  for,  as  we  shall 
presently  see,  the  connection  is  most  intimate  at  an  early  period,  before 
the  hyaline  investments  of  the  cells  have  increased  so  much  as  to  separate 
the  masses  of  endochrome  to  a  distance  from  one  another  (figs.  2,  3,  4); 
whilst  in  a  mature  individual,  in  which  the  separation  has  taken  place 
to  its  full  extent,  and  the  nutritive  processes  have  become  less  active, 
the  masses  of  endochrome  very  commonly  assume  an  angular  form,  and 
the  connecting  processes  are  drawn-out  into  threads  (as  seen  in  fig.  5), 
or  they  retain  their  globular  form,  and  the  connecting  processes  alto- 
gether disappear.  The  influence  of  re-agents,  or  the  infiltration  of 
water  into  the  interior  of  the  hyaline  investment,  will  sometimes  cause 
the  connecting  process  (as  in  Protococcus,  §  231)  to  be  drawn  back  into 
the  central  mass  of  endochrome;  and  they  will  also  retreat  on  the  mere 
rupture  of  the  hyaline  investment:  from  these  circumstances  it  may  be 
inferred  that  they  are  not  inclosed  in  any  definite  membrane.  On  the 
other  hand,  the  connecting  threads  are  sometimes  seen  as  double  lines, 
which  seem  like  tubular  prolongations  of  a  consistent  membrane,  with- 
out any  protoplasmic  granules  in  their  interior.  It  is  obvious,  then, 
that  an  examination  of  a  considerable  number  of  specimens,  exhibiting 
various  phases  of  conformation,  is  necessary  to  demonstrate  the  nature 
of  these  communications;  but  this  may  be  best  made-out  by  attending  to 
the  history  of  their  development,  which  we  shall  now  describe. 

239.  The  spherical  body  of  the  young  Volvox  (Plate  ix.,  fig.  1)  is 
composed  of  an  aggregation  of  somewhat  angular  masses  of  endochrome 
(5),  separated  by  the  interposition  of  hyaline  substance;  and  the  whole 
seems  to  be  inclosed  in  a  distinctly  membranous  envelope,  which  is  pro- 
bably the  distended  hyaline  investment  of  the  ^  primordial  ^  cell,  within 
which,  as  will  presently  appear,  the  entire  aggregation  originated.  In 
the  midst  of  the  polygonal  masses  of  endochrome,  one  mass  {a),  rather 
larger  than  the  rest,  is  seen  to  present  a  circular  form;  and  this,  as  will 
presently  appear,  is  the  originating  cell  of  what  is  hereafter  to  become  a 
new  sphere.  The  growing  Volvox  at  first  increases  in  size,  not  only  by 
the  interposition  of  new  hyaline  substance  between  its  component  masses 
of  endochrome,  but  also  by  an  increase  in  these  masses  themselves  (fig. 
2,  a),  which  come  into  continuous  connection  with  each  other  by  the  coa- 


^  The  existence  of  rhythmically  contracting  vacuoles  in  Volvox  (though  con- 
firmed by  the  observations  of  Prof.  Stein)  is  denied  by  Mr.  Saville  Kent  (*' Manual 
of  the  Infusoria,"  p.  47);  but  it  may  be  fairly  presumed  that  he  has  not  looked  for 
them  at  the  stage  of  development  at  which  their  action  was  vritnessed  by  Mr. 
Busk. 


MICROSCOPIC  FORMS  OF  VEGETABLE  LIFE.  241 

lescence  of  processes  (b)  which  they  severally  put- forth ;  at  the  same  time 
an  increase  is  observed  in  the  size  of  the  globular  cell  (c),  which  is  pre- 
liminary to  its  binary  subdivision.  A  more  advanced  stage  of  the  same 
developmental  process  is  seen  in  fig.  3;  in  which  the  connecting  pro- 
cesses {ay  a)  are  so  much  increased  in  size,  as  to  establish  a  most  in- 
timate union  between  the  masses  of  endochrome,  although  the  increase 
of  the  intervening  hyaline  substance  carries  these  masses  apart  from  one 
another;  whilst  the  endochrome  of  the  central  globular  cell  has  under- 
gone segmentation  into  two  halves.  In  the  stage  represented  in  fig.  4, 
the  masses  of  endochrome  have  been  still  more  widely  separated  by  the 
interposition  of  hyaline  substance;  each  has  become  furnished  with  its 
pair  of  flagella;  and  the  globular  cell  has  undergone  a  second  segmenta- 
tion. Finally,  in  fig.  5,  which  represents  a  portion  of  the  spherical  wall 
of  a  mature  Volvox,  the  endochome-masses  are  observed  to  present  a 
more  scattered  aspect,  partly  on  account  of  their  own  reduction  in  size, 
and  partly  through  the  interposition  of  a  greatly-increased  amount  of 
hyaline  substance,  which  is  secreted  from  the  surface  of  each  mass;  and 
that  portion  which  belongs  to  each  cell,  standing  to  the  endochrome- 
mass  in  the  relation  of  the  cellulose  coat  of  an  ordinary  cell  to  its  ecto- 
plasm, is  frequently  seen  to  be  marked-out  from  the  rest  by  delicate  lines 
of  hexagonal  areolation  (c,  c)  which  indicate  the  boundaries  of  each. 
Of  these  it  is  often  difficult  to  obtain  a  sight,  a  nice  arrangement  of  the 
light  being  usually  requisite  with  fresh  specimens;  but  the  prolonged 
action  of  water  (especially  when  it  contains  a  trace  of  iodine),  or  of  gly- 
cerine, will  often  bring  them  into  clear  view.  The  prolonged  action  of 
glycerine,  moreover,  will  often  show  that  the  boundary  lines  are  double, 
being  formed  by  the  coalescence  of  two  contiguous  cell-walls;  and  they 
sometimes  retreat  from  each  other  so  far  that  the  hexagonal  areolae  be- 
come rounded.  As  the  primary  sphere  approaches  maturity,  the  large 
secondary  germ-mass,  or  macro-gonidium,  whose  origin  has  been  traced 
from  the  beginning,  also  advances  in  development;  its  contents  under- 
going multiplication  by  successive  segmentations,  so  that  we  find  it  to  con- 
sist of  8,  16,  32,  64,  and  still  more  numerous  divisions,  as  shown  in  figs. 
6,  7,  8.  Up  to  this  stage,  at  which  first  the  sphere  appears  to  become 
hollow,  it  is  retained  within  the  hyaline  envelope  of  the  cell  within 
which  it  has  been  produced;  a  similar  envelope  can  be  easily  distin- 
guished, as  shown  in  fig.  10,  just  when  the  segmentation  has  been  com- 
pleted, and  at  that  stage  the  flagella  pass  into  it,  but  do  not  extend 
beyond  it;  and  even  in  the  mature  Volvox  it  continues  to  form  an  invest- 
ment around  the  hyaline  envelopes  of  the  separate  cells,  as  shown  in 
fig.  11.  It  seems  to  be  by  the  adhesion  of  the  hyaline  investment  of  the 
new  sphere  to  that  of  the  old,  that  the  secondary  sphere  remains  for  a 
time  attached  to  the  interior  wall  of  the  primary;  at  what  exact  period, 
or  in  what  precise  manner,  the  separation  between  the  two  takes  place, 
has  not  yet  been  determined.  At  the  time  of  the  separation,  the  devel- 
opmental process  has  generally  advanced  as  far  as  the  stage  represented 
in  fig.  1;  the  foundation  of  one  or  more  tertiary  spheres  being  usually 
distinguishable  in  the  enlargement  of  certain  of  its  cells. 

240.  The  development  and  setting-free  of  these  composite  ^macro- 
gonidia,'  which  is  essentially  a  process  of  cell-subdivision  or  gemmiparous 
extension  (§  226),  is  the  ordinary  mode  of  multiplication  in  Volvox; 
taking  place  at  all  times  of  the  year,  except  when  the  sexual  generation 
(now  to  be  described)  is  in  progress.  The  mode  in  which  this  process  is 
here  performed  (for  our  knowledge  of  which  we  are  indebted  to  the  per- 
16 


242 


THE  MICROSCOPE  AND  ITS  REVELATIONS. 


severing  investigation  of  Prof.  Cohn')  shows  a  great  advance  upon  the 
simple  ^conjugation'  of  two  similar  cells  (§  229),  and  closely  resembles 
that  which  prevails  not  only  among  the  higher  Algce,  but  (under  some 
form  or  other)  through  a  large  part  of  the  Cryptogamic  series.  As 
autumn  advances,  the  Volvox-spheres  usually  cease  to  multiply  them- 
selves by  the  formation  of  '  macro-gonidia; '  and  certain  of  their  ordinary 
cells  begin  to  undergo  changes  by  which  they  are  converted,  some  into 
male  or  ^ sperm-cells/ others  into  female  or  ^germ-cells/ — the  greater 
number,  however,  remaining  ^sterile.'  Each  sphere  of  Volvox  gloiator 
(Frontispiece,  fig.  1)  contains  both  kinds  of  sexual  cells,  so  that  this 
species  ranks  as  monoecious;  but  F.  aureus  is  dioecious^  the  ^sperm-cells' 
and  ^germ-cells'  occurring  in  separate  spheres.  Both  kinds  of  ^ sexual^ 
cells  are  at  first  distinguishable  from  the  ordinary  ^sterile'  cells  by 
their  larger  size  (fig.  2,  a),  in  this  respect  resembling  ^macro-gonidia' 
in  an  early  stage;  but  their  subsequent  history  is  altogether  different. 
The  'sperm-cells'  begin  to  undergo  subdivision  when  they  attain  about 
three  times  the  size  of  the  ^sterile'  cells;  this,  however,  takes  place  not 
on  the  ^  binary'  plan,  but  in  such  a  manner  that  the  endochrome  of  the 
primary  cell  resolves  itself  into  a  cluster  of  very  peculiar  secondary  cells 
(fig.  1,  a,  a%  fig.  5),  each  consisting  of  an  elongated  ^body'  containing 
an  orange-colored  endochrome  with  a  red  corpuscle,  and  of  along  color- 
less beak,  from  the  base  of  which  proceeds  a  pair  of  long  flagella  (figs. 
6,  7), — as  in  the  ^  antherozoids '  of  the  higher  Cryptogams  (Fig.  154,  h). 
As  the  sperm-cells  approach  maturity,  the  aggregate  clusters  may^be 
seen  to  move  within  them,  at  first  slowly,  and  afterwards  more  rapidly; 
the  bundles  then  separate  into  their  component  ^anterozoids,'  which 
show  an  active  independent  movement  whilst  still  within  the  cavity  of 
the  primary  cell  (fig.  1,  a^);  and  finally  escape  by  the  giving- way  of  its 
wall  {a^)y  diffusing  themselves  through  the  cavity  of  the  Volvox-sphere. 
The  germ-^cells  (fig,  1,  5,  5),  on  the  other  hand,  continue  to  increase  in 
size  without  undergoing  subdivision;  at  first  showing  large  vacuoles  in 
their  protoplasm  (b%  V^),  but  subsequently  becoming  filled  with  dark- 
green  endochrome.  The  form  of  the  ^germ-cell'  gradually  changes  from 
its  original  flask-shape  to  the  globular  {p^)\  and  it  projects  into  the  cav- 
ity of  the  Volvox-sphere,  at  the  same  time  acquiring  a  gelatinous  envel- 
ope. Over  this  the  swarming  antherozoids  diffuse  themselves  (fig.  3), 
penetrating  its  substance,  so  as  to  find  their  way  to  the  interior;  and  in 
this  situation  they  seem  to  dissolve-away,  so  as  to  become  incorporated 
with  the  endochrome.    The  product  of  this  fusion  (which  is  only  '  con- 


^  The  original  observations  of  Cohn  on  the  sexuality  of  Volvox,  published  in 
the  **  Ann.  des  Sci.  Nat.,"  4ieme  Ser.,  Botan.,  Tom.  v.  (1857),  p.  323,  were  con- 
firmed  by  Mr.  Carter,  Ann.  Nat.  Hist.,"  3d  Ser.,  Vol.  iii.  (1859),  p.  4,  who  ob- 
served the  sexual  process  in  Eudorina  also.— In  the  well-known  form  Pandorina 
morum,  the  generative  process  is  performed,  according  to  the  observations  of 
Pringsheim,  in  a  manner  curiously  intermediate  between  the  lower  and  the 
higher  types  referred  to  above.  For  within  each  cell  of  the  original  sixteen  of 
which  its  mulberry -like  mass  is  composed,  a  brood  of  sixteen  secondary  cells 
is  formed  by  ordinary  binary  subdivision;  and  these,  when  set  free  by  the  dis- 
solution of  their  containing  cell-wall,  swim  forth  as  *  swarm-spores,'  each  being  fur- 
nished with  a  pair  of  flagella.  Among  the  crowd  of  these  swarm-spores  may  be 
observed  some  which  approach  in  pairs,  as  if  seeking  one  another;  when  they 
meet,  their  points  at  first  come  together,  but  gradually  their  whole  bodies  coa- 
lesce ;  and  a  globular  *  zygospore '  is  thus  formed,  which  germinates  after  a  period 
of  rest,  reproducing  by  binary  subdivision  the  original  sixteen-celled  mulberry- 
like  Pandorina.   (See  Sachs'    Botany,"  p.  219.) 


MICROSCOPIC  FORMS  OF  VEGETABLE  LIFE. 


243 


jngation  ^  under  another  form)  is  a  reproductive  globule  or  oospore; 
which  speedily  becomes  enveloped  by  an  internal  smooth  membrane,  and 
with  a  thicker  external  coat  which  is  usually  beset  with  conical-pointed 
processes  (tig.  4);  and  the  contained  chlorophyll  gives  place,  as  in  Pal- 
mogloea  (§  229),  to  starch  and  a  red  or  orange  colored  oil.  As  many  as 
forty  of  such  'oo-spores'  have  been  seen  by  Dr.  Cohn  in  a  single  sphere 
of  Volvox,  which  thus  acquires  the  peculiar  appearance  that  has  been  dis- 
tinguished by  Ehrenberg  by  a  different  specific  name,  Volvox  stellatus. 
Soon  after  the  ^oo-spores^  reach  maturity,  the  parent-sphere  breaks  up, 
and  the  oo-spores  fall  to  the  bottom,  where  they  remain  during  the  win- 
ter. Their  further  history  has  since  been  traced  out  by  Kirchner;  who 
found  that  their  germination  commenced  in  February  with  the  liberation 
of  the  spherical  '  endospore^  from  its  envelope,  and  with  its  division  into 
four  cells  by  the  formation  of  two  partitions  at  right  angles  to  one  an- 
other. These  partially  separate,  holding  together  only  at  one  end,  which 
becomes  one  pole  of  the  globular  cluster  subsequently  formed  by  cell- 
multiplication;  the  other  pole  only  closing-in  when  a  large  number  of 
cells  have  been  formed.  The  cells  are  then  carried  apart  from  one  an- 
other by  the  hyaline  investment  formed  by  each;  and  the  characteristic 
Vol  vox-sphere  is  thus  completed.^ 

241.  There  are  other  points  in  the  life-history  of  Volvox  which  must 
not  be  left  without  mention,  although  their  precise  import  is  as  yet  un- 
certain. Thus,  according  to  Mr.  G.  Busk  (with  whom  Prof.  Cohn  is  in 
accord  on  this  point),  the  body  designated  by  Prof.  Ehrenberg  Sphcero- 
sira  volvox  is  an  ordinary  Volvox  in  a  different  phase  of  development;  its 
only  marked  feature  of  dissimilarity  being  that  a  large  proportion  of 
the  green  cells,  instead  of  being  single  (as  in  the  ordinary  form  of  VoU 
vox),  save  where  they  are  developing  themselves  into  young  spheres,  are 
very  commonly  double,  quadruple,  or  multiple;  and  the  groups  of  ciliated 
cells  thus  produced,  instead  of  constituting  a  hollow  sphere,  form  by 
their  aggregation  discoid  bodies,  of  which  the  separate  fusiform  cells  are 
connected  at  one  end,  whilst  at  the  other  they  are  free,  each  being  fur- 
nished with  a  single  flagellum.  These  clusters  separate  themselves  from 
the  primary  sphere,  and  swim  forth  freely,  under  the  forms  which  have 


1  The  doctrine  of  the  vegetable  nature  of  Volvox,  which  has  been  suggested 
by  Siebold,  Braun,  and  other  German  Naturalists,  was  first  distinctly  enunciated 
by  Prof.  Williamson,  on  the  basis  of  the  history  of  its  development,  in  the 
Transactions  of  the  Philosophical  Society  of  Manchester,"  Vol.  ix.  Subse- 
quently Mr.  G.  Busk,  whilst  adducing  additional  evidence  of  the  Vegetable 
nature  of  Volvox,  in  his  extremely  valuable  Memoir  in  the  ''Transactions  of  the 
Microscopical  Society,"  N.  S.,  Vol.  i.  (1853),  p.  31,  called  in  question  some  of  the 
views  of  Prof.  Williamson,  which  were  justified  by  that  gentleman  in  his  "  Fur- 
ther Elucidations"  in  the  same  "Transactions."  The  description  above  given 
by  the  Author,  on  the  basis  of  the  facts  in  which  these  excellent  observers  were 
agreed  (their  original  differences  having  been  in  great  degree  reconciled  by  their 
mutual  admissions),  is  in  entire  harmony  with  the  most  recentaccount  of  this  most 
interesting  organism  given  by  Prof.  Cohn  ("  Beitrage  zur  Biologie  der  Pflanzen," 
Btl.  i..  Heft  3,  1875),  to  whom  we  owe  the  discovery  of  its  generative  process. 
The  observations  of  Dr.  Kirchner  on  its  germination  will  be  found  in  Bd.  iii., 
Heft  1  (1879),  of  the  same  serial. — An  extremely  interesting  Volvocine  form  de- 
scribed by  Cohn  under  the  name  Stephanosphcera  pluvialis  exhibits  all  the  phe- 
nomena of  reproduction  by  macro-gonidia  or  composite  masses  of  adherent  cells, 
by  micro-gonidia  or  active  zoospores,  by  '  still '  or  stato-spores,  and  by  oospores 
produced  by  true  sexual  action,  in  a  very  characteristic  manner;  and  his  account 
of  its  life-history  should  be  consulted  by  every  one  who  desires  to  study  that  of 
any  of  the  Protophyta.  See  "Ann.  of  Nat.  Hist.,"  2d  Ser.,  Vol.  x,  (1853),  pp.  321, 
401,  and  ''Quart.  Journ.  of  Microsc.  Sci.,"  Vol.  vi.  (1858),  p.  131. 


244 


THE  MICROSCOPK  AND  ITS  KEVELA.TIONS. 


been  designated  by  Prof.  Ehrenberg  as  Uvella  and  Syncrypta, — Again, 
it  has  been  noticed  by  Dr.  Hicks^  that  towards  the  end  of  the  autumn, 
the  bodies  formed  by  the  binary  subdivision  of  the  single  cells  of  VolvoXy 
instead  of  forming  spherical  flagellated  ^  macro-gonidia^  which  tend  to 
escape  outwards,  form  clusters  of  irregular  shape,  each  composed  of  an 
indefinite  mass  of  gelatinous  substance  in  which  the  green  cells  lie  sepa- 
rately imbedded.  These  clusters,  being  without  motion,  may  be  termed 
statO'Spores;  and  it  is  probable  that  they  constitute  one  of  the  forms  in 
which  the  existence  of  this  organism  is  prolonged  through  the  winter. 

242.  Another  phenomenon  of  a  very  remarkable  nature,  namely,  the 
conversion  of  the  contents  of  an  ordinary  Vegetable  cell  into  a  free  mov- 
ing mass  of  protoplasm  that  bears  a  strong  resemblance  to  the  animal 
Avimba  (Fig.  289),  is  affirmed  by  Dr.  Hicks  ^  to  take  place  place  in  Voh 
vox,  under  circumstances  that  leave  no  reasonable  ground  for  that  doubt 
of  its  reality  which  has  been  raised  in  regard  to  the  accounts  of  similar 
phenomena  occurring  elsewhere.  The  endrochrome-mass  of  one  of  the 
ordinary  cells  increases  to  nearly  double  its  usual  size;  but  instead  of 
undergoing  duplicative  subdivision  so  as  to  produce  a  ^macro-goni- 
dium,^  as  in  Fig.  142,  it  loses  its  color  and  its  regularity  of  form,  and 
becomes  an  irregular  mass  of  colorless  protoplasm,  containing  a  number 
of  brown  or  reddish-brown  granules  (a,  a),  and  capable  of  altering  its 
form  by  protruding  or  retracting  any  portion  of  its  membraneous  wall, 
exactly  like  a  true  Amceba.  By  this  self-moving  power,  each  of  these 
bodies,  c,  c  (of  which  twenty  may  sometimes  be  counted  within  a  single 
Volvox)  glides  independently  over  the  inner  surface  of  the  sphere  among 

its  unchanged  green  cells, 
bending  itself  round  any 
one  of  these  with  which 
it  may  come  into  contact, 
precisely  after  the  manner 
of  an  Amceba.  After  the 
^  amoeboid  Mias  begun  to 
travel,  it  is  always  noticed 
that  for  every  such  mov- 
ing body  in  the  Volvox 
there  is  the  empty  space 
of  a  missing  cell;  and  this 
confirms  the  belief — 
founded  on  observation 
of  the  gradational  transi- 
tion from  the  one  condi- 
TTnrrv^of?^    p  K     1,     ^  .  "^i^n  other,  and  on 

Jormation  of  Amoeboid  bodies  in  Volvox:— a,  a  ordinarv  +1^^   4^ 

parasitically  from  without-that  the  '  amcBboid 'l^allyj^^^^^^^ 

may  take  place,  according  to  Dr.  Hicks,  even  after  the  process  of  binary 

imXid  bo2'T"''"r'^\  ^"bsequent  destination  of  thesl 

AmcBboid  bodies,  has  not  yet  been  ascertained.' 


»  "  Quart.  Journ.  of  Microsc.  Science,"  N.S.,  Vol.  i.  (1861)  x>  281 
A  similar  produotion  o(  '  i,mosbol<i''hai  been  observ<«i  by  Mr.  Archer  in 


MICROSCOPIC  FORMS  OF  VEGETABLE  LIFE. 


245 


243.  With  the  two  Protophytes  just  described  may  be  ranked  under 
the  general  designation  Falmellacew  a  number  of  others  scarcely  less  sim- 
ple in  their  essential  characters,  but  sometimes  attaining  considerable 
dimensions.  They  all  grow  either  on  damp  surfaces,  or  in  fresh  or  salt 
water;  and  they  may  either  form  (1)  a  mere  powdery  layer,  of  which  the 
component  particles  have  little  or  no  adhesion  to  each  other,  or  they 
may  present  themselves  (2)  in  the  condition  of  an  indefinite  slimy  film, 
or  (3)  in  that  of  a  tolerably  firm  and  definitely  bounded  membranous 
^  frond.' — The  fi^^st  of  these  states  we  have  seen  to  be  characteristic  of 
PalmoglcBa  and  Froctococcus;  the  new  cells,  which  are  originated  by  the 
process  of  binary  subdivision,  usually  separating  from  each  other  after  a 
short  time;  and,  even  where  they  remain  in  cohesion,  not  forming  a 
'  frond  ^  or  membranous  expansion.  The  ^red  snow,^  which  sometimes 
colors  extensive  tracts  in  Arctic  or  Alpine  regions,  penetrating  even  to 
the  depth  of  several  feet,  and  vegetating  actively  at  a  temperature  which 
reduces  most  plants  to  a 
state  of  torpor,  is  generally 
considered  to  be  a  species  of 
Protococcus;  but  as  its  cells 
are  connected  by  a  tolerably 
firm  gelatinous  investment, 
it  Avould  rather  seem  to  be 
a  Palmella. — The  second  is 
the  condition  of  the  Pal- 
mella proper;  of  which  one 
species,  the  P,  cruenta^  usu 
ally  known  under  the  name 
of  '  gory  dew,'  is  common  on 
damp  walls  and  in  shady 
places,  sometimes  extending 
itself  over  a  considerable  area 
as  a  tough  gelatinous  mass, 
of  the  color  and  general  ajD- 
pearance  of  coagulated 
blood.  A  characteristic  il- 
lustration of  it  is  also  afford- 
ed by  the  Hcematococcus 
sang  ui  n  eus  (Fig.  143), 
which  chiefly  differs  from 
Palmella  in  the  partial  persistence  of  the  walls  of  the  parent-cells,  so  that 
the  whole  mass  is  subdivided  by  partitions,  which  inclose  a  larger  or 
smaller  number  of  cells  originating  in  the  subdivision  of  their  contents. 
Besides  increasing  in  the  ordinary  mode  of  binary  multiplication,  the 
Palmella-cells  seem  occasionally  to  rupture  and  diffuse  their  granular 
contents  through  the  gelatinous  stratum,  and  thus  to  give  origin  to  a 
whole  cluster  at  once,  as  seen  at  after  the  manner  of  other  simple 
Plants  to  be  presently  described  (§  245),  save  that  these  minute  segments 
of  the  endochrome,  having  no  power  of  spontaneous  motion,  cannot  bo 
ranked  as  zoosj)ores.'  The  gelatinous  masses  of  the  Palmellse  are  fre- 
quently found  to  contain  parasitic  growths  formed  by  the  extension  of 
other  plants  through  their  substance;  but  numerous  Ijranched  filaments 
sometimes  present  themselves,  which,  being  traceable  into  absolute  con- 

Stephanosphcera  pluvialis;  and  is  scarcely  now  to  be  considered  an  exceptional 
phenomenon. 


Hcematococcus  sanguineus  in  various  stages  of  devel- 
opment :— a,  single  cells,  inclosed  in  their  mucous  envelope ; 
6,  c,  cluster  formed  by  subdivision  of  parent-cell;  d,  more 
numerous  cluster,  its  component  cells  in  various  stages  of 
division;  e,  large  mass  of  young  cells,  formed  by  the  sub- 
division of  the  parent  endochrome,  and  inclosed  within  a 
common  mucous  envelope. 


246 


tHE  MICROSCOPE  AND  ITS  REVELATIONS. 


tinuity  with  the  cells,  must  be  considered  as  proi^erly  appertaining  to 
them.  Sometimes  these  filaments  radiate  in  various  directions  from  a 
single  central  cell,  and  must  at  first  be  considered  as  mere  extensions  of 
this;  their  extremities  dilate,  however,  into  new  cells;  and  when  these  are 
fully  formed,  the  tubular  connections  close  up,  and  the  cells  become 
detached  from  each  other.  ^ — Of  the  third  condition,  we  have  an  example 
in  the  curious  Palmodictyon  described  by  Kutzing;  the  frond  of  which 
appears  to  the  naked  eye  like  a  delicate  network  consisting  of  anastomos- 
ing branches,  each  composed  of  a  single  or  double  row  of  large  vesicles, 
Avithin  every  one  of  which  is  produced  a  pair  of  elliptical  cellules  that 
ultimately  escape  as  ^zoospores.'  The  alternation  between  the  ^motile' 
form  and  the  '  still '  or  resting  form,  which  has  been  described  as  occur- 
ring in  Protococcus  (§  231),  has  been  observed  in  several  other  forms  of 
this  group;  and  it  seems  obviously  intended,  like  the  production  of 
zoospores,^  to  secure  the  dispersion  of  the  plant,  and  to  prevent  it  from 
choking  itself  by  overgrowth  in  any  one  locality. — It  is  very  commonly 
by  plants  of  this  group,  that  the  Algal  portions  of  Lichens  are  formed 
(§  325). 

244.  Notwithstanding  the  very  definite  form  and  large  size  attained 
by  the  fronds  or  leafy  expansions  of  the  UlvacecBy  to  which  group  belong 
the  grass-green  Sea- weeds  (or  Pavers')  found  on  every  coast,  yet  their 
essential  structure  differs  but  very  little  from  that  of  the  preceding 
group;  and  the  principal  advance  is  shown  in  this,  that  the  cells,  when 
multiplied  by  binary  subdivision,  not  only  remain  in  firm  connection 
with  each  other,  but  possess  a  very  regular  arrangement  (in  virtue  of  the 
determinate  plan  on  which  the  subdivision  takes  place),  and  form  a  de- 
finite membranous  expansion.  The  mode  in  which  this  frond  is  pro- 
duced may  be  best  understood  by  studyiug  the  history  of  its  develop- 
ment, some  of  the  principal  phases  of  which  are  seen  in  Fig.  144;  for  the 
isolated  cells  (a),  in  which  it  originates,  resembling  in  all  points  those  of 
a  Protococcus,  give  rise  by  their  successive  subdivisions  in  determinate 
directions,  to  such  regular  clusters  as  those  seen  at  B  and  c,  or  to  such 
Converfoid  filaments  as  that  shown  at  d.  A  continuation  of  the  same 
regular  mode  of  subdivision,  taking  place  alternately  in  two  directions, 
may  at  once  extend  the  clusters  b  and  c  into  leaf -like  expansions;  or,  if 
the  filamentous  stage  be  passed  through  (different  species  presenting 
variations  in  the  history  of  their  development),  the  filament  increases  in 
breadth  as  well  as  in  length  (as  seen  at  e),  and  finally  becomes  such  a 
'  frond  ^  as  is  shown  at  f,  g.  In  the  simple  membranous  expansion,  or 
thalhts,  thus  formed,  there  is  no  approach  to  a  differentiation  of  parts  by 
even  the  semblance  of  a  formation  of  root,  stem,  and  leaf,  such  as  the 
higher  Algse  present;  every  portion  is  the  exact  counterpart  of  every 
other;  and  every  portion  seems  to  take  an  equal  share  in  the  operations 
of  growth  and  reproduction.  Each  cell  is  very  commonly  found  to  ex- 
hibit an  imperfect  partitioning  into  four  parts,  preparatory  to  multipli- 
cation by  double  subdivision;  and  the  entire  frond  usually  shows  the 
groups  of  cells  arranged  in  clusters  containing  some  multiple  of  four. 

245.  Besides  this  continuous  increase  of  the  individual  frond,  how- 
ever, we  find  in  most  species  of  Ulva  a  provision  for  extending  the  plant 
by  the  dispersion  of  ^zoospores;'  for  the  endrochrome  (Fig.  145,  a)  sub- 


^This  fact,  first  made  public  by  Mr.  Thwaites  (*'Ann.  of  Nat.  Hist.,"  2d 
Series,  Vol.  ii.,  1848,  p.  313),  is  one  of  fundamental  importance  in  the  determina- 
tion of  the  real  character  of  this  group. 


MICROSCOPIC  FORMS  OF  VEGETABLE  LIFE. 


247 


divides  into  numerous  segments  (as  at  5  and  c),  which  at  first  are  seen  to 
lie  in  close  contact  within  the  cell  that  contains  them,  then  begin  to  ex- 
hibit a  kind  of  restless  motion,  and  at  last  pass  forth  through  an  aperture 
in  the  cell-wall,  acquire  four  or  more  flagelia  (d),  and  swim  freely 
through  the  water  for  some  time.  At  last,  however,  they  come  to  res 
attach  themselves  to  some  fixed  point, 
and  begin  to  grow  into  clusters  or 
filaments  {e),  in  the  manner  already 
described.  The  walls  of  the  cells 
which  have  thus  discharged  their  en- 
dochrome,  remain  as  colorless  spots 
on  the  frond;  sometimes  these  are 
intermingled  with  the  portions  still 
vegetating  in  the  usual  mode;  but 
sometimes  the  whole  endochrome  of 
one  portion  of  the  frond  may  thus 
escape  in  the  form  of  zoospores,  thus 
leaving  behind  it  nothing  but  a  white 
flaccid  membrane.  If  the  Micro- 
scopist  who  meets  with  a  frond  of  an 
XJlva  in  this  condition  should  ex- 
amine the  line  of  separation  between 
its  green  and  its  colored  portions, 
he  may  not  improbably  meet  with 
cells  in  the  .  very  act  of  discharging 
their  zoospores,  which  ^  swarm ' 
around  their  points  of  exit  very  much 
in  the  manner  that  Animalcules  are 
often  seen  to  do  around  particular 
spots  of  the  field  of  view,  and  which 
might  easily  be  taken  for  true  Infu- 
soria; but  on  carrying  his  observations 
further,  he  would  see  that  similar 
bodies  are  moving  withiii  cells  a  little 
more  remote  from  the  dividing  line, 
and  that,  a  little  farther  still,  they  are  obviously  but  masses  of  endochrome 
in  the  act  of  subdivision.' — Of  the  true  Generative  process  in  the  Ulvacece, 
nothing  whatever  is  known. 

246.  The  Oscillator iacem  constitute  another  tribe  of  Protophytes  of 
great  interest  to  the  Microscopist,  on  account  both  of  the  extreme  sim- 
plicity of  their  structure,  and  of  the  peculiar  Animal-like  movements 
which  they  exhibit.  They  are  continuous  tubular  filaments,  formed  by 
the  elongation  of  their  primordial  cells,  usually  lying  together  in  bundles 
or  in  strata,  sometimes  quite  free,  and  sometimes  invested  by  gelatinous 
sheaths.  The  cellulose  envelope  (Fig.  146,  A,  a,  d)  usually  exhibits  some 
degree  of  transverse  striation,  as  if  the  tube  were  undergoing  division 
into  cells;  but  this  division  is  never  perfected  by  the  formation  of  com- 
plete partitions,  though  the  endochrome  shows  a  disposition  to  separate 
into  regular  segments  (b,  c),  especially  when  treated  with  re-agents. 


^  Such  an  observation  the  Author  had  the  good  fortune  to  make  in  the  year 
1842,  when  the  emission  of  zoospores  from  the  Ulvacece,  although  it  had  been  de- 
scribed by  the  Swedish  Algologist  Agardh  had  not  been  seen  (he  believes)  by  any 
British  naturalist. 


248 


THE  MICROSCOPE  AND  ITS  KEVELATIONS. 


According  to  Dr.  F.  d'Alquen/  each  filament — at  least  in  certain  species 
— has  an  axis  of  different  composition  from  the  surrounding  endoclirome; 
being  solid,  highly  refractive,  but  slightly  affected  by  iodine,  and  nearly 
colorless  when  moist,  though  slightly  greenish  when  dry.  And  he  gives 
reasons  for  the  belief  that  it  is  in  this  (protoplasmic?)  axis  that  the  pecu- 
liar motor  power  of  the  filament  specially,  if  not  exclusively,  resides. 
The  filaments  ultimately  break  up  into  distinct  joints;  the  fragments  of 
endochrome,  which  are  to  be  regarded  as  gonidia,  usually  escaping  from 
their  sheaths,  and  giving  origin  to  new  filaments. — These  plants  are 
commonly  of  some  shade  of  green,  often  mingled,  however,  with  blue; 
but  not  unfrequently  they  are  of  a  puriolish  hue,  and  are  sometimes  so 
dark  as  when  in  mass  to  seem  nearly  black.  They  occur  not  only  in 
fresh,  stagnant,  brackish,  and  salt  waters  (certain  species  being  peculiar 
to  each),  but  also  in  mud,  on  wet  stones,  or  on  damp  ground.  Their 
movements  are  described  by  Dr.  Harvey^  as  of  three  kinds;  first,  a  pen- 


FiG.  145. 


Formation  of  Zoospores  in  Phycoseris  gigantea  {Viva,  latissima) :— a,  portion  of  the  ordinary- 
frond;  6,  cells  in  which  the  endochrome  is  beginning  to  break  up  into  segments;  c,  cells  from  the 
boundary  between  the  colored  and  colorless  portions,  some  of  them  containing  zoospores,  others 
being  empty;  d»  flagellated  zoospores,  as  in  active  motion;  e,  subsequent  development  of  the  zoo- 
spores. 

dulum-like  movement  from  side  to  side,  performed  by  one  end,  whilst 
the  other  remains  fixed  so  as  to  form  a  sort  of  pivot;  second,  a  movement 
of  flexure  of  the  filament  itself,  the  oscillating  extremity  bending  over, 
first  to  one  side  and  then  to  the  other,  like  the  head  of  a  worm  or  cater- 
pillar seeking  something  on  its  line  of  march;  and  third,  a  simple  onward 
movement  of  progression.  "  The  whole  phenomenon,'^  continues  Dr.  H., 
"  may  perhaps  be  resolved  into  a  spiral  onward  movement  of  the  filament. 
If  a  piece  of  the  stratum  of  an  Oscillatoria  be  placed  in  a  vessel  of  water, 
and  allowed  to  remain  there  for  some  hours,  its  edge  will  first  become 
fringed  with  filaments,  radiating  as  from  a  central  point,  with  their  tips 
outwards.  These  filaments,  by  their  constant  oscillatory  movements,  are 
continually  loosened  from  their  hold  on  the  stratum,  cast  into  the  water, 


»  Quart.  Journ.  of  Microsc.  Science,"  Vol.  iv.  (1856),  p.  245. 
2    Manual  of  British  Marine  Algse,"  p.  220. 


MICROSCOPIC  FORMS  OF  VEGETABLE  LIFE. 


249 


and  at  the  same  time  propelled  forward;  and  as  the  oscillation  continues 
after  the  filament  has  left  its  nest,  the  little  swimmer  gradually  moves 
along,  till  it  not  only  reaches  the  edge  of  the  vessel,  but  often — as  if  in 
the  attempt  to  escape  confinement — continues  its  voyage  up  the  sides, 
till  it  is  stopped  by  dryness.  Thus  in  a  very  short  time  a  small  piece  of 
Oscillatoria  will  spread  itself  over  a  large  vessel  of  water/^  This  rhyth- 
mical movement,  impelling  the  filaments  in  an  undeviating  onward 
course,  is  greatly  influenced  by  temperature  and  light,  being  much  mere 
active  in  warmth  and  sunshine  than  in  cold  and  shade;  and  it  is  checked 
by  any  strong  chemical  agents. — The  true  Generation  of  Oscillatoriacem 
is  as  yet  completely  unknown. 

247.  Nearly  allied  to  the  preceding  is  the  little  tribe  of  NostocliacecB ; 
Avliich  consists  of  distinctly-beaded  filaments,  lying  in  firmly-gelatinous 
fronds  of  definite  outline  (Fig.  147).    The  filaments  are  usually  simple. 


if 

ii 

Hi 

Jiil 

II 

ii 


I 

M 
i 


structure  of  Oscillatoria  contexta. 
— A,  portion  of  a  filament,  showing 
the  striations  on  the  cellulose-coat,  a, 
a,  where  the  endochrome  is  wanting; 
B,  portion  of  filament  treated  with 
weak  syrup,  showing  a  disposition  to 
a  regular  breaking- up  of  the  endo- 
chrome into  masses;  c,  portion  of  fila- 
ments treated  with  strong  solution  of 
chloride  of  calcium,  showing  a  more 
advanced  stage  of  the  same  separa- 
tion. 


Portion  of  gelatinous  frond  of 
Nostoc. 


though  sometimes  branched;  and  are  almost  always  curved  or  twisted, 
often  taking  a  spiral  direction.  The  masses  of  jelly  in  which  they  are 
imbedded  are  sometimes  globular  or  nearly  so,  and  sometimes  extend  in 
more  or  less  regular  branches;  they  frequently  attain  a  very  considerable 
size;  and  as  they  occasionally  present  themselves  quite  suddenly  (espe- 
cially in  the  latter  part  of  autumn,  on  damp  garden-walks),  they  have 
received  the  name  of  ^fallen  stars. ^  They  are  not  always  so  suddenly 
produced,  however,  as  they  appear  to  be;  for  they  shrink  up  into  mere 
films  in  dry  v^eather,  and  expand  again  with  the  first  shower.*  Nostoos 
multiply,  like  the  Oscillatoriacese,  by  the  subdivision  of  their  filaments, 
portions  of  which  escape  from  the  gelatinous  mass  wherein  they  were  im- 
bedded, and  move  slowly  through  the  water  in  the  direction  of  their 
length:  after  a  time  they  cease  to  move,  and  a  new  gelatinous  envelope 
is  formed  around  each  piece,  which  then  begins  not  only  to  increase  in 
length  by  the  transverse  subdivision  of  its  segments,  but  also  to  double 


^  See  Hicks  in    Quart.  Journ.  of  Microsc.  Science.,"  N.S.,  Vol.  i.  (1861),  p.  90. 


250 


THE  MICROSCOPE  AND  ITS  REVELATIONS. 


itself  by  longitudinal  fission,  so  that  each  filament  splits  lengthways  (as 
it  were)  into  two  new  ones.  By  the  repetition  of  this  process  a  mass  of 
new  filaments  is  produced,  the  parts  of  which  are  at  first  confused,  but 
afterwards  become  more  distinctly  separated  by  the  interposition  of  the 
gelatinous  substance  developed  between  them. — Besides  the  ordinary  cells 
of  the  beaded  filaments,  two  other  kinds  are  occasionally  observable;  but 
it  has  not  yet  been  ascertained  whether  these  are  in  any  way  subservient 
to  the  true  Generative  act. 

248.  Although  many  of  the  plants  belonging  to  the  Family  Sipho- 
nacecB  attain  a  considerable  size,  and  resemble  the  higher  Sea  weeds  in 
their  general  mode  of  growth,  yet  they  retain  a  simplicity  of  structure  so 
extreme  that  it  apparently  requires  them  to  be  ranked  among  the  Proto- 
phytes.  They  are  inhabitants  both  of  fresh-water  and  of  the  sea;  and 
consist  of  very  large  tubular  cells,  which  commonly  extend  themselves 
into  branches,  so  as  to  form  an  aborescent  frond.  These  branches,  how- 
ever, are  seldom  separated  from  the  stem  by  any  intervening  partition; 
but  the  whole  frond  is  composed  of  a  simple  continuous  tube,  the  entire 
contents  of  which  may  be  readily  pressed -out  through  an  orifice  made  by 
wounding  any  part  of  the  wall.  The  Vauoheria,  named  after  the  Gene- 
vese  botanist  by  whom  the  fresh-water  Confervse  were  first  carefully 
studied,  may  be  selected  as  a  particularly  good  illustration  of  this  family; 
its  history  having  been  pretty  completely  made  out.  Most  of  its  species 
are  inhabitants  of  fresh  water;  but  some  are  marine;  and  they  commonly 
present  themselves  in  the  form  of  cushion-like  masses,  composed  of  irreg- 
ularly branching  filaments,  which,  although  they  remain  distinct,  are 
densely  tufted  together  and  variously  interwoven. — The  formation  of 
motile  gonidia  or  ^  zoospores  ^  may  be  readily  observed  in  these  plants,  the 
whole  process  usually  occupying  but  a  very  short  time.  The  extremity  of 
one  of  the  filaments  usually  swells  up  in  the  form  of  a  club,  and  the  en- 
dochrome  accumulates  in  it  so  as  to  give  it  a  darker  hue  than  the  rest;  a 
separation  of  this  part  from  the  remainder  of  the  filament,  by  the  inter- 
position of  a  transparent  space,  is  next  seen;  a  new  envelope  is  then 
formed  around  the  mass  thus  cut  off;  and  at  last  the  membranous  wall  of 
the  investing  tube  gives  way,  and  the  ^zoospore'  escapes,  not,  however, 
until  it  has  undergone  marked  changes  of  form,  and  exhibited  curious 
movements.  Its  motions  continue  for  some  time  after  its  escape,  and  are 
then  plainly  seen  to  be  due  to  the  action  of  the  cilia  with  which  its  whole 
surface  is  clothed.  If  it  be  placed  in  water  in  which  some  carmine  or  indigo 
has  been  rubbed,  the  colored  granules  are  seen  to  be  driven  in  such  a  manner 
as  to  show  that  a  powerful  current  is  produced  by  their  propulsive  action, 
and  a  long  track  is  left  behind  it.  When  it  meets  with  an  obstacle,  the 
ciliary  action  not  being  arrested,  the  zoospore  is  flattened  against  the  ob- 
ject; and  it  may  thus  be  compressed,  even  to  the  extent  of  causing  its  en- 
dochrome  to  be  discharged.  The  cilia  are  best  seen  when  their  move- 
ments have  been  retarded  or  entirely  arrested  by  means  of  opium,  iodine, 
or  other  chemical  re-agents.  The  motion  of  the  spore  continues  for  about 
two  hours;  but  after  the  lapse  of  that  time  it  soon  comes  to  an  end,  and 
the  spore  begins  to  develop  itself  into  a  new  plant.  It  has  been  observed 
by  linger,  that  the  escape  of  the  zoospores  generally  takes  place  towards 
8  A.M.;  to  watch  this  phenomenon,  therefore,  the  plant  should  be  gath- 
ered the  day  before,  and  its  tufts  examined  early  in  the  morning.  It  is 
stated  by  Dr.  Hassall,  that  he  has  seen  the  same  filament  give  off  two  or 
three  zoospores  successively. 

249.  Kecent  discoveries  have  shown  that  there  exists  in  this  humble 


MICROSCOPIC  FORMS  OF  VEGETABLE  LIFE. 


251 


plant  a  true  process  of  sexual  Generation,  as  was,  indeed,  long  ago  sus- 
pected by  Vaucher,  though  upon  no  sufficient  grounds.  The  branching 
filaments  are  o'ften  seen  to  bear  at  their  sides  peculiar  globular  or  oval 
capsular  protuberances,  sometimes  separated  by  the  interposition  of  a 
stalk,  which  are  filled  with  dark  endochrome;  and  these  give  exit  to  largo ' 
bodies  covered  with  a  firm  envelope, 
from  which,  after  a  time,  new  plants 
arise.  In  the  immediate  neighborhood  ' 
of  these  ^capsules'  are  always  found 
certain  other  projections,  which,  from 
being  usually  pointed  and  somewhat 
curved,  have  been  named  '  horns  ^  (Fig. 
148,  A,  a);  and  these  have  been  shown 
by  Pringsheim  to  be  ^  antheridia,'  which 
produce,  ^antherozoids^  in  their  inte- 
rior; whilst  the  capsules  (a,  o)  are 
^  oogonia,^  each  containing  a  mass  of  en- 
dochrome which  constitutes  a  ^  germ- 
cell  '  that  is  destined  to  become,  when 
fertilized,  the  primordial  cell  of  a  new 
generation.  The  antherozoids  (b,  c,  d), 
when  set  free  from  the  antheridium  a, 
swarm  over  the  exterior  of  the  oogon- 
ium b,  and  have  actually  been  seen  to 
penetrate  its  cavity  through  an  aperture 
which  opportunely  forms  in  its  wall, 
and  to  come  into  contact  with  the  sur- 
face of  its  endochrome-mass,  over  which 
they  diffuse  themselves;  there  they  seem 
to  undergo  dissolution,  their  substance 
mingling  itself  Avith  that  of  the  germ- 
cell;  and  the  endoplasm  of  the  ^  oospore' 
thus  formed,  which  had  previously  no 
proper  investment  of  its  own,  soon 
begins  to  form  an  envelope  (c,  b),  which 
increases  in  thickness  and  strength,  un- 
til it  has  acquired  such  a  density  as 
enables  it  to  afford  a  firm  protection  to 
its  contents. 

250.  The  Microscopist  who  wishes 
to  study  the  development  of  ^  zoospores,' 
as  well  as  several  other  phenomena  of 
this  low  type  of  vegetation,  may  ad- 
vantageously have  recourse  to  the  little 
plant  termed  Achlya  proUfera,  whicli 
grows  parasitically  upon  the  bodies  of 
dead  Flies  lying  in  the  water,  but  also 
not  unfrequentljr  attaches  itself  to  the 
gills  of  Fish,  and  is  occasionally  found  on  the  bodies  of  Frogs.  ^    Its  tufts 


Successive  phases  of  Generative  process 
in  Vaucheria  sessilis:— At  A  are  seen  one 
of  the  'horns'  or  Antheridia  (a)  and  one 
of  the  Capsules  (6),  as  yet  unopened;  at  b 
the  antheridium  is  seen  in  the  act  of  emit- 
ting the  antherozoids  (c),  of  which  many 
enter  the  opening  at  the  apex  of  the  cap- 
sule, whilst  others  (d)  which  do  not  enter 
it,  display  their  flagella  when  they  become 
motionless;  at  c  the  orifice  of  the  capsule 
is  closed  again  by  the  formation  of  a  pro- 
per coat  around  its  endochrome,  thus  con- 
stituting an  oospore. 


^  This  Plant,  though,  as  an  inhabitant  of  water,  formerly  ranked  among  Algce, 
is  now  more  generaUy  regarded  as  belonging  to  the  group  of  Fungi,  on  account 
of  its  incapacity  for  the  production  of  chlorophyll,  and  its  parasitism  on  the 
bodies  of  Animals,  from  whose  juices  its  ceUs  seem  to  draw  their  nourishment. 


252 


THE  MICROSCOPE  AND  ITS  REVELATiONS. 


are  distinguishable  by  the  naked  eye  as  clusters  of  minute  colorless  fila- 
ments; and  these  are  found,  when  examined  by  the  microscope,  to  be  long 
tubes,  devoid  of  all  partitions,  extending  themselves  in  various  directions. 
The  tubes  contain  a  colorlesss  lightly-granular  protoplasm,  the  particles  of 
which  are  seen  to  move  slowly  in  streams  along  the  walls,  as  in  Chara,  the 
currents  occasionally  anastomosing  with  each  other  (Fig.  149,  c).  Within 
about  thirty- six  hours  after  the  first  appearance  of  the  parasite  on  anyi 
body,  the  protoplasm  begins  to  accumulate  in  the  dilated  ends  of  the  fil- 
aments, each  of  which  is  cut  off  from  the  remainder  by  the  formation  of 
a  partition;  and  within  this  dilated  cell  the  movement  of  the  protoplasm 
continues  for  a  time  to  be  distinguishable.  Very  speedily,  however,  its 
endoplasm  shows  the  appearance  of  being  broken  up  into  a  large  number 
of  distinct  masses,  which  are  at  first  in  close  contact  with  each  other, 
and  with  the  walls  of  the  cell  (Fig.  149,  a),  but  which  gradually  become 
more  isolated,  each  seeming  to  acquire  a  proper  cell-wall;  they  then  begin 
to  move  about  within  the  parent-cell;  and,  when  quite  mature,  they  are 
set  free  by  the  ruptusre  of  its  wall  (b),  to  go  forth  and  form  new  attach- 
ments, and  to  develop  themselves  into  tubiform  cells  resembling  those 
from  which  they  sprang.  Each  of  these  '  motile  gonidia'  is  possessed  of 
two  flagella;  their  movements  are  not  so  powerful  as  those  of  the  zoo- 
spores of  Vaucheria,  and  come  to  an  end  sooner. — The  Generative  process 
in  this  type  is  performed  in  a  manner  that  may  be  regarded  as  an  advance 
upon  ordinary  conjugation.  The  end  of  one  of  the  long  tubiform  cells 
enlarges  into  a  globular  dilatation,  the  cavity  of  which  becomes  shut  off 
by  a  transverse  partition.  Its  contained  endoplasm  divides  into  two, 
three,  or  four  segments,  each  of  which  takes  a  globular  form,  and  is  then 
fertilized  by  the  penetration  of  an  antheridial  tube  which  comes  off  from 
the  filament  a  little  below  the  partition.^  The  '  oospores'  thus  produced, 
escaping  from  the  globular  cavities,  acquire  firm  envelopes,  and  may  re- 
main unchanged  for  a  long  time  even  in  Avater,  when  no  appropriate 
nidus  exists  for  them;  but  will  quickly  germinate  if  a  dead  Insect  or 
other  suitable  object  be  thrown-in. 

251.  One  of  the  most  curious  forms  of  this  group  is  the  Hydrodidyon 
utriculatum,  which  is  found  in  fresh-water  pools  in  the  midland  and 
southern  counties  of  England.  Its  frond  consists  of  a  green  open  net- 
work of  filaments,  acquiring,  when  full  grown,  a  length  of  from  four  to 
six  inches,  and  composed  of  a  vast  number  of  cylindrical  tubular  cells, 
which  attain  the  length  of  four  lines  or  more,  and  adhere  to  each  other 
by  their  rounded  extremities,  the  points  of  junction  corresponding  to  the 
knots  or  intersections  of  the  network.  Each  of  these  cells  may  form 
within  itself  an  enormous  multitude  (from  7,000  to  20,000)  of  ^swarm- 
spores,'  which,  at  a  certain  stage  of  their  development,  are  observed  in 
active  motion  in  its  interior;  but  of  which  clusters  are  afterwards  formed 
by  their  mutual  adhesion,  that  are  set  free  by  the  dissolution  of  their 
envelopes,  each  cluster,  or  ^macro-gonidium,'  giving  origin  to  a  new 
plant-net.  Besides  these  bodies,  however,  certain  cells  produce  from 
30,000  to  100,000  ^micro-gonidia'  of  longer  shape,  each  furnished  with 
four  long  flagella  and  a  red  spot;  these  escape  from  the  cell  in  a  swarm, 
move  freely  in  the  water  for  some  time,  and  then  come  to  rest  and  sink 
to  the  bottom,  where  they  remain  heaped  in  green  masses.  It  appears 
from  the  observations  of  Pringsheim,^  that  they  become  surrounded  with 

1  See  Prof.  Sachs's  '^Text-book  of  Botany"  (translated  by  A.  W.  Bennett), 
p.  12. 

8    Quart.  Joum.  of  Microsc.  Science,"  N.S.,  Vol.  ii.  (1862),  pp.  54,  104. 


MICROSCOPIC  FORMS  OF  VEGETABLE  LIFE. 


253 


a  firm  cellulose  envelope,  and  may  remain  for  a  considerable  length  of 
time  in  a  dormant  condition,  in  which  they  are  known  as  ^  statospores;' 
and  that  in  this  state  they  are  able  to  endure  being  completely  dried  up 
without  the  loss  their  vitality,  provided  that  they  are  secluded  from  the 
action  of  Light,  which  causes  them  to  wither  and  die.  In  this  state 
they  bear  a  strong  resemblance  to  the  cells  of  Protococcus, — The  first 
change  that  manifests  itself  in  them  is  a  simple  enlargement;  next,  the 
endochrome  divides  itself  successively  into  distinct  masses,  usually  from 
two  to  five  in  number;  and  these,  when  set  free  by  the  giving  way  of  the 
enveloping  membrane,  present  the  characters  of  ordinary  ^zoospores,' 
each  of  them  possessing  one  or  two  flagella  at  its  anterior  semi-trans- 
parent extremity.    Their  motile  condition,  however,  does  not  last  long. 


Development  of  Achlya  proUfera : — a,  dilated 
extremity  of  a  filament  b,  separated  from  the 
rest  by  a  partition  a,  and  containing^  gonidia  in 
progress  of  formation  ; — b,  conceptacle  discharg- 
ing itself,  and  setting  free  gonidia,  a,  b,  c c, 
portion  of  filament,  showing  the  course  of  the 
circulation  of  granular  protoplasm. 


FiQ.  150. 

A 


Process  of  cell-multiplication 
in  Conferva  glomerata a,  por- 
tion of  filament  with  incomplete 
separation  at  a,  and  complete  par- 
tition at  b;  B,  the  separation  com- 
pleted, a  new  cellulose  partition 
being  formed  at  a ;  c,  formation 
of  additional  layers  of  cellulose 
wall  c,  beneath  the  mucous  invest- 
ment d,  and  around  the  ectoplasm 
a,  which  incloses  the  endochrome 
6. 


often  giving  place  to  the  motionless  stage  before  they  have  quite  freed 
themselves  from  the  parent-cell;  they  then  project  long  angular  pro- 
cesses, so  as  to  assume  the  form  of  irregular  polyhedra,  at  the  same  time 
augmenting  in  size;  and  the  endochrome  contained  within  each  of  these 
breaks-up  into  a  multitude  of  gonidia,  which  are  at  first  quite  independ- 
ent and  move  actively  within  the  cell-cavity,  but  soon  unite  into  a 
network  that  becomes  invested  with  a  gelatinous  envelope,  and  speedily 
increases  so  much  in  size  as  to  rupture  the  containing  cell-wall,  on 
escaping  from  which  it  presents  all  the  essential  characters  of  a  young 
Hydrodictyon.  Thus,  whilst  this  plant  multiplied  itself  by  ^macro- 
gonidia'  during  the  period  of  its  most  active  vegetation,  this  method  of 
multiplication  by  ^micro-gonidia'  appears  destined  to  secure  its  perpetu- 
ation under  conditions  that  would  be  fatal  to  it  in  its  perfect  form. — 


254 


THE   MICROSCOPE   AND   ITS  KEVELATIONS. 


The  rapidity  of  the  growth  of  this  curious  organism  is  not  one  of  the 
least  remarkable  parts  of  its  history.  The  individual  cells  of  which  the 
net  is  composed,  at  the  time  of  their  emersion  as  ^gonidia/  measure  no 
more  than  l-2500th  of  an  inch  in  length;  but  in  the  course  of  a  few 
hours,  they  grow  to  a  length  of  from  l-12th  to  l-3d  of  an  inch. — Nothing 
has  been  as  yet  ascertained  respecting  the  sexual  Generation  of  this 
type. 

252.  Almost  every  pond  and  ditch  contains  some  members  of  the 
Family  Confervacm  ;  but  they  are  especially  abundant  in  moving  water; 
and  they  constitute  the  greater  part  of  those  green  threads  which  are  to 
be  seen  attached  to  stones,  with  their  free  ends  floating  in  the  direction 
of  the  current,  in  every  running  stream,  and  upon  almost  every  part  of 
the  sea-shore,  and  which  are  commonly  known  under  the  name  of  ^silk- 
weeds,' or  ^crow-silk.'  Their  form  is  usually  very  regular,  each  thread 
being  a  long  cylinder  made  up  by  the  union  of  a  single  file  of  short  cylin- 
drical cells  united  to  each  other  by  their  flattened  extremities;  sometimes 
these  threads  give  off  lateral  branches,  which  have  the  same  structure. 
The  endochrome,  though  usually  green,  is  occasionally  of  a  brown  or 
purple  hue;  it  is  sometimes  distributed  uniformly  throughout  the  cell 
(as  m  Fig.  150),  whilst  in  other  instances  it  is  arranged  in  a  pattern  of 
some  kind,  as  a  network  or  spiral;  but  this  may  be  only  a  transitory 
stage  in  its  development. — The  plants  of  this  family  are  extremely  favor- 
able subjects  for  the  study  of  the  method  of  cell-multiplication  by  Unary 
subdivision.  This  process  usually  takes  place  only  in  the  terminal  cell; 
and  it  may  be  almost  always  observed  there  in  some  one  of  its  stages. 
The  first  step  is  seen  to  be  the  subdivision  of  the  endochrome,  and  the 
inflexion  of  the  ectoplasm  around  it  (Fig:.  150,  A,  a)\  and  thus  there  is 
gradually  formed  a  sort  of  hour-glass  contraction  across  the  cavity  of  the 
parent-cell,  by  which  it  is  divided  into  two  equal  halves  (b).  The  two 
surfaces  of  the  infolded  utricle  produce  a  double  layer  of  cellulose-mem- 
brane between  them;  this  is  not  confined,  however,  to  the  contiguous 
surfaces  of  the  young  cells,  but  extends  over  the  whole  of  their  exterior, 
so  that  the  new  septum  becomes  continuous  with  a  new  layer  that  is 
formed  throughout  the  interior  of  the  cellulose  wall  of  the  original  cell 
(c).  Sometimes,  however,  as  in  Conferva  glomerata  (a  common  species), 
new  cells  may  originate  as  branches  from  any  part  of  the  surface,  by  a 
process  of  budding;  which,  notwithstanding  its  difference  of  mode,  agrees 
with  that  just  described  in  its  essential  character,  being  the  result  of  the 
subdivision  of  the  original  cell.  A  certain  portion  of  the  ectoplasm  seems 
to  undergo  increased  nutrition,  for  it  is  seen  to  project,  carrying  the 
cellulose  envelope  before  it,  so  as  to  form  a  little  protuberance;  and  this 
sometimes  attains  a  considerable  length,  before  any  separation  of  its  cavity 
from  that  of  the  cell  which  gave  origin  to  it  begins  to  take  place.  This 
separation  is  gradually  effected,  however,  by  the  infolding  of  the  ecto- 
plasm, just  as  in  the  preceding  case:  and  thus  the  endochrome  of  the 
branch-cell  becomes  completely  severed  from  that  of  the  stock.  The 
branch  then  begins  to  elongate  itself  by  the  subdivision  of  its  first-formed 
cell;  and  this  process  may  be  repeated  for  a  time  in  all  the  cells  of  the 
filament,  though  it  usually  comes  to  be  restricted  at  last  to  the  terminal 
cell. — The  Conf ervacem  multiply  themselves  by  zoospores,  which  are  pro- 
duced within  their  cells,  and  are  then  set  free,  just  as  in  the  Ulvaceae 
(§  245). 

253.  A  true  sexual  Generation  has  been  observed  in  several  Oonfer- 
vacese,  and  is  probably  universal  throughout  the  group.    It  is  presented 


MICROSCOPIC  FORMS  OF  VEGETABLE  LIFE. 


255 


under  a  very  interesting  form  in  a  plant  termed  Sphceroplea  annulina, 
the  development  and  generation  of  which  have  been  specially  studied  by 
Dr.  F.  Cohn.^  The  *  oospore/  which  is  the  product  of  the  sexual  pro- 
cess to  be  presently  described,  is  filled  when  mature  with  a  red  oil,  aud 
is  enveloped  by  two  membranes,  of  which  the  outer  one  is  furnished  with 
stellate  prolongations  (Plate  x.,  fig.  1).  When  it  begins  to  vegetate,  its 
endochrome  breaks  up — first  into  two  halves  (fig.  2),  and  then  by  succes- 
sive subdivisions  into  numerous  segments  (figs.  3,  4),  at  the  same  time 
becoming  green  towards  its  margin.  These  segments,  set  free  by  the 
rupture  of  their  containing  envelope,  escape  as  ^  micro-gonidia,^  which 
are  at  first  rounded  or  oval,  each  having  a  semi-transparent  beak  whence 
proceed  two  flagella,  but  which  gradually  elongate  so  as  to  become  fusi- 
form (fig.  5),  at  the  same  time  changing  their  color  from  red  to  green. 
These  move  actively  for  a  time,  and  then,  losing  their  motile  power, 
begin  to  develop  themselves  into  filaments.  The  first  stage  in  this 
development  consists  in  the  elongation  of  the  cell,  and  the  separation  of 
the  endochrome  of  its  two  halves  by  the  interposition  of  a  vacuole  (fig. 
6);  and  in  more  advanced  stages  (figs.  7,  8)  a  repetition  of  the  like 
interposition  gives  to  the  endochrome  that  annular  arrangement  from 
which  the  plant  derives  its  specific  name.  This  is  seen  p-t  a,  fig.  9,  as  it 
presents  itself  in  the  filaments  of  the  adult  plant;  whilst  at  J,  in  the  same 
figure,  we  see  a  sort  of  frothy  appearance  which  the  endochrome  comes 
to  possess  through  the  multiplication  of  the  vacuoles.  The  next  stage  in 
the  development  of  the  filaments  that  are  to  produce  the  oospores,  con- 
sists in  the  aggregation  of  the  endochrome  into  definite  masses  (as  seen 
at  fig.  10,  a),  which  soon  become  star-shaped  (as  seen  at  5),  each  one 
being  contained  within  a  distinct  compartment  of  the  cell.  In  a  some- 
what more  advanced  stage  (fig.  11,  a),  the  masses  of  endochrome  begin 
to  draw  themselves  together  again;  and  they  soon  assume  a  globular  or 
ovoidal  shape  (Z*),  whilst  at  the  same  time  definite  openings  (c)  are 
formed  in  their  containing  cell-wall.  Through  these  openings  the 
'  antherozoids  ^  developed  within  other  filaments  gain  admission,  as 
shown  at  rf,  fig.  12;  and  they  seem  to  dissolve  away  (as  it  were)  upon  the 
surface  of  the  before-mentioned  masses,  which  soon  afterwards  become 
invested  with  a  firm  membranous  envelope,  as  shown  in  the  lower  part 
of  fig.  12.  These  undergo  further  changes  whilst  still  contained  within 
their  tubular  parent-cells;  their  color  passing  from  green  to  red,  and  a 
second  investment  being  formed  within  the  first,  which  extends  itself 
into  stellate  prolongations,  as  seen  in  fig.  13;  so  that,  when  set  free,  they 
precisely  resemble  the  mature  oospores  which  we  have  taken  as  the 
starting-point  in  this  curious  history.  Certain  of  the  filaments  (fig.  14), 
instead  of  giving  origin  to  spores,  have  their  annular  collections  of  endo- 
chrome converted  into  *  antherozoids,^  which,  as  soon  as  they  have  disen- 
gaged themselves  from  the  mucilaginous  sheath  that  envelops  them, 
move  about  rapidly  in  the  cavity  of  their  containing  cell  (a,  h)  around 
the  large  vacuoles  which  occupy  its  interior;  and  then  make  their  escape 
through  apertures  {c,  d)  which  form  themselves  in  its  wall,  to  find  their 
way  through  similar  apertures  into  the  interior  of  the  spore-bearing  cells, 
as  already  described.  These  antherozoids  are  shown  in  fig.  15,  as  they 
appear  when  swimming  actively  through  the  water  by  means  of  the  two 
flagella  which  each  possesses. — The  peculiar  interest  of  this  history  con- 
sists in  the  entire  absence  of  any  special  organs  for  the  Generative  process. 


^    Ann.  des  Sci.  Nat.,"  4eme  Ser.,  Botan.,  Tom.  v.,  p.  187. 


256 


THE  MICBOSCOPE  AND  ITS  REVELATIONS. 


DEVELOPMENT  AND  REPRODUCTION  OP  SPaffiJROPLEA  ANNULINA  (after  Cohn). 

Fig.  1.  Oospore,  of  a  red  color,  having  its  outer  membrane  furnished  with  stellate  prolonga' 
taons. 

2,  3,  4.  Successive  stages  of  segmentation  of  the  oospore. 

5.  Fusiform  flagellated  zoospores  set  free  by  the  rupture  of  the  coats  of  the  oospore. 

6,  7,  8.  Successive  stages  of  development  of  zoospore  into  a  filament. 

9.  Immature  filament,  showing  at  a  the  annulation  of  the  endochrome  produced  by  the  regular 
arrangement  of  vacuoles,  and  at  6,  the  frothy  appearance  produced  by  the  multiplication  of 
vacuoles. 

10.  More  advanced  stage,  showing  at  a  the  aggregation  of  the  endochrome  into  definite  masses 
which  become  star-shaped  as  seen  at  h. 

11.  The  star-shaped  masses  of  endochrome,  a,  draw  themselves  together  again  and  become 
ovoidal,  as  at  6  ;  definite  openings,  c,  show  themselves  in  the  cell-wall. 

12.  Entrance  of  the  antherozoids,  d,  through  the  openings,  c,  c. 

13.  Formation  of  mature  oospores  within  the  filament. 

14.  Contents  of  another  filament,  a,  becoming  converted  into  antherozoids,  which  move  about 
at  b  within  their  containing  cell,  and  escape  (as  seen  at  d)  through  the  opening  c. 

15.  Antherozoids  swimming  freely  by  means  of  two  flagella. 


MICROSCOPIC  FORMS  OF  VEGETABLE  LIFE. 


257 


the  ordinary  filamentous  cells  developing  oospores  on  the  one  hand,  and 
antherozoids  on  the  other;  and  in  the  simplicity  of  the  means  by  which 
the  fecundating  process  is  accomplished. 

254.  The  CEdogoniecB  resemble  Confervacece  in  general  aspect  and 
habit  of  life,  but  differ  from  them  in  some  curious  particulars.  As  the 
component  cells  of  the  filaments  extend  themselves  longitudinally,  new 
rings  of  cellulose  are  formed  successively,  and  intercalated  into  the  cell- 
wall  at  its  upper  end,  giving  it  a  ringed  appearance.  Only  a  single  large 
zoospore  is  set  free  from  each  cell;  and  its  liberation  is  accomplished  by 
the  almost  complete  fission  of  the  wall  of  the  cell  through  one  of  these 
rings,  a  small  part  only  remaining  uncleft,  which  serves  as  a  kind  of 
hinge  whereby  the  two  parts  of  the  filament  are  prevented  from  being 


A.  Sexual  generation  of  (Edogonium  ciliatum ;— 1,  filament  with  two  oospores  in  process  of 
formation,  the  lower  one  having  two  androspores  attached  to  its  exterior,  the  contents  of  the 
upper  one  in  the  act  of  being  fertilized  by  the  entrance  of  an  antherozoid  set  free  from  the  interior 
of  its  androspore ;  2  free  antherozoids;  3,  mature  oospore,  still  invested  with  the  cell-membrane 
of  the  parent  filament;  4,  portions  of  a  filament  bearing  sperm  cells,  from  one  of  which  an  andro- 
spore is  being  set  free;  5,  liberated  androspore. 

B.  Branches  of  Choetophora  elegans,  in  the  act  of  discharging  flagellated  zoopores,  which  are 
seen,  as  in  motion,  on  the  right. 

altogether  separated.  Sometimes  the  zoospore  does  not  completely  extri- 
cate itself  from  the  parent-cell;  and  it  may  begin  to  grow  in  this  situa- 
tion, the  root-like  processes  which  it  puts  forth  being  extended  into  the 
cavity.  Professor  A.  M.  Edwards  (U.  S.)  states  that  he  has  seen  the 
so-called  ^motile  spores'  of  the  (Edogo7iium  develop  into  objects  exactly 
resembling  Euglenm,  and  finally  reproducing  "  a  filament  exactly  like 
that  from  which  the  original  green  spore  was  projected."  He  further 
a'sserts  he  has  seen  the  cell-contents  of  (Edogonium  develop  into  forms 
identical  with  several  genera  of  Ehrenberg's  Polygastric  Animalcules.^ 


1  Monthly  Microsc.  Journal,"  Vol.  viii.  (1872),  p.  28. 
17 


258 


THE  MICROSCOPE  AND  ITS  REVELATIONS. 


Observations  of  an  analogous  character  were  previously  made  by  Oohn 
and  Itzigsohn. 

255.  In  their  generative  process,  also,  the  CEdogoniece  show  a  curious 
departure  from  tlie  ordinary  type;  for  whilst  the  '  oospores'  are  formed 
within  certain  dihited  cells  of  the  ordinary  filament  (Fig.  151,  A,  i),  and 
are  fertilized  by  the  penetration  of  '  antherozoids '  (2),  tliese  antherozoids 
are  not  the  immediate  product  of  the  sperm-cells  of  the  same  or  of 
another  filament,  but  are  developed  within  a  body  termed  an  '  andro- 
spore'  (5),  which  is  to  be  set  free  from  within  a  sperm-cell  (4),  and 
which,  being  furnished  with  a  circular  fringe  of  cilia,  and  having  motile 
powers,  very  strongly  resembles  an  ordinary  zoospore.  This  androspore, 
after  its  period  of  activity  has  come  to  an  end,  attaches  itself  to  the  outer 
surface  of  a  germ-cell,  as  shown  at  i,  b  ;  it  then  undergoes  a  change  of 
shape,  and  a  sort  of  lid  drops  off  from  its  free  extremity  as  seen  in  the 
upper  part  of  i,  by  which  its  contained  antherozoids  (2)  are  set  free;  and 
at  the  same  time  an  aperture  is  formed  in  the  wall  of  the  germ-cell,  by 
which  the  antherozoid  enters  its  cavity,  and  fertilizes  its  endoplasm  by 
dissolving  upon  it  and  blending  with  it.  This  mass  then  becomes  an 
oospore  (3),  invested  with  a  thick  wall  of  its  own,  but  still  retains  more 
or  less  of  the  envelope  derived  from  the  cell  within  which  it  was  devel- 
oped.^ It  is  probable  that  the  same  thing  happens  in  many  Confer vacese, 
and  that  some  of  the  bodies  which  have  been  termed  ^  micro-gonidia '  are 
really  androspores. — The  offices  of  these  different  classes  of  reproductive 
bodies  are  only  now  beginning  to  be  understood;  and  the  inquiry  is  one 
so  fraught  with  Physiological  interest,  and,  from  the  facility  of  growing 
these  plants  in  Aquaria,  may  be  so  easily  pursued,  that  it  may  be  hoped 
that  the  zeal  of  Microscopists  will  not  long  leave  any  part  of  it  in 
obscurity. 

256.  The  CJicBtoplioracece  constitute  a  beautiful  and  interesting  little 
group  of  Confervoid  plants,  of  which  some  species  inhabit*  the  sea,  whilst 
others  are  found  in  fresh  and  pure  water, — rather  in  that  of  gently  mov- 
ing streams,  however,  than  in  strongly  flowing  currents.  Generally 
speaking,  their  filaments  put  forth  lateral  branches,  and  extend  them- 
selves into  arborescent  fronds;  and  one  of  the  distinctive  characters  of 
the  group  is  afforded  by  the  fact,  that  the  extremities  of  these  branches 
are  usually  prolonged  into  bristle-shaped  processes  (Fig.  151,  b).  As  in 
many  preceding  cases,  these  plants  multiply  themselves  by  the  conversion 
of  the  endochrome  of  certain  of  their  cells  into  ^zoospores;'  and  these, 
when  set  free,  are  seen  to  be  furnished  with  four  flagella.  '  Kesting- 
spores  ^  have  also  been  seen  in  many  species;  and  it  is  probable  that  these, 
as  in  Confervacem,  are  really  oospores. 

257.  The  BatracliospermecBy  whose  name  is  indicative  of  the  strong 
resemblance  which  their  beaded  filaments  bear  to  frog  spawn,  are  now 
ranked  as  humble  fresh-water  forms  of  a  far  higher  marine  group  of 
Algge,  the  RhodospermecB  or  Ked  Sea-weeds  (§  330).  But  they  deserve 
special  notice  here  on  account  of  the  simplicity  of  their  structure,  and 
the  extreme  beauty  of  the  objects  they  afford  to  the  Microscopist  (Fig. 
152).    They  are  chiefly  found  in  water  which  is  pure  and  gentle-flowing. 

They  are  so  extremely  flexible,"  says  Dr.  Hassall,  "  that  they  obey  the 
slightest  motion  of  the  fluid  which  surrounds  them;  and  nothing  can 
surpass  the  ease  and  grace  of  their  movements.  When  removed  from  the 


^  See  Pringsheim  in  Ann.  des  Sci.  Nat.,"  4eme  Ser.,  Botan.,  Tom.  v.  (1856), 
p  187. 


MICROSCOPIC  FORMS  OF  VEGETABLE  LIFE. 


259 


water  they  lose  all  form,  and  appear  like  pieces  of  jelly,  without  trace  of 
organization;  on  immersion,  however,  the  branches  quickly  resume  their 
former  disposition/'  Their  color  is  for  the  most  part  of  a  brownish- 
green;  but  sometimes  they  are  of  a  reddish  or  bluish  purple.  The  central 
axis  of  each  plant  is  originally  composed  of  a  single  file  of  large  cylindri- 
cal cells  laid  end  to  end;  but  this  is  subsequently  invested  by  other  cells,  i 
in  the  manner  to  be  presently  described.  It  bears,  at  pretty  regular  in- ! 
tervals,  whorls  of  short  radiating  branches,  each  of  them  composed  of 
rounded  cells,  arranged  in  a  bead-like  row,  and  sometimes  subdividing 
again  into  two,  or  themselves  giving  off  lateral  branches.  Each  of  the 
•primary  branches  originates  in  a  little  protuberance  from  the  primitive 
cell  of  the  central  axis,  precisely  after  the  manner  of  the  lateral  cells  of 
Conferva  glornerata  (§  252);  as  this  protuberance  increases  in  size,  its 
cavity  is  cut  off  by  a  septum,  so  as  to  render  it  an  independent  cell; 
and  by  the  continual  repetition  of  the  process  of  binary  subdivision,  this 
single  cell  becomes  converted  into  a  beaded  filament.  Certain  of  these 
branches,  however,  instead  of  radiating  from  the  main  axis,  grow  down- 
wards upon  it,  so  as  to  form  a  closely-fitting  investment  that  seems  prop- 
erly to  belong  to  it.  Some  of  the  radiating  branches  grow  out  into  long 
transparent  points,  like  those  of  Chse- 
tophoraceae;  and  within  those  are  pro- 
duced ^antherozoids,'  which,  though 
not  endowed  with  the  power  of  spon- 
taneous movement,  find  their  way  to 
the  germ-cells  contained  in  other 
parts  of  the  filaments;  and  by  the  fer- 
tilization of  the  contents  of  these  are 
produced  '  oospores,'  which  are  seen  as 
dark  bodies  lying  in  the  midst  of  the 
whorls  of  branches  (Fig.  152). 

258.  Among  the  highest  of  the 
Alg88  in  regard  to  the  complexity  of 
their  Generative  apparatus,  which 
contrasts  strongly  with  the  general 
simplicity  of  their  structure,  is  the 
Family  of  CharacecB  (ranked  by  some 
Botanists  as  a  group  of  primary  im- 
portance); some  members  of  which 

have  received  a  large  amount  of  atten-  Batrachospermum  moniUforme. 

tion  from  Microscopists,  on  account 

of  the  interesting  phenomena  they  exhibit.  These  humble  plants  are  for 
the  most  part  inhabitants  of  fresh  waters,  and  are  found  rather  in  such 
as  are  still,  than  in  those  which  are  in  motion;  one  species,  however,  may 
be  met  with  in  ditches  whose  waters  are  rendered  salt  by  communication 
with  the  sea.  They  may  be  easily  grown  for  the  purposes  of  observation 
in  large  glass  jars  exposed  to  the  light;  all  that  is  necessary  being  to  pour 
off  the  water  occasionally  from  the  upper  part  of  the  vessel  (thus  carry- 
ing away  a  film  that  is  apt  to  form  on  its  surface),  and  to  replace  this  by 
fresh  water.  Each  plant  is  composed  of  an  assemblage  of  long  tubif orm 
cells,  placed  end  to  end;  with  a  distinct  central  axis,  around  which  the 
branches  are  disposed  at  intervals  with  great  regularity  (Fig.  153,  a).  In 
the  genus  Nitella,  the  stem  and  branches  are  simple  cells,  which  some- 
times attain  the  length  of  several  inches;  whilst  in  the  true  Chara  each 
central  tube  is  surrounded  by  an  envelope  of  smaller  ones,  which  is 


260  THE  MICROSCOPE  AND  ITS  REVELATIONS. 

formed  as  in  Batrachospermeae,  save  that  the  investing  cells  grow  up- 
wards as  well  as  downwards  from  each  node,  and  meet  each  other  on  the 
stem  half-way  between  the  nodes.  Some  species  have  the  power  of 
secreting  carbonate  of  lime  from  the  water  in  which  they  grow,  if  this  be 
at  all  impregnated  with  calcareous  matter;  and  by  the  deposition  of 
it  beneath  their  teguments  they  have  gained  their  popular  name  of 
'  stoneworts.^  The  long  tubiform  cells  of  Nitella  afford  a  very  beautiful 
and  instructive  display  of  the  phenomenon  of  cyclosis,  or  rotation  of  fluid 
in  their  interior.  Each  cell,  in  the  healthy  state,  is  lined  by  a  layer  of 
green  oval  granules,  which  cover  every  part,  except  two  longitudinal  lines 
that  remain  nearly  colorless  (Fig.  153,  b);  and  a  constant  stream  of  semi* 
fluid  matter  containing  numerous  jelly-like  globules  is  seen  to  flow  over 


Tjg:  153. 


Nitella  fiexilis:— A,  stem  and  branches  of  the  natural  size;  a,  6,  c,  d,  four  verticils  of  branches 
issuing  from  the  stem;  e,  /,  subdivision  of  the  branches;— b,  portion  of  the  stem  and  branches  en- 
larged; a,  6,  joints  of  stem;  c,  d,  verticils;  e,  /,  new  cells  sprouting  from  the  sides  of  the  branches; 
gr,  h,  new  ceils  sprouting  at  the  extremities  of  the  branches. 

the  green  layer,  the  current  passing  up  one  side,  changing  its  direction 
at  the  extremity,  and  flowing  down  the  other  side,  the  ascending  and  de- 
scending spaces  being  bounded  by  the  transparent  lines  just  mentioned. 
That  the  currents  are  in  some  way  directed  by  the  layer  of  granules,  ap- 
pears from  the  fact  noticed  by  Mr.  Varley,^  that  if  accident  damages  or 
removes  them  near  the  boundary  between  the  ascending  and  descending 
currents,  a  portion  of  the  fluid  of  the  two  currents  will  intermingle  by 
passing  the  boundary;  whilst,  if  the  injury  be  repaired  by  the  develop- 
ment of  new  granules  on  the  part  from  which  they  had  been  detached, 
the  circulation  resumes  its  regularity,  no  part  of  either  current  passing 
the  boundary.    In  the  young  cells,  however,  the  rotation  may  be  seen 


^    Transactions  of  the  Microscopical  Society,"  3d  Series,  Vol.  ii.,  p.  99. 


MICROSCOPIC  FORMS  OF  VEGETABLE  LIFE. 


261 


before  this  granular  lining  is  formed.  The  rate  of  the  movement  is  af- 
fected by  anything  that  influences  the  vital  activity  of  the  Plant;  thus,  it 
is  accelerated  by  moderate  warmth,  whilst  it  is  retarded  by  cold;  and  it 
may  be  at  once  checked  by  a  slight  electric  discharge  through  the  plant. 
The  moving  globules,  which  consist  of  starchy  matter,  are  of  various 
sizes;  being  sometimes  very  small  and  of  definite  figure,  whilst  in  other 
instances  they  are  seen  as  large  irregular  masses,  which  appear  to  be 
formed  by  the  aggregation  of  the  smaller  particles.^  The  production  of 
new  cells  for  the  extension  of  the  stem  or  branches,  or  for  the  origination 
of  new  whorls,  is  not  here  accomplished  by  the  subdivision  of  the  parent- 


Generative  organs  of  Chara  fragiUs:—A,  antheridium  or  'globule  '  developed  at  the  base  of  pis- 
tillidium  or  'nucule;' — b,  nucule  enlarged,  and  globule  laid  open  by  the  separation  of  its  valves;— c, 
one  of  the  valves,  with  its  group  of  antheridial  filaments,  each  composed  of  a  linear  series  of  cells, 
within  every  one  of  which  an  antherozoid  is  formed;— in  d,  e,  and  f,  the  successive  stages  of  this 
formation  are  seen :— and  at  g  is  shown  the  escape  of  the  mature  anther ozoids.  h. 

cell,  but  takes  place  by  the  method  of  out-growth  (Fig.  153,  B,  6,  /,  g,  7i), 
which,  as  already  shown  (§  252),  is  nothing  but  a  modification  of  the 
usual  process  of  cell-multiplication:  in  this  manner,  the  extension  of  the^ 

This  interesting  phenomenon  may  be  readily  observed,  by  taking  a  small' 
portion  of  the  plant  out  of  the  water  in  which  it  is  growing,  and  either  placing  it 
in  a  large  Aquatic  box  (§  122)  or  in  the  Zoophyte-trough  (§  124),  or  laying  it  on  the 
glass  Stage-plate  (§  120)  and  covering  it  with  thin  glass.  A  portion  of  Chara  or 
Nitella  placed  in  the  Growing-slide  (§  121)  may  be  kept  under  observation  for 
many  days  together. 


262 


THE  MICROSCOPE  AND  ITS  REVELATIONS. 


individual  plant  is  effected  with  considerable  rapidity.  "When  these  plants 
are  well  supplied  with  nutriment,  and  are  actively  vegetating  under  the 
influence  of  light,  warmth,  etc.,  they  not  unfrequently  develop  ^  bulbels/ 
or  ^gonidia/  which  are  little  clusters  of  cells,  filled  with  starch,  that 
sprout  from  the  sides  of  the  central  axis,  and  then,  falling  off,  evolve  the 
long  tubiform  cells  characteristic  of  the  plant  from  which  they  were  pro- 
duced. The  Characece  may  also  be  multiplied  by  artificial  subdivision; 
the  separated  parts  continuing  to  grow  under  favorable  circumstances, 
and  gradually  developing  themselves  into  the  typical  form. 

259.  The  Generative  apparatus  of  Characece  consists  of  two  sets  of 
bodies,  both  of  which  grow  at  the  bases  of  the  branches  (Fig.  154,  a,  b): 
one  set,  formerly  known  as  ^globules,'  are  really  antheridia;  whilst  the 
other,  known  as  ^  nucules,' contain  the  germ -cells,  and  are  trviQ  pistil- 
lidia.  The  ^globules,'  which  are  nearly  spherical,  have  an  envelope 
made  up  of  eight  triangular  valves  (b,  c),  often  curiously  marked,  which 
incloses  a  nucleus  of  a  light  reddish  color:  this  nucleus  is  principally 
composed  of  a  mass  of  filaments  rolled  up  compactly  together;  and  each 
of  these  filaments  {c)  consists,  like  a  Conferva,  of  a  linear  succession  of 
cells.  In  every  one  of  these  cells  there  is  formed,  by  a  gradual  change  in 
its  contents  (the  successive  stages  of  which  are  seen  at  d,  e,  f),  a  spiral 
thread  of  two  or  three  coils,  which,  at  first  motionless,  after  a  time 
begins  to  move  and  revolve  within  the  cell;  and  at  last  the  cell-wall  gives 
way,  and  the  spiral  thread  makes  its  escape  (g),  partially  straightens 
itself,  and  moves  actively  through  the  water  for  some  time  (h)  in  a  tol- 
erably determinate  direction,  by  the  lashing  action  of  two  long  and  very 
delicate  filaments  with  which  they  are  furnished.  The  exterior  of  the 
'  nucule'  (a,  b)  is  formed  by  five  spirally-twisted  tubes,  that  give  it  a  very 
peculiar  aspect;  and  these  inclose  a  central  sac  containing  protoplasm, 
oil,  and  starch-globules.  At  a  certain  period  the  spirally-twisted  tubes, 
which  form  a  kind  of  crown  around  the  summit,  separate  from  each 
other,  leaving  a  canal  that  leads  down  to  the  central  germ-cell  or  oo- 
sphere;  and  it  is  probable  that  through  this  canal  the  antherozoids  make 
their  way  down  to  perform  the  act  of  fertilization.  Ultimately  the  nucule 
falls  off  like  a  seed;  and  the  fertilized  germ-cell,  or  ^oospore/  gives  ori- 
gin to  a  single  new  plant  by  a  kind  of  germination. 

DESMIDIACE^  AND  DIATOMACE^. 

260.  Among  those  simple  Algce  whose  Generative  process  consists  in 
the  ^conjugation' of  two  similar  cells  (§  235),  there  are  two  groups  of 
such  peculiar  interest  to  the  Microscopist  as  to  need  a  special  notice; 
these  are  the  Desinidiacece  and  the  DiatomacecB.  Both  of  them  were 
ranked  by  Ehrenberg  and  many  other  Naturalists  as  Animalcules;  but 


1  A  full  account  of  the  Characece  will  be  found  in  Prof.  Sachs's  Text-book  of 
Botany  "  (translated  by  A.  W.  Bennett),  p.  278. — Various  observers  have  asserted 
that  particles  of  the  protoplasmic  contents  of  the  cells  of  the  Characece^  when  set 
free  by  the  rupture  of  their  cells,  may  continue  to  live,  move,  and  grow  as  inde- 
pendent Rhizopods.  But  the  writer  is  disposed  to  think  that  the  phenomena  thus 
represented  are  rather  to  be  regarded  as  cases  of  parasitism;  the  decaying  cells  of 
^itella  having  been  found  by  Cienkowski  {'  Beitrage  zur  Kentniss  der  Monaden,' 
in  "  Arch.  f.  Mikr.  Anat."  Bd.  i.,  1865,  p.  203)  to  be  inhabited  by  minute  spindle- 
shaped  ciliated  bodies,  which  seem  to  correspond  with  the  *  spores '  of  the  Myxo- 
mycetes  (§  322),  going  through  an  amoeboid  stage,  and  then  producing  a  Plasmo- 
dium, which,  after  undergoing  a  sort  of  encysting  process,  finally  '  breaks  up ' 
into  spindle-shaped  particles  resembling  those  found  in  the  iVi^e^^a-cells. 


MICROSCOPIC  FORMS  OF  VEGETABLE  LIFE. 


263 


the  fuller  knowledge  of  their  life-history,  and  the  more  extended  ac- 
quaintance with  the  parallel  histories  of  other  simple  forms  of  Vegeta- 
tion, which  have  been  gained  during  the  last  twenty  years,  are  now  gen- 
erally accepted  as  decisive  of  their  Vegetable  nature. 

261.  The  Desmidiace^  ^  are  minute  plants  of  a  green  color,  growing 
in  fresh  water;  generally 
speaking,  the  cells  are  in- 
dependent of  each  other 
(Figs.  155-158);  but  some- 
times those  which  have 
been  produced  by  binary 
subdivision  from  a  single 
primordial  cell,  remain  ad- 
herent one  to  another  in 
linear  series,  so  as  to  form 
a  filament  (Fig.  160). 
The  tribe  is  distinguished 
by  two  peculiar  features: 
one  of  these  being  the 
semblance  of  a  division 
of  each  cell  into  two  sym- 
metrical halves  by  a  '  su- 
tural  line,^  which  is  some- 

f  imPQ  rlppi'rlprl  fn  Viqvo  Various  species  of  Staurastrum:—A,S.  vestitum;  b,  S.  aculea- 
llUltJb  bO  ueciuua  ab  to  nave  turn;  c,  S.  paradoxum;  d,  e,  S.  brachiatum. 

led  to  the  belief  that  the  cell 

is  really  double  (Fig.  158,  a),  though  in  other  cases  it  is  merely  indicated  by 
a  slight  notch;  whilst  the  other  is  the  frequency  of  projections  from  the 
surface,  which  are  sometimes  short  and  inconspicuous  (Fig.  158),  but  are 
often  elongated  into  spines,  presenting  a  very  symmetrical  arrangement 
(Fig.  155).  These  projections  are  generally  formed  by  the  cellulose  en- 
velope alone;  which  possesses  an  almost  horny  consistence,  so  as  to  retain 
its  form  after  the  discharge  of  its  contents  (Figs.  158,  B,  D,  162,  e),  but 
does  not  include  any  mineral  ingredient,  either  calcareous  or  siliceous,  in 
its  composition;  in  other  instances,  however,  they  are  formed  by  a  notch- 
ing of  the  margin  of  the  cell  (Fig.  157)  which  may  affect  only  the  outer 
casing,  or  may  extend  into  the  cell-cavity.  The  outer  coat  is  surrounded 
by  a  very  transparent  sheet  of  gelatinous  substance,  which  is  sometimes 
very  distinct  (as  shown  in  Fig.  160),  whilst  in  other  cases  its  existence 
is  only  indicated  by  its  preventing  the  contact  of  the  cells.  The  outer 
coat  incloses  a  primordial  utricle,  which  is  not  always  closely  adherent 
to  it;  and  this  immediately  surrounds  the  endoclirome,  which  occupies 
nearly  the  whole  interior  of  the  cell,  and  in  certain  stages  of  its  growth 
is  found  to  contain  starch-granules. — Many  of  these  Plants  have  a  power 
of  slowly  changing  their  place,  so  that  they  approach  the  light  side  of 
the  vessels  in  which  they  are  kept,  and  will  even  ticivorse  the  field  of  the 
Microscope  under  the  eye  of  the  observer;  by  what  agency  this  movement 
is  effected  has  not  yet  been  certainly  made  out. 

262.  A  ^  cyclosis^  may  be  readily  observed  in  many  Desmidiacece;  and 
is  particularly  obvious  along  the  convex  and  concave  edges  of  the  cell  of 
any  vigorous  specimen  of  Closterium,  with  a  magnifying  power  of  250  or 

1  Our  first  accurate  Jjnowledge  of  this  group  dates  from  the  publication  of  Mr. 
Ralfs's  admirable  Monograph  in  1848.  Later  information  in  regard  to  it  v^ill  be 
found  in  the  Section  contributed  by  Mr.  W.  Archer  to  the  4th  Edition  of  Prit- 
chard's  *  Infusoria.' 


264 


THE  MICROSCOPE  AND  ITS  REVELATIONS. 


300  diameters  (Fig.  156,  A,  b).  By  careful  focussing,  the  flow  may  be 
seen  in  broad  streams  over  the  whole  surface  of  the  endochrome;  and 
these  streams  detach  and  carry  with  them,  from  time  to  time,  little  oval 
or  globular  bodies  (a,  i),  which  are  put-forth  from  it,  and  are  carried  by 
the  course  of  the  flow  to  the  transparent  spaces  at  the  extremities,  where 
they  join  a  crowd  of  similar  bodies.  In  each  of  these  spaces  (b),  a  proto- 
plasmic flow  proceeds  from  the  somewhat  abrupt  termination  of  the  en- 
dochrome, towards  the  obtuse  end  of  the  cell  (as  indicated  by  the  interior 

arrows) ;  and  the  globules  it 
Tis.;lyI5ii  contains  are  kept  in  a  sort 

^ .  of  twisting  movement  on  the 

inner  side  {a)  of  the  primor- 
dial utricle.  Other  currents 
are  seen  apparently  external 
to  it,  which  form  three  or 
four  distinct  courses  of  glob- 
ules, passing  towards  and 
away-from  c  (as  indicated 
by  the  outer  arrows),  where 
they  seem  to  encounter  a 
fluid  jetted  towards  them  as 
if  through  an  aperture  in 
the  primordial  utricle  at  the 
apex  of  the  chamber;  and 

Cyclosis  in  Closterium  lunula: — a,  cell  showing  central  here  some  communication 
separation  at  a,  in  which  the  large  globules,  6,  are  not  Vwofwoon  +Vio  inTmi*  onrl  fVio 
seen ;-B,  one  extremity  enlarged,  showing  the  movement  'J^l^ween  XUe  mnei  dUU  tue 
of  globules  in  the  colorless  space ;— c,  external  jet  produced  Outcr  CUrrCU  ts  appears  to 
by  pressure  on  the  cell,  probably  through  an  opening  in  the  x^t^o  -r»lor>Q  i  Av»/^ilickT  /^mn' 
cellulose  envelope  ;—D,  cell  in  a  state  of  self-division.  taKe  pidCG.       iinOLnei  CUll- 

ous  movement  is  often  to  be 
witnessed  in  the  interior  of  the  cells  of  members  of  this  family,  especially 
the  various  species  of  Cosmariuju,  which  has  been  described  as  '  the  swarm- 
ing of  the  granules/  from  the  extraordinary  resemblance  which  the  mass 
of  particles  of  endochrome  in  active  vibratory  motion  bears  to  a  swarm 
of  bees.  This  motion  continues  for  some  time  after  the  particles  have 
been  expelled  by  pressure  from  the  interior  of  the  cell;  and  it  does  not 
seem  to  depend  (like  that  of  true  ^zoospores')  upon  the  action  of  cilia  or 
flagella,  but  rather  to  be  a  more  active  form  of  the  molecular  movement 
common  to  other  minute  particles  freely  suspended  in  fluid  (§  155).  It 
has  been  supposed  that  the  ^  swarming^  is  related  to  the  production  of 
zoospores;  but  for  this  idea  there  does  not  seem  any  adequate  founda- 
tion. 


1  See  Lord  S.  G.  Osborne's  communications  to  the  Quart.  Journ.  of  Microsc. 
Sci.,  Vol.  ii.  (1854),  p.  234,  and  Vol.  iii.  (1855),  p.  54  —Although  the  movement  is 
an  unquestionable  fact,  yet  there  can  be  no  hesitation  in  regarding  the  appear- 
ance of  ciliary  action  described  by  that  observer  as  an  optical  illusion ;  as  was 
early  pointed  out  by  Mr.  Wenham  in  the  same  Journal,  Vol.  iv.,  1856,  p.  158. — 
The  character  of  this  movement  has  been  described  by  a  recent  observer  (Mr.  A. 
W.  Wills)  as  one  of  ebb  and  flow,  alternately  towards  and  from  the  ends,  in  deli- 
cate longitudinal  bands  or  streams;  its  direction  in  any  one  band  being  usually 
reversed  every  few  seconds;  while  in  different  bands  the  flow  may  be  in  opposite 
directions  at  the  same  time.  The  clear  spaces  at  the  ends  of  the  cell  he  affirms  to 
be  contractile  vesicles;  and  these  (he  says)  can  be  seen  under  a  l-4th  or  l-6th  inch 
objective  to  be  undergoing  incessant  though  slight  changes  in  form,  with  which 
the  flow  of  the  currents  can  be  distinctly  connected.  (See  Midland  Naturalist," 
1880,  p.  187,  quoted  in    Journ.  of  Roy.  Microsc.  Soc,"  Vol.  iii.  (1880),  p.  845. 

2  See  Archer  in    Quart.  Journ.  of  Microsc.  Sci.,"  Vol.  viii.  (1860),  p.  215. 


MICROSCOPIC  FORMS  OF  VEGETABLE  LIFE. 


265 


263.  When  the  single  cell  has  come  to  its  full  maturity,  it  commonly 
multiplies  itself  by  Unary  subdivision  ;  but  the  plan  on  which  this  takes 
place  is  often  peculiarly  modified,  so  as  to  maintain  the  symmetry  charac- 
teristic of  the  tribe.  In  a  cell  of  the  simple  cylindrical  form  of  those  of 
Didymoprium  (Fig.  160),  little  more  is  necessary  than  the  separation  of 
the  two  halves  at  the  sutural  line,  and  the  formation  of  a  partition  be- 
tween them  by  the  infolding  of  the  primordial  utricle  ;  and  in  this  man- 
ner, out  of  the  lowest  cell  of  the  filament  A,  a  double  cell,  b,  is  produced. 
But  it  will  be  observed  that  each  of  the  simple  cells  has  a  bifid  wart-like 
jirojection  of  the  cellulose  wall  on  either  side,  and  that  the  half  of  this 
projection,  which  has  been  appropriated  by  each  of  the  two  new  cells, 
is  itself  becoming  bifid,  though  not  symmetrically  ;  in  process  of  time, 
however,  the  increased  development  of  the  sides  of  the  cells  which  remain 
in  contiguity  with  each  other  brings  up  the  smaller  projections  to  the 
dimensions  of  the  larger,  and  the  symmetry  of  the  cells  is  restored. — In 
Closterinm  (Fig.  156,  d),  the  two  halves  of  the  endochrome  first  retreat 
from  one  another  at  the  sutural  line,  and  a  constriction  takes  place 
round  the  cellulose  wall ;  this  constriction  deepens  until  it  becomes  an 
hour-glass  contraction,  which  proceeds  until  the  cellulose  wall  entirely 
closes  round  the  primordial  utricle  of  the  two  segments  ;  in  this  state, 
one  half  commonly  remains  passive,  whilst  the  other  has  a  motion  from 
side  to  side,  which  gradually  becomes  more  active  ;  and  at  last  one  seg- 
ment quits  the  other  with  a  sort  of  jerk.  At  this  time  a  constriction  is 
seen  across  the  middle  of  the  primordial  utricle  of  each  segment,  indicat- 
ing the  formation  of  the  sutural  band  ;  but  there  is  no  division  of  the 
cell-cavity,  which  is  that  belonging  to  one  of  the  halves  of  the  original 
entire  cell.  The  cyclosis,  for  some  hours  previously  to  subdivision,  and 
for  a  few  hours  afterwards,  runs  quite  round  the  obtuse  end  a,  of  the 
endochrome  ;  but  gradually  a  transparent  space  is  formed,  like  that  at 
at  the  opposite  extremity,  by  the  retreat  of  the  colored  layer  ;  whilst, 
at  the  same  time,  its  obtuse  form  becomes  changed  to  a  more  elongated 
and  contracted  shape.  Thus,  in  five  or  six  hours  after  the  separation,  the 
aspect  of  each  extremity  becomes  the  same,  and  each  half  resembles  the 
cell  in  whose  self-division  it  originated. 

264.  The  process  is  seen  to  be  performed  after  nearly  the  same  method 
in  Staurastrum  (Fig.  155,  d,  e);  the  division  taking  place  across  the  cen- 
tral constriction,  and  each  half  gradually  acquiring  the  symmetry  of  the 
original. — In  such  forms  as  Cosmarium^  however,  in  which  the  cell  con- 
sists of  two  lobes  united  together  by  a  narrow  isthmus  (Fig.  158),  the 
division  takes  place  after  a  dilferent  method  ;  for  when  the  two  halves  of 
the  outer  wall  separate  at  the  sutural  line,  a  semiglobular  protrusion  of 
the  endochrome  is  put  forth  from  each  half;  these  protrusions  are  sepa- 
rated from  one  another,  and  from  the  two  halves  of  the  original  cell 
(which  their  interposition  carries  apart),  by  a  narrow  neck  ;  and  they  pro- 
gressively increase  until  they  assume  the  appearance  of  the  half-segments 
of  the  original  cell.  In  this  state,  therefore,  the  plant  consists  of  a  row 
of  four  segments,  lying  end  to  end,  the  two  old  ones  forming  the  extremes, 
and  the  two  new  ones  (which  do  not  usually  acquire  the  full  size  or  the 
characteristic  markings  of  the  original  before  the  division  occurs)  occu- 
pying the  intermediate  place.  At  last  the  central  fission  becomes 
complete,  and  two  bipartite  fronds  are  formed,  each  having  one  old  and 
one  young  segment :  the  young  segment,  however,  soon  acquires  the  full 
size  and  characteristic  aspect  of  the  old  one  ;  and  the  same  process,  the 
whole  of  which  may  take  place  within  twenty-four  hours,  is  repeated  ere 


266 


THE  MICROSCOPE  AND  ITS  REVELATIONS. 


long.*  The  same  general  plan  is  followed  in  Micrasterias  denticulata 
(Fig.  157);  but  as  the  small  hyaline  hemisphere,  put  forth  in  the  first 
instance  from  each  frustule  (a),  enlarges  with  the  flowing  in  of  the 
endochrome,  it  undergoes  progressive  subdivision  at  its  edges,  first  into 
three  lobes  (b),  then  into  five  (c),  then  into  seven  (d),  then  into 
thirteen  (e),  and  finally  at  the  time  of  its  separation  (f),  acquires  the 
characteristic  notched  outline  of  its  type,  being  only  distinguishable  from 
the  older  half  by  its  smaller  size.  The  whole  of  this  process  may  take 
place  within  three  hours  and  a  half.^ — In  Splmrozosma^  the  cells  thus 


Successive  stages  of  Binary  subdivision  of  Micrasterias  denticulata. 


produced  remain  connected  in  rows  within  a  gelatinous  sheath,  like  those 
of  Didymoprium  (Fig.  160);  and  different  stages  of  the  process  may 
commonly  be  observed  in  the  different  parts  of  any  one  of  the  filaments 
thus  formed.  In  any  such  filament,  it  is  obvious  that  the  two  oldest 
segments  are  found  at  its  opposite  extremities,  and  that  each  subdivision 
of  the  intermediate  cells  must  carry  them  farther  and  farther  from  each 
other.  This  is  a  very  different  mode  of  increase  from  that  of  the  Con- 
fervacecBy  in  which  the  terminal  cell  alone  undergoes  subdivision  (§  252), 
and  is  consequently  the  one  last  formed. 

'  See  the  observations  of  Mrs.  Herbert  Thomas  on  Cosmarium  margariti- 
ferum,  in  Transact,  of  Microsc.  Society,"  N.S.,  Vol.  ill.  (1855),  pp.  33-36.— 
Several  varieties  in  the  mode  of  subdivision  are  described  in  this  short  record  of 
long-continued  observations,  as  of  occasional  occurrence. 

^SeeLobb  in   Transact,  of  Microsc.  Society,"  N.S.,  Vol.  ix.  (1861),  p.  1 


MICROSCOPIC  FORMS  OF  VEGETABLE  LIFE. 


267 


265.  Although  it  is  probable  that  the  Desmidiacem  generally  multiply 
themselves  also  by  the  subdivision  of  their  endochrome  into  a  number 
of  zoospores,  only  one  undoubted  case  of  the  kind  has  yet  been  recorded 
(the  PediastrecB,  §  270,  being  no  longer  ranked  within  this  group);  that, 
namely,  of  Docidhim  Ehreribergi,  whose  elongated  cell  puts  forth  from 
the  vicinity  of  the  sutural  line,  one,  two,  or  three  tubular  extensions 
resembling  the  finger  of  a  glove,  through  which  there  pass  out  from  20 
to  50  motile  microgonidia  formed  by  the  breaking  up  of  the  endochrome 
of  the  neighboring  portion  of  each  segment.^ 

266.  Whether  there  is  in  this  group  anything  that  corresponds  to  the 
^encysting'  process  (§  228  note)  or  the  formation  of  ^ stato-spores ' 
(§  241)  in  other  Protophytes,  has  not  yet  been  certainly  ascertained;  but 
the  following  observations  may  have  reference  to  such  a  condition.  It  is 
stated  by  Focke  that  the  entire  endochrome  of  Closterium  sometimes 
retracts  itself  from  the  cell-wall,  and.  breaks  itself  up  into  a  number  of 
globules,  every  one  of  which  acquires  a  very  firm  envelope.  And  it  is 
affirmed  by  Mr.  Jenner  that  "  in  all  the  Desmidiaceae,  but  especially  in 
Closterium  and  Micrasterias,  small,  compact,  seed-like  bodies  of  a 
blackish  color  are  at  times  to  be  met  with.  Their  situation  is  uncertain, 
and  their  number  varies  from  one  to  four.  In  their  immediate  neigh- 
borhood the  endochrome  is  wanting,  as  if  it  had  been  required  to  form 
them;  but  in  the  rest  of  the  frond  it  retains  its  usual  color  and  appear- 
ance.'^  It  seems  likely  that,  when  thus  inclosed  in  a  firm  cyst,  the 
gonidia  are  more  capable  of  preserving  their  vitality,  than  they  are  when 
destitute  of  such  a  protection;  and  that  in  this  condition  they  may  be 
taken  up  and  wafted  through  the  air,  so  as  to  convey  the  species  into  new 
localities. 

267.  The  proper  Generative  process  in  the  Desmidiacece  is  always 
accomplished  by  the  act  of  ^conjugation;'  which  commences  with  the 
dehiscence  of  the  firm  external  envelope  of  each  of  the  conjugating  cells, 
so  as  to  separate  it  into  two  valves  (Fig.  158,  c,  d;  Fig.  159,  c).  The 
contents  of  each  cell  thus  set  free  without  any  distinct  investment,  blend 
with  those  of  the  other;  and  a  ^  zygospore '  is  formed  by  their  union, 
which  soon  acquires  a  truly  membranous  envelope.^  This  envelope  is  at 
first  very  delicate,  and  is  filled  with  green  and  granular  contents;  by 
degrees  the  envelope  acquires  increased  thickness,  and  it  contents  become 
brown  or  red.  The  surface  of  the  zygospore  is  sometimes  smooth,  as  in 
Closterium  and  its  allies  (Fig.  159);  but  in  the  Cosmariece,  it  becomes 
granular,  tuberculated,  or  even  spinous  (Fig.  158,  d),  the  spines  being 
sometimes  simple  and  sometimes  forked  at  their  extremities.^ — The  mode 
in  which  conjugation  takes  place  in  the  filamentous  species  constituting 
the  DesmidiecB  proper,  is,  however,  in  many  respects  different.  The 
filaments  first  separate  into  their  component  joints;  and  when  two  cells 
approach  in  conjugation,  the  outer  cell- wall  of  each  splits  or  gapes  at 
that  part  which  adjoins  the  other  cell,  and  a  new  growth  takes  place, 
which  forms  «  sort  of  connecting  tube  that  unites  the  cavities  of  the  two 
cells  (Fig.  160,  D,  e).  Through  this  tube  the  entire  endochrome  of  one 
cell  passes  over  into  the  cavity  of  the  other  (d);  and  the  two  are  com- 


iSee  Archer  in    Quart.  Journ.  of  Microsc.  Sci.,"  Vol.  viii.  (1860),  p.  227. 

2  In  certain  species  of  Closterium,  as  in  many  of  the  Diatomacece  (§  280),  the 
act  of  conjugation  gives  origin  to  two  sporangia. 

2  Bodies  precisely  resembling  these,  and  almost  certainly  to  be  regarded  as  of 
like  kind,  are  often  found  fossilized  in  Flints,  and  have  been  described  by  Ehren- 
berg  as  the  remains  of  Animalcules,  under  the  name  of  Xanthidia, 


268 


THE  MICROSCOPE  AND  ITS  REVELATIONS. 


mingled  so  as  to  form  a  single  mass  (e),  as  is  the  case  in  many  of  the 
ConjugatecB  (§  235).  The  Joint  which  contains  the  zygospore  can  scarcely 
be  distinguished  at  first  (after  the  separation  of  the  empty  cell),  save  by 
the  greater  density  of  its  contents;  but  the  proper  coats  of  the  zygospore 
gradually  become  more  distinct,  and  the  enveloping  cell-wall  disappears. 
— The  subsequent  history  of  the  zygospores  has  hitherto  been  made  out 
in  only  a  few  cases.  From  the  observations  of  Mrs.  H.  Thomas  {loc.  cit.) 
on  Cosmarizim,  it  appeared  that  each  zygospore  gives  origin,  not  to  a 
single  cell  but  to  a  brood  of  cells;  and  this  view  is  fully  confirmed  by 
Hoffmeister/  who  speaks  of  it  as  beyond  doubt  that  the  contents  of  the 
zygospores  are  transformed  by  repeated  binary  subdivisions  into  8  or  16 
cells,  which  assume  the  original  form  of  the  parent  before  they  are  set 
free  by  the  rupture  or  diffluence  of  the  inclosing  wall.  The  observations 
of  Jenner  and  Pocke  render  it  probable  that  the  same  is  the  case  in  Clos- 
terium  ;  but  much  has  still  to  be  learned  in  regard  to  the  development 
of  the  products  of  the  Generative  process,  as  it  is  by  no  means  certain 


Conjugation  of  Cosmarium  botrytis :  Conjugation  of  Closterium  striatulum:—A,  or- 

— A,  mature  cells;  b,  empty  cell-enve-  dinary  cell;  b,  empty  cell;  c,  two  cells  in  conju- 

lope;  c,  transverse  view;  d,  zygospore  gation,  with  incipient  zygospore, 
with  empty  cell-envelopes 


that  they  always  resemble  the  parent  forms.  For  it  is  affirmed  by  Mr. 
Ealfs  that  there  are  several  Desmidiaceae  which  never  make  their  appear- 
ance in  the  same  pool  for  two  years  successively,  although  their  zygo- 
spores are  abundantly  produced — a  circumstance  which  would  seem  to 
indicate  an  ^alternation  of  generations.^  It  is  a  subject,  therefore,  to 
which  the  attention  of  Microscopists  cannot  be  too  sedulously  directed. 

268.  The  subdivision  of  this  Family  into  Genera,  according  to  the 
method  of  Mr.  Ralfs  (^^  British  Desmidiese"),  as  modified  by  Mr. 
Archer  (Pritchard's  Infusoria ''),  is  based  in  the  first  instance  upon 
zhe  connection  or  disconnection  of  the  individual  cells;  two  groups 
being  thus  formed,  of  which  one  includes  all  the  genera  whose  cells, 
when  multiplied  by  binary  subdivision,  remain  united  into  an  elongated 
filament;  whilst  the  other  comprehends  all  those  in  which  the  cells 
become  separated  by  the  completion  of  the  fission.    The  further  division 


1    Ann.  of  Nat.  Hist.,"  3d  Ser.,  Vol.  i.  (1858),  p.  2. 


MICROSCOPIC  FORMS  OF  VEGETABLE  LIFE. 


269 


Cro.  ICQ; 


of  the  filamentous  group,  in  which  the  zygospores  are  always  globular 
and  smooth,  is  based  on  the  fact  that  in  one  set  of  genera  the  joints  are 
many  times  longer  than  they  are  broad,  and  that  they  are  neither 
constricted  nor  furnished  with  lateral  teeth  or  projections;  whilst  in  the 
other  set  (of  which  Didymojprmm,  Fig.  160,  i's  an  example)  the  length 
and  breadth  of  each  joint  are  nearly  equal,  and  the  joints  are  more  or 
less  constricted,  or  have  lateral  teeth  or  projecting  angles,  or  are  other- 
wise figured;  and  it  is  for  the  most  part  upon  the  variations  in  these  last 
particulars,  that  the  generic  characters  are  based.  The  solitary  group 
presents  a  similar  basis  for  primary 
division  in  the  marked  difference 
in  the  proportions  of  its  cells;  such 
elongated  forms  as  Closterium 
(Figs.  156,  159),  in  which  the 
length  is  many  times  the  breadth, 
being  thus  separated  from  those  in 
which,  as  in  Micrasterias  (Fig. 
157),  Cosmarmm  (Fig.  158),  and 
Staiirastr^im  (Fig.  155),  the 
breadth  more  nearly  equals  the 
length.  In  the  former  the  sporan- 
gia are  smooth,  whilst  in  the  latter 
they  are  very  commonly  spinous  and 
are  sometimes  quadrate.  In  this 
group,  the  chief  secondary  charac- 
ters are  derived  from  the  degree  of 
constriction  between  the  two  halves 
of  the  cell,  the  division  of  its  mar- 
gin into  segments  by  incisions  more 
or  less  deep,  and  its  extension  into 
teeth  or  spines. 

269.  The  Desmidiacece  are  not 
found  in  running  streams,  unless 
the  motion  of  the  water  be  very 
slow;  but  are  to  be  looked-for  in 
standing  though  not  stagnant 
waters.  Small  shallow  pools  that 
do  not  dry  up  in  summer,  especi- 
ally in  open  exposed  situations,  tinary  subdivision  and  Conjugation  of  Didy- 
•^1  ^  '  moprium  Grevillii:—A,  portion  of  filament,  sur- 

SUCn  as  boggy  moors,  are  most  rounded  by  gelatinous  ebvelope;  b,  dividing  cell; 
■nrorlnpHvp  THa  Inro-pr  anrl  bpnv  c,  single  cell  viewed  transversely;  d,  two  cells  in 
pi  oauCUiye.      L  ne  lai  ger  ana  neav-  conjugation;  e,  formation  of  zygospore. 

ler  species  commonly  lie  at  the 

bottom  of  the  pools,  either  spread-out  as  a  thin  gelatinous  stratum,  or 
collected  into  finger-like  tufts.  By  gently  passing  the  fingers  beneath 
these,  they  may  be  caused  to  rise  towards  the  surface  of  the  waters,  and 
may  then  be  lifted  out  by  a  tin-box  or  scoop.  Other  species  form  a 
greenish  or  dirty  cloud  upon  the  stems  and  leaves  of  other  aquatic 
plants;  and  these  also  are  best  detached  by  passing  the  band  beneath 
them,  and  ^  stripping  ^  the  plant  between  the  fingers,  so  as  to  carry  off 
upon  them  what  adhered  to  it.  If,  on  the  other  hand,  the  bodies  of 
which  we  are  in  search  should  be  much  diffused  through  the  water,  there 
is  no  other  course  than  to  take  it  up  in  large  quantities  by  the  box  or 
scoop,  and  to  separate  them  by  straining  through  a  piece  of  linen.  At 
first,  nothing  appears  on  the  linen  but  a  mere  stain  or  a  little  dirt;  but 


270 


THE  MICROSCOPE  ANB  ITS  REVELATIONS. 


by  the  straining  of  repeated  quantities,  a  considerable  accumulation  may- 
be gradually  made.  This  should  be  then  scraped-off  with  a  knife,  and 
transferred  into  bottles  with  fresh  water.  If  what  has  been  brought  up 
by  hand  be  richly  charged  with  these  forms,  it  should  beat  once  deposited 
in  a  bottle;  this  at  first  seems  only  to  contain  foul  water,  but  by  allowing 
it  to  remain  undisturbed  for  a  little  time,  the  Desmids  will  sink  to  the 
bottom,  and  most  of  the  water  may  then  be  poured-off,  to  be  replaced  by 
a  fresh  supply.  If  the  bottles  be  freely  exposed  to  solar  light,  these  little 
plants  will  flourish,  apparently  as  well  as  in  their  native  pools;  and  their 
various  phases  of  multiplication  and  reproduction  may  be  observed  during 
successive  months  or  even  years. — If  the  pools  be  too  deep  for  the  use  of 
the  hand  and  the  scoop,  a  collecting-bottle  attached  to  a  stick  (§  216)  may 
be  employed  in  its  stead.  The  ring-net  (§  216)  may  also  be  advantage- 
ously employed,  especially  if  it  be  so  constructed  as  to  allow  of  the  ready 
substitution  of  one  piece  of  muslin  for  another.  For  by  using  several 
pieces  of  previously  wetted  muslin  in  succession,  a  large  number  of  these 
minute  organisms  may  be  separated  from  the  water;  the  pieces  of  muslin 


Various  phases  of  development  of  Pediastrum  granulatum. 


may  be  brought  home  folded-up  in  wide-mouthed  bottles,  either  sepa- 
rately, or  several  in  one,  according  as  the  organisms  are  obtained  from 
one  or  from  several  waters;  and  they  are  then  to  be  opened  out  in  jars  of 
filtered  river-water,  and  exposed  to  the  light,  when  the  Desmids  will 
detach  themselves. 

270.  Pediastrece. — The  members  of  this  family  were  formerly  included 
in  the  preceding  group;  but,  though  doubtless  related  to  the  true  Des- 
midiacem  in  certain  particulars,  they  present  too  many  points  of  differ- 
ence to  be  properly  associated  with  them.  Their  chief  point  of  resem- 
blance consists  in  the  firmness  of  the  outer  casing,  and  in  the  frequent 
interruption  of  its  margin  either  by  the  protrusion  of  '  horns'  (Fig.  161, 
a),  or  by  a  notching  more  or  less  deep  (Fig.  162,  b);  but  they  differ  in 
these  two  important  particulars,  that  the  cells  are  not  made  up  of  two  syin- 
metrical  halves,  and  that  they  are  always  found  in  aggregation,  which  is 
not — except  in  such  genera  as  Scenodesmus  {Arthrodesmus,  Ehr.),  which 


MICROSCOPIC  FORMS  OF  VEGETABLE  LIFE. 


271 


connect  this  group  with  the  preceding — in  linear  series,  but  in  the  form 
of  discoidal  fronds.  In  this  tribe  we  meet  with  a  form  of  multiplication 
by  zoospores  aggregated  into  macro-gonidia,^  which  reminds  us  of  the 
formation  of  the  motile  spheres  of  Volvox  (§  239),  and  which  takes  place 
in  such  a  manner  that  the  resultant  product  may  vary  greatly  in  number 
of  its  cells,  and  consequently  both  in  size  and  in  form.  Thus  in,  Pedi- 
astrum  gra?iulatu?n  {Fig.  161),  the  zoospores  formed  by  the  subdivision 
of  the  endochrome  of  one  cell  into  gonidia,  which  may  be  4,  8,  16,  32,  or 
64  in  number,  escape  from  the  parent  frond  still  inclosed  in  the  inner 
tunic  of  the  cell;  and  it  is  within  this  that  they  develop  themselves  into 
a  cluster  resembling  that  in  which  they  originated,  so  that  whilst  the 
frond  normally  consists  of  sixteen  cells,  it  may  be  composed  of  either  of 
the  just-mentioned  multiples  or  sub-multiples  of  that  number.  At  A,  is 
seen  an  old  disk,  of  irregular  shape,  nearly  emptied  by  the  emission  of  its 
macro-gonidia,  which  had  been  seen  to  take-place  within  a  few  hours  pre- 
viously from  the  cells  by  c,  d,  e  ;  most  of  the  empty  cells  exhibit  the 
cross  slit  through  which  their  contents  had  been  discharged;  and  where 
this  does  not  present  itself  on  the  side  next  the  observer,  it  is  found  on 
the  other.  Three  of  the  cells  still  possess  their  colored  contents,  but  in 
different  conditions.  One  of  them  exhibits  an  early  stage  of  the  subdivi- 
sion of  the  endochrome,  namely,  into  two  halves,  one  which  already  ap- 
pears halved  again.  Two  others  are  filled  by  sixteen  very  closely-crowded 
gonidia,  only  half  of  which  are  visible,  as  they  form  a  double  layer. 
Besides  these,  one  cell  is  in  the  very  act  of  discharging  its  gonidia;  nine 
of  which  have  passed  forth  from  its  cavity,  though  still  enveloped  in  a 
vesicle  formed  by  the  extension  of  its  innermost  membrane;  whilst 
seven  yet  remain  in  its  interior.  The  new-born  family,  as  it  appears  im- 
mediately on  its  complete  emersion,  is  shown  at  b;  the  gonidia  are  actively 
moving  within  the  vesicle;  and  they  do  not  as  yet  show  any  indication 
either  of  symmetrical  arrangement,  or  of  the  peculiar  form  which  they 
are  subsequently  to  assume.  Within  a  quarter  of  an  hour,  however,  the 
gonidia  are  observed  to  settle-down  into  one  plane,  and  to  assume  some 
kind  of  regular  arrangement,  most  commonly  that  seen  at  c,  in  which 
there  is  a  single  central  body  surrounded  by  a  circle  of  five,  and  this  again 
by  a  circle  of  ten;  they  do  not,  however,  as  yet  adhere  firmly  together. 
The  gonidia  now  begin  to  develop  themselves  into  new  cells,  increase  in 
size,  and  come  into  closer  approximation  (d);  and  the  edge  of  each,  espe- 
cially in  the  marginal  row,  presents  a  notch,  which  foreshadows  the  pro- 
duction of  its  characteristic  '  horns.'  Within  about  four  or  five  hours  after 
the  escape  of  the  gonidia,  the  cluster  has  come  to  assume  much  more  of  the 
distinctive  aspect  of  the  species,  the  marginal  cells  having  grown-out  into 
horns  (e);  still,  however,  they  are  not  very  closely  connected  with  each 
other;  and  between  the  cells  of  the  inner  row  considerable  spaces  yet 
intervene.  It  is  in  the  course  of  the  second  day  that  the  cells  become 
closely  applied  to  each  other,  and  that  the  growth  of  the  horns  is  com- 
pleted, so  as  to  constitute  a  perfect  disk  like  that  seen  at  f,  in  which, 
however,  the  arrangement  of  the  interior  cells  does  not  follow  the  typical 
plan.^ 

271.  The  varieties  which  present  themselves,  indeed,  both  as  to  the 


\  Solitary  zoospores  or  micro-gonidia  have  been  observed  by  Braun  to  make 
their  way  out  and  swim  away;  but  their  subsequent  history  is  unknown. 

^  See  Prof.  Braun  on  *'  The  Phenomenon  of  Eejuvenescence  in  Nature,"  pub- 
lished by  the  Ray  Society  in  1853;  and  his  subsequent  Memoir,  Algarum  Unicel- 
lularum  Genera  nova  aut  minus  cognita,"  1855. 


272 


THE  MICROSCOPE  AND  ITS  REVELATIONS. 


number  of  cells  in  each  cluster,  and  the  plan  on  which  they  are  disposed, 
are  such  as  to  baffle  all  attempts  to  base  specific  distinctions  on  such 
grounds;  and  the  more  attentively  the  life-history  of  any  one  of  these 
plants  is  studied,  the  more  evident  does  it  appear  that  many  reputed 
^  species  ^  have  no  real  existence.  Some  of  these,  indeed,  are  nothing 
else  than  mere  transitory  forms;  thus  it  can  scarcely  be  doubted  that  the 
specimen  represented  in  Fig.  ]62,  D,  under  the  name  of  Pediastrum 
pertusum,  is  in  reality  nothing  else  than  a  young  frond  of  P.  granu- 
latum,  in  the  stage  represented  in  Fig.  161,  e,  but  consisting  of  32  cells. 
On  the  other  hand,  in  Fig.  162,  E,  we  see  an  emptied  frond  of  P.  granu- 
latum,  exhibiting  the  peculiar  surface-marking  from  which  the  name  of 
the  species  is  derived,  but  composed  of  no  more  than  eight  cells.  And 
instances  every  now  and  then  occur  in  which  the  frond  consists  of  only 
four  cells,  each  of  them  presenting  the  two-horned  shape.  So,  again,  in 
Fig.  162,  B  and  c,  are  shown  two  varieties  of  Pediastrum  Mradiatum, 
whose  frond  is  normally  composed  of  sixteen  cells;  whilst  at  A  is  figured 
a  form  which  is  designated  as  P.  tetras,  but  which  may  be  strongly  sus- 


Various  species  (?)  of  Pediastrum.— a,  P.  tetras ;  b,  c,  P.  biradiatum ;     P,  pertusum ; 
empty  frond  of  P.  granulatum. 

pected  to  be  merely  a  four-celled  variety  of  B  and  c.  Many  similar  cases 
might  be  cited;  and  the  Author  would  strongly  urge  those  Microscopists 
who  have  the  requisite  time  and  opportunities  to  apply  themselves  to  the 
determination  of  the  real  species  of  these  groups,  by  studying  the  entire 
life-history  of  whatever  forms  may  happen  to  lie  within  their  reach,  and 
noting  all  the  varieties  which  present  themselves  among  the  offsets  from 
any  one  stock.  The  characters  of  such  varieties  are  diffused  by  the 
process  of  binary  subdivision  amongst  vast  multitudes  of  so-called  indi- 
viduals. Thus  it  happens  that,  as  Mr.  Ealfs  has  remarked,  ^^one  pool 
may  abound  with  individuals  of  Staurastrum  dejectum  or  ArtTirodesmus 
incus,  having  the  mucro  curved  outwards;  in  a  neighboring  pool,  every 
specimen  may  have  it  curved  inwards;  and  in  another  it  may  be  straight. 
The  cause  of  the  similarity  in  each  pool  no  doubt  is,  that  all  its  plants 
are  offsets  from  a  few  primary  fronds."  Hence  the  universality  of  any 
particular  character,  in  all  the  specimens  of  one  gathering,  is  by  no 
means  sufficient  to  entitle  these  to  take  rank  as  a  distinct  species;  since 


MICROSCOPIC  FORMS  OF  VEGETABLE  LIFE. 


2T3 


they  are,  properly  speaking,  but  repetitions  of  the  same  variety  by  U  pro- 
cess of  simple  multiplication,  really  representing  in  their  entire  aggregate 
the  one  plant  or  tree  that  grows  from  a  single  seed. 

272.  DiATOMACE^. — These,  like  the  Desmidiaceae,  are  simple  cells, 
having  a  firai  external  coating,  within  which  is  included  an  ^endo- 
chrome'  whose  superficial  layer  constitutes  a  ^primordial  utricle:'  but 
their  external  coat  is  consolidated  by  silex,  the  presence  of  which  is  one 
of  the  most  distinctive  characters  of  the  group,  and  gives  rise  to  the 
peculiar  surface-markings  of  its  members  (§  277).  It  has  been  thought 
by  some  that  the  solidifying  mineral  forms  a  distinct  layer  exuded  from 
the  exterior  of  the  cellulose-wall;  but  there  seems  good  reason  for  regard- 
ing that  wall  as  itself  interpenetrated  by  the  silex,  since  a  membrane 
bearing  the  characteristic  surface-markings  is  found  to  remain  after  its 
removal  by  hydrofluoric  acid.  The  ^endochrome'  of  Diatoms  consists, 
as  in  other  plants,  of  a  viscid  protoplasm,  in  which  float  the  granules  of 
coloring  matter.  In  the  ordinary  condition  of  the  cell,  these  granules 
are  diffused  through  it  with  tolerable  uniformity,  except  in  the  central 
spot,  which  is  occupied  by  a  nucleus  ;  round  this  nucleus  they  commonly 
form  a  ring,  from  which  radiating  lines  of  granules  may  be  seen  to 
diverge  into  the  cell  cavity.  Instead  of  being  bright  green,  however,  the 
endochrome  is  of  a  yellowish-brown.  The  principal  coloring  substance 
appears  to  be  a  modification  of  ordinary  chlorophyll;  it  takes  a  green  or 
greenish-blue  tint  with  sulphuric  acid,  and  often  assumes  this  hue  in 
drying;  but  with  it  is  combined  in  greater  or  less  proportion  a  yellow 
coloring  matter  termed  phycoxantliin,  which  is  very  unstable  in  the  light, 
and  fades  in  drying.^  At  certain  times,  oil-globules  are  observable  in  the 
protoplasm;  these  seem  to  represent  the  starch-granules  of  the  Desmi- 
diace88  (§  261)  and  the  oil-globules  of  other  Protophytes  (§  229).  A 
distinct  movement  of  the  granular  particles  of  the  endochrome,  closely 
resembling  the  cyclosis  of  the  Desmidiaceao  (§  262),  has  been  noticed  by 
Prof.  W.  Smith in  some  of  the  larger  species  of  DiatomaceaB,  such  as 
Surirella  iiseriata,  Nitzscliia  scalaris,  and  Camjjylodiscus  spiralis  ;  and 
i>y  Prof.  Max  Schultze^  in  Coscinodiscus,  Denticella^  and  KJiizosolenia ; 
but  this  movement  has  not  the  regularity  so  remarkable  in  the  preceding 
group. 

273.  The  Diatomacem  seem  to  have  received  their  name  from  the 
readiness  with  which  those  forms  that  grow  in  coherent  masses  (which 
were  those  with  which  Naturalists  first  became  acquainted)  may  be  cut  or 
broken  through  ;  hence  they  have  been  also  designated  by  the  vernacular 
term  ^brittle-worts.'  Of  this  we  have  an  example  in  the  common 
Diatoma  (Fig.  173),  whose  component  cells  (which  in  this  tribe  are 


^  A  full  account  by  M.  Petit  of  recent  Chemical  and  Spectroscopic  investiga- 
tions on  the  endochrome  of  Diatoms,  will  be  found  in  Journ.  of  Roy.  Microsc. 
Soc,"  Vol.  iii.  (1880),  p.  680. 

2  The  account  of  the  Diatomacece  here  given,  is  chiefly  based  on  the  valuable 
Synopsis  of  the  British  Diatomacese,"  by  the  late  Prof.  W.  Smith;  of  which, 
and  of  its  beautiful  illustrations  by  Mr.  Tuff  en  West,  the  Author  has  been  enabled 
to«make  free  use  by  the  liberality  of  Messrs.  Beck.  In  the  sketch  he  has  given  of 
the  Systematic  arrangement  of  the  group,  however,  he  has  followed  the  Classifi- 
cation of  Mr.  Ralfs  (Pritchard's  Infusoria,"  4th  edition).  A  more  recent  Classi- 
fication proposed  by  M.  Paul  Petit  will  be  found  in  the  "  Monthly  Journal  of  the 
Microscopical  Society,"  Vol.  xviii.  (1877),  pp.  10,  65.  The  new  Monograph  of  the 
group  by  Prof.  Hamilton  Smith  (U.S.),  announced  as  forthcoming,  will  doubtless 
supersede  all  previous  descriptions  of  it. 

.    ^    Quart.  Journ.  of  Microsc.  Science,"  Vol.  vii.  (1859),  p.  13. 
18 


THE  MICROSCOPE  AND  ITS  REVELATIONS. 


usually  designated  as  fritstules)  are  sometimes  found  adherent  side  by 
side  (as  at  b)  so  as  to  form  filaments,  but  are  more  commonly  met  with 
in  a  state  of  partial  separation,  remaining  connected  at  their  angles  only 
(usually  the  alternate  angles  of  the  contiguous  frustules)  so  as  to  form  a 
zigzag  chain.  A  similar  cohesion  at  the  angles  is  seen  in  the  allied  genus 
Grammatopliora  (Fig.  174),  in  Isthmia  (Fig.  181),  and  in  many  other 
Diatoms ;  in  Biddulphia  (Fig.  167)  there  even  seems  to  be  a  special 
organ  of  attachment  at  these  points.  In  some  Diatoms,  however,  the 
frustules  produced  by  successive  acts  of  binary  subdivision  habitually 
remain  coherent  one  to  another,  and  thus  are  produced  filaments  or 
clusters  of  various  shapes.  Thus  it  is  obvious  that  when  each  frustule  is 
a  short  cylinder,  an  aggregation  of  such  cylinders,  end  to  end,  must  form 
a  rounded  filament,  as  in  Melosira  (Figs.  177,  178);  and  whatever  may 
be  the  form  of  the  sides  of  the  frustules,  if  they  be  parallel  one  to  the 
other,  a  straight  filament  will  be  produced,  as  in  Achnanthes  (Fig.  185). 
But  if,  instead  of  being  parallel,  the  sides  be  somewhat  inclined  towards 
each  other,  a  curved  band  will  be  the  result;  this  may  not  continue 
entire,  but  may  so  divide  itself  as  to  form  fan-shaped  expansions,  as 
those  of  Lichmophora  flalellata  (Fig.  172);  or  the  cohesion  may  be 
sufficient  to  occasion  the  band  to  wind  itself  (as  it  were)  round  a  central 
axis,  and  thus  to  form,  not  merely  a  complete  circle,  but  a  spiral  of 
several  turns,  as  in  Meridian  circulare  (Fig.  170).  Many  Diatoms, 
again,  possess  a  stipes^  or  stalk-like  appendage,  by  which  aggregations  of 
frustules  are  attached  to  other  plants,  or  to  stones,  pieces  of  wood,  etc. ; 
and  this  may  be  a  simple  foot-like  appendage,  as  in  Aclinantlies  longipes 
(Fig.  185),  or  it  may  be  a  composite  plant-like  structure,  as  in  Lichmo- 
phora (Fig.  172),  Gomphonema  (Fig.  186),  and  Mastogloia  (Fig.  189). 
Little  is  known  respecting  the  nature  of  this  stipes;  it  is,  however,  quite 
flexible,  and  may  be  conceived  to  be  an  extension  of  the  cellulose  coat 
unconsolidated  by  silex,  analogous  to  the  prolongations  which  have  been 
seen  in  the  Desmidiacem  (§  261),  and  to  the  filaments  which  sometimes 
connect  the  cells  of  the  FalmellacecB  (§  243).  Some  Diatoms,  again, 
have  a  mucous  or  gelatinous  investment,  which  may  even  be  so  substan- 
tial that  their  frustules  lie — as  it  were — in  a  bed  of  it,  as  in  Mastogloia 
(Figs.  189  B,  190),  or  may  form  a  sort  of  tubular  sheath  to  them,  as  in 
Schizonema  (Fig.  188).  In  a  large  proportion  of  the  group,  however, 
the  frustules  are  always  met  with  entirely /r^^;  neither  remaining  in  the 
least  degree  coherent  one  to  another  after  the  process  of  binary  subdi- 
vision has  once  been  completed,  nor  being  in  any  way  connected  either 
by  a  stipes  or  by  a  gelatinous  investment.  This  is  the  case,  for  example, 
with  Triceratium  (Fig.  164),  Pleurosigma  (Fig.  165),  Actinocyclus  (Fig. 
191,  5),  Actinoptychus  (Fig.  1^0),  Arachjioidiscus  (Plate  xi.),  Cam- 
•  pylodiscus  (Fig.  176),  SurireUa  (Fig.  175),  Coscinodisctts  (Fig.  191,  a,  a 
a),  Heliopelta  (Plate  i..  Fig.  3),  and  many  others.  The  solitary  discoidal 
forms,  however,  when  obtained  in  their  living  state,  are  commonly  found 
cohering  to  the  surface  of  Seaweeds. 

274.  We  have  now  to  examine  more  minutely  into  the  curious  struc- 
ture of  the  silicified  casing  which  incloses  every  Diatom-cell  or  '  frus- 
tule,' and  the  presence  of  which  imparts  a  peculiar  interest  to  the  group; 
not  merely  on  account  of  the  elaborately-marked  pattern  which  it  often 
exhibits  (Fig.  277),  but  also  through  the  perpetuation  of  the  minutest 
details  of  that  pattern  in  the  specimens  obtained  from  fossilized  deposits 
(Fig.  299).  This  casing  consists  of  two  valves  or  plates,  usually  of  the 
most  perfect  symmetry,  closely  applied  to  each  other,  like  the  two  valves 


MICROSCOPIC  FORMS  OF  VEGETABLE  LIFE. 


275 


of  a  Pecten,  or  other  bivalve  shell,  along  a  line  of  junction  or  suture; 
and  as  each  valve  is  more  or  less  concavo-convex,  a  cavity  is  left  between 
the  two,  which  is  occupied  by  the  cell-contents.  The  form  of  this  cav- 
ity, however,  varies  widely  in  different  Diatoms;  for  sometimes  each  valve 
is  hemispherical,  so  that  the  cavity  is  globular;  sometimes  it  is  a  smaller 
segment  of  a  sphere  resembling  a  watch-glass,  so  that  the  cavity  is  len- 
ticular; sometimes  the  central  portion  is  completely  flattened  and  the 
sides  abruptly  turned-up,  so  that  the  valve  resembles  the  cover  of  a  pill- 
box, in  which  case  the  cavity  will  be  cylindrical;  and  these  and  other 
varieties  may  co-exist  with  any  modifications  of  the  contour  of  the  valves, 
which  may  be  square,  triangular  (Fig.  164),  heart-shaped  (Fig.  176), 
boat-shaped  (Fig.  175,  a),  or  very  much  elongated  (Fig.  171),  and  may 
be  furnished  (though  this  is  rare  among  Diatoms)  with  projecting  out- 
growths (Figs.  182,  183).  Hence  the  shape  presented  by  the  frustule 
differs  completely  with  the  aspect  under  which  it  is  seen.  In  all  instances, 
the  frustule  is  considered  to  present  its  ^  front  ^  view  when  its  suture  is 
turned  towards  the  eye,  as  in  Fig.  175,  B,  c;  whilst  its  ^side'  view  is 
seen  when  the  centre  of  either  valve  is  directly  beneath  the  eye  (a).  Al- 
though the  two  valves  meet  along  the  suture  in  those  newly-formed  frus- 
tules  which  have  been  just  produced  by  binary  subdivision  (as  shown  in 
Fig.  167,  A,^),  yet  as  soon  as  they  begin  to  undergo  any  increase,  the 
valves  separate  from  one  another;  and  by  the  silicification  of  the  cell- 
membrane  thus  left  exposed,  a  pair  of  hoops  is  formed,  each  of  which  is 
attached  by  one  edge  to  the  adjacent  valve,  while  the  other  edge  is  free. 
As  will  be  presently  explained,  one  of  the  valves  is  always  older  than  the 
other;  and  the  hoop  of  the  older  valve  partly  incloses  that  of  the  younger, 
just  as  the  cover  of  a  pill-box  surrounds  the  upper  part  of  the  box  itself.' 
As  the  newly-formed  cell  increases  in  length,  separating  the  valves  from 
one  another,  both  hoops  increase  in  breadth  by  additions  to  their  free 
edges;  and  the  outer  hoop  slides  off  the  inner  one,  until  there  is  often 
but  a  very  small  ^overlap.'  As  growth  and  self -division  are  continually 
going  on  when  the  frustules  are  in  a  healthy  vigorous  condition,  it  is  rare 
to  find  a  specimen  in  which  the  valves  are  not  in  some  degree  separated 
by  the  interposition  of  the  hoops. 

275.  The  impermeability  of  the  silicified  casing  renders  necessary 
some  special  aperture,  through  which  the  surrounding  water  may  come 
into  relation  with  the  contents  of  the  cell.  Such  apertures  are  found 
along  the  whole  line  of  suture  in  disk-like  frustules;  but  when  the  Dia- 
tom is  of  an  elongated  form,  they  are  found  at  the  extremities  of  the 
frustules  only.  They  do  not  appear  to  be  absolute  perforations  in  the 
envelope,  but  are  merely  points  at  which  its  siliceous  impregnation  is 
wanting;  and  these  are  usually  indicated  by  slight  depressions  of  its 


^  This  was  long  since  pointed  out  by  Dr.  Wallich  in  his  important  Memoir  on 
the  'Development  and  Structure  of  the  Diatom-valve Transact,  of  Microsc. 
Soc,"  N.S.,  Vol.  viii.,  1860,  p.  129);  but  his  observation  seems  not  to  have  at- 
tracted the  notice  of  Diatomists,  until  in  1877  he  called  attention  to  it  in  a  more 
explicit  manner  (** Monthly  Microsc.  Journ.,"  Vol.  xvii.,  p.  61).  The  correctness 
of  his  statement  has  been  confirmed  by  the  distinguished  American  Diatomist, 
Prof.  W.  Hamilton  Smith;  but  as  it  has  been  called  in  question  by  Mr.  J.  D.  Cox 
American  Journal  of  Microscopy,"  Vol.  iii.,  1878,  p.  100),  who  asserts  that  in 
Isthmia  there  are  three  hoops— two  attached  to  the  two  valves,  and  the  third 
overlapping  them  both  at  their  line  of  junction, — the  Author  has  himself  made  a 
very  careful  examination  of  a  large  series  of  specimens  of  Isthmia  and  BidduU 
phia,  the  result  of  which  has  fully  satisfied  him  of  the  correctness  of  Dr.  Wal. 
lich's  original  description. 


276 


THE  MICROSCOPE  AND  ITS  KEYELATIONS. 


surface.  In  some  Diatoms,  as  S^trirella  (Fig.  175)  and  Campylodiscus 
(Fig.  176),  these  interruptions  are  connected  with  what  were  thought, 
t)j  Prof.  W.  Smith,  to  be  minute  canals  hollowed  out  between  the  sili- 
cified  casing  and  the  membrane  investing  the  endochrome;  but  the  ap- 
parent canals  are  really  internal  ribs,  or  projections  of  the  shell,  showing 
its  characteristic  ^ beaded^  structure  under  sufficiently  good  objectives. 
. — In  many  genera  the  surface  of  each  valve  is  distinguished  by  the  pres- 
ence of  a  longitudinal  band  on  which  the  usual  markings  are  deficient, 
and  this  is  widened  into  small  expansions  at  the  extremities,  and  some- 
times at  the  centre  also,  as  we  see  in  Pleurosigma  (Fig.  164)  and  Gom- 
plionema  (Fig.  186);  but  this  band  is  merely  a  portion  in  which  the  sili- 
cified  casing  is  thicker  than  it  is  elsewhere,  forming  a  sort  of  rib  that 
gives  firmness  to  the  valve,  its  expansions  being  solid  nodules  of  the  same 
substance. — These  nodules  were  mistaken  by  Prof .  Ehrenberg  for  aper- 
tures; and  in  this  error  he  has  been,  followed  by  Kiitzing.  There  cannot 
any  longer,  however,  be  a  doubt  as  to  their  real  nature. 

276.  The  nature  of  the  delicate  and  regular  markings  with  which 
probably  every  Diatomaceous  valve  is  beset,  has  been  of  late  years  a  sub- 
ject of  much  discussion  among  Microscopists;  but  on  certain  points 
there  is  now  a  general  convergence  of  opinion. — There  can  be  no  longer 
any  question  as  to  the  nature  of  the  comparatively  coarse  areolation  seen  in 
the  larger  forms,  such  as  Isthmia  (Fig.  163),  Triceratium  (Fig.  164), 
and  Biddulpliia  (Fig.  167);  in  all  of  which  that  structure  can  be  dis- 
tinctly seen  under  a  low  magnifying  power  and  with  ordinary  light.  In 

each  of  these  instances,  we  see  a 
;/ :  T'la.ifiai  number  of  symmetrically  disposed 

areolcB,  rounded,  oval,  or  hexago- 
nal, with  intervening  boundaries; 
and  these  have  now  been  unmis- 
takably proved  to  be  depressions, 
lying  in  the  interspaces  of  an 
elevated  reticulation.  The  retic- 
ulation presents  itself  in  clear 
relief,  when  viewed  Binocularly 
with  a  sufficiently  high  power;  and 
the  depression  of  its  interspaces 

-/wo    v^-^  or^---- -  -  ^^-^w  becomes  manifest  when  an  edge- 

^n<:Pa''Oo^OoQo^a^oo.:Poool  ^^^^^  is  obtained  of  a  curved  sur- 

portion  of  valve  of^/s^^^^^^^  g^^^j^        ^j^^^^        ^  ^^^yq  of 

'     ^  Isthmia,^ — Both    the  depressed 

areol88  and  the  intervening  network  of  Diatoms  presenting  this  areo- 
lation, when  examined  with  a  sufficient  magnifying  power,  show 
th^  'beaded^  aspect  characteristically  displayed  in  Pleurosigma  angula- 
tum  (Fig,  166);  and  this  is  also  well  seen  in  some  species  of  Actinocyclus 
and  Coscinodiscus,  and  in  the  beautiful  Heliopelta  (Plate  i.  Fig.  3). — 
The  observations  of  Mr.  Stephenson  on  Coscinodiscus  oculus  Iridis  (§ 

*  When  specimens  of  Diatoms  which  exhibit  this  areolation  are  examined 
by  the  test  of  Focal  adjustment  (§  152),  it  is  found  that  if  they  are  mounted  in 
Canada  balsam,  the  optical  effects  are  reversed;  the  areolae  being  made  to  look 
bright  {W^e  elevations)  when  the  distance  of  the  objective  is  increased,  and  dark 
Mien  it  is  diminished.  This,  however,  is  readily  explicable  by  the  fact  that  the 
.refractive  power  of  the  Balsam  is  greater  than  that  of  the  Silicified  valve;  so 
that  the  predominant  effect  will  be  produced  by  the  convexities  formed  in  the 
medium^by, the  concavities  of  the  object.  (See  Schultze  in ''Quart.  Journ.  of 
Microsc.  Science,"  Vol.  iii.,  N.S.,  1863,  p.  131.) 


MICROSCOPIC  FORMS  OF  VEGETABLE  LIFE. 


277 


289),  and  of  Mr.  Shadbolt  on  Arachnoidiscus  (§  291)  leave  no  doubt 
that  in  those  Diatoms  the  silicified  valve  is  composed  of  two  layers;  and 
the  same  is  probably  the  case  in  all  those  forms  which  present  a  surface- 
areolation.  Appearances  are  seen,  too,  in  other  Diatoms,  which  seems  to 
indicate  that  in  them  also  the  valve  consists  of  two  layers.^ 

277.  The  ^ beaded^  aspect  (Fig.  166,  a),  which  is  generally,  if  not 
universally,  discernible  in  the  silicified  envelopes  of  Diatoms,  when  ex- 
amined under  a  sufficiently  high  magnifying  power,  and  with  an  illumi- 
nation specially  adapted  to  display  them,  is  now  usually  regarded  as  in- 
dicating that  the  silicified  envelope  is  composed  of  globular  particles  of 
silex,  closely  set  together  in  regular  rows.^  And  on  this  view  of  their 
nature,  it  is  on  the  dimensions  of  their  component  spherules,  and  on  the 
mode  in  which  they  are  disposed,  that  those  peculiar  markings  of  certain 
Diatom- valves  depend,  which  render  them  of  special  value  as  Test-objects 
(§  161).  Such  valves  have  been  commonly  spoken  of  as  marked  by  strice, 
longitudinal,  transverse,  or  oblique,  as  the  case  may  be;  but  this  term 
does  not  express  the  real  nature  of  the  markings  (the  apparent  lines 
being  resolvable  by  Objectives  of  sufficient  magnifying  power  and  angular 
aperture  into  imvs  of  dots),  and  should  only  be  used  for  the  sake  of  con- 

(  Jig.  104, 


Triceratium  favus:—A,  side  view  ;  b,  front  view. 


cisely  indicating  the  degree  of  their  approximation.  If  we  examine 
rieurosigma  angulatum,  one  of  the  easier  tests,  with  an  Objective  of 
l-4th  inch  focus  (having  an  angular  aperture  of  90°  and  a  magnifying 


1  See  Mr.  C.  Stodder  (of  Boston,  U.  S.),  On  the  Structure  of  the  Valve  of  the 
DiafomaceoB,"  in  Quart.  Journ.  of  Microsc.  Science,"  Vol.  iii.,  N.S.  (1863),  p. 
214;  also  Ralfs,  Op.  cit.,  Vol.  vi.  (1858),  p.  214;  and  Rylands,  Op.  cit.,  Vol.  viii. 
(I860),  p.  27. 

2  See  Dr.  Wallich's  Papers  on  this  subject  in  Quart.  Journ.  of  Microsc.  Sci- 
ence," Vol.  vi.  (1858),  p.  247;  "  Annals  of  Nat.  Hist.,"  Vol.  v.  Ser.,  4  (Feb.,  1860), 
p.  122;  and  Trans,  of  Microsc.  Soc,"  Vol.  viii.,  N.S.  (1860),  p.  129.  See  also 
Norman  in  Quart.  Journ.  of  Microsc.  Sci.,"  Vol.  ii.,  N.S.  (1862),  p.  212.— Mr. 
"Wenham,  who  at  one  time  inclined  to  the  opposite  belief,  stated  (when  Dr.  Wal- 
lich's Paper  was  read  before  the  Microscopical  Society),  as  the  result  of  observa- 
tions made  with  an  Objective  of  l-50th  inch  focus  and  large  aperture,  that  the 
valves  are  composed  wholly  of  spherical  particles  of  silex,  possessing  high  refrac- 
tive power;  and  he  showed  how  all  the  various  optical  appearances  presented  by 
the  different  species  could  be  reconciled  with  the  supposition  that  their  structure 
is  universally  the  same. — Recourse  has  been  had,  with  a  certain  measure  of  suc- 
cess, to  the  production  of  '  artificial  Diatoms '  by  the  deposit  of  silex  from  its  fluo- 
ride; thin  films  being  formed,  which  exhibit  a  *  beaded '  structure,  often  arranged 
in  very  regular  patterns.    See  the  Memoir  of  Prof.  Max  Schultze,  abstracted  in 

Quart.  Journ.  of  Microsc.  Sci.,  N.S.,  Vol.  iii.  (1863),  p.  120;  and  Mr.  Slack's 
Paper  in    Monthly  Microsc.  Journ.,"  Vol.  iv.  (1870),  p.  181. 


278 


THE  MICROSCOPE  AND  ITS  REVELATIONS. 


power  of  500  diameters),  we  shall  see  very  much  what  is  represented  in 
Fig.  165,.  e;  namely,  a  double  series  of  somewhat  interrupted  lines, 
crossing  each  other  at  an  angle  of  60  degrees,  so  as  to  have  between  them 
imperfectly-defined  lozenge-shaped  spaces.  When,  however,  the  valve  is 
examined  with  an  Objective  of  higher  power,  having  an  angular  aperture 

of  120°  or  more,  and  a  mag- 
pie. nifying  power  of  1,200  diam 

eters,  an  appearance  like  that 
represented  in  Fig.  166,  a) 
may  be  obtained,  namely,  a 
hexagonal  areolation,  in 
which  the  areolae  can  be  made 
to  appear  light,  and  the  di- 
viding network  dark,  or  vice 
versa,  according  to  the  ad- 
justment of  the  focus  (Fig. 
115).  That  the  areolae  are 
here  elevations,  and  not  (like 
those  of  Triceratum)  depres- 
sions, is  indicated  by  the 
comparative  results  of  the  ex- 
amination of  fractured  valves. 
For  in  Triceratium  the  frac- 
tures pass  through  the  aj:)- 
parent  depressions,  and  co- 
incide with  various  optical 
indications  in  establishing 
their  reality.  Fractured 
valves  of  P.  angulatum  and 
allied  species  show  that  the 


Outline  of  Pleurosigma  quadratum,  as  seen  under  a  Weakest  parts  are  between  tllC 
power  of  400  diameters:-at  A,  B,  D,  are  sh^^^^  and    siuP'le  beads 

tions  of  the  lines  seen  under  a  power  of  1,300,  the  lUumin-  i  j 

ating  rays  falling  obliquely  (in  each  case)  in  a  direction  at  may  Oltcn    DC  SCCU  tcrmiua- 
right  angles  to  the  lines;  at  e  are  shown  two  sets  of  j--        „  c>,orn  «no-nlnr  -nnvfinn 
lines,  as  seen  when  the  oblique  rays  fall  in  the  direction  bHdip  angular  porcion. 


of  the  midrib;  and  at  c  is  shown  the  appearance  of  the  Further, 
markings  when  illuminated  with  an  Achromatic  Conden-  • 
ser  of  large  angular  aperture,  the  spherules  being  within  JrieurOStffina 
the  focus,  and  the  portion  left  blank  showing  the  obli-  tipq+Vi  glaSS 


and  the  portion  left  blank  showing  the  obli-  neath 
teration  of  the  markings  by  moisture.  ,  .   -        ,  ^ 

markings  obscured 


when  specimens  of 
mounted  be- 
have had  their 
by  mois- 
ture, the  obscurity  is  dissipated  by  the  application  of  a  gentle  heat,  in  a 
way  that  is  readily  explicable  on  the  supposition  that  the  markings  are 
elevations,  but  seems  unintelligible  on  the  idea  of  their  being  depres- 
sions.^— Notwithstanding  these  considerations,  however,  it  must  be  freely 
admitted  that  there  is  still  considerable  uncertainty  respecting  the  real 
structure  of  the  Diatom-valve.  For  it  cannot  be  positively  asserted  that 
the  focal  adjustment  which  gives  the  image  represented  in  Fig.  166,  A,  is 
more  correct  than  that  which  gives  the  equally  distinct  images  B,  c,  D  of 
other  parts  of  the  same  valve,  of  which  the  last  departs  in  the  most 
marked  manner  from  what  is  commonly  regarded  as  the  normal  type. 
And  now  that  it  has  been  shown  that  these  images  are  not  formed  diop- 
trically,  but  are  resultants  of  the  combination  of  numerous  ^  diffraction- 
spectra'  (§  157),  it  is  impossible  to  entertain  the  same  confidence  as 
before  that  they  truly  picture  the  surface  marking  they  are  supposed  to 


*  See  Mr.  G.  Hunt  in  -*  Quart.  Journ.  of  Microsc.  Sci.,"  Vol.  iii.  (1855),  p.  174. 


MICROSCOPIC  FORMS  OF  VEGETABLE  LIFE. 


2T9 


represent. — By  Mr.  Stephenson,  who  has  made  a  special  stndy  of  the 
effects  of  the  immersion  of  Diatom-valves  in  very  highly-refracting  media, 
it  is  believed  that  the  light  spaces  really  represent  apertures  (§  ?389). 
The  question  must  be  regarded,  therefore,  as  still  an  open  one. 

278.  Multiplication  by  Binary  subdivision  takes  place  among  the 
Diatomacece  on  the  same  general  plan  as  in  the  Desmidiaceae,  but  with 
some  modifications  incident  to  the  pecularities  of  the  structure  of  the 
former  group. — The  first  stage  consists  in  the  elongation  of  the  cell,  and 
the  formation  of  a  '  hoop'  adherent  to  each  end-valve  (§  274),  so  that  the 
two  valves  are  separated  by  a  band,  which  progressively  increases  in 
breadth  by  addition  to  the  free  edges  of  the  hoops,  as  is  well  seen  in  Fig. 
167  A.    In  the  newly  formed  cell    the  two  valves  are  in  immediate  ap- 


Portions  of  Valve  of  Pleurosigma  angulatum,  as  seen  under  a  magnifying:  power  of  2,000  dia- 
meters, with  central  illuminalion;  from  a  Photograph  by  Carl  Gunther  in  the  possession  ot  the 
Royal  Microscopical  Society.  r. 

A.  Normal  hexagonal  areolation;  areolae  bright  circles,  surrounded  by  dark  hexagons.  ^ 

B.  In  upper  part,  areolge  and  their  dark  borders  graduating  from  circular  to  elliptical;  in  lower 
part,  dark  borders  coalescing  latterly,  so  as  to  give  the  appearance  of  contmuous  vertical 
lineation.  ,  , 

c.  Areolae  larger,  brighter,  and  more  elliptical;  their  dark  bodies  coalescing  laterally,  so  as  to 
form  very  decided  vertical  lineation.  .      ^  ,    ,  • 

D.  Transition  from  hexagonal  to  triangular  areolation,  with  three  series  of  dark  lines,  one  hori- 
zontal and  two  oblique. 

position;  in  d,  a  band  intervenes;  in  a,  this  band  has  become  much 
wider;  and  in  b,  the  hicrease  has  gone- on  until  the  original  form  of  the 
cell  is  completely  changed,  xit  the  same  time,  the  endochrome  se^iarates 
into  two  halves;  the  nucleus  also  subdivides  in  the  manner  formerly 
fihown  (Plate  viii.,  fig.  1,  G,  H,  i);  and  the  primordial  utricle  folds-in, 
first  forming  a  mere  constriction,  then  an  hour-glass  contraction,  and 


280 


THE  MICROSCOPE   AND  ITS  KEVELATION8* 


finally  a  complete  double  partition,  a?  in  other  instances  (§  252).  From 
each  of  its  adjacent  surfaces  a  new  siliceous  yalve  is  formed,  as  shown  at 
Fig.  A,  c,  just  as  a  new  cellulose-wall  is  generated  in  the  subdivision 
of  other  cells;  and  this  yalve  is  usually  the  exact  counterpart  of  the  one 
to  which  it  is  opposed,  and  forms  with  it  a  complete  cell,  so  that  the 
original  frustule  is  replaced  by  two  frustules,  each  of  which  has  one  old 
and  one  new  yalve,  just  as  in  Desmidiaceae  (§  2G4).  Generally  speaking, 
the  new  valves  are  a  little  smaller  than  their  predecessors;  so  that  after 
repeated  subdivisions  (as  in  chains  of  Isthmia),  a  diminution  of  diameter 
becomes  obvious.  But  sometimes  the  new  valves  are  a  little  larger  than 
their  predecessors;  so  that,  in  the  filamentous  species,  there  may  be  an 
increase  sufficient  to  occasion  a  gradual  widening  of  the  filament,  although 
not  perceptible  when  two  contiguous  frustules  are  compared;  whilst,  in 
the  free  forms,  frustules  of  different  sizes  may  be  met  with,  of  which  the 
larger  are  more  numerous  than  the  smaller,  the  increase  in  number 
having  taken  place  in  geometrical  progression,  whilst  that  of  size  was 
uniform.  It  is  not  always  clear  what  becomes  of  the  '  hoop.'  In  Melosira 
(Figs.  177,  178),  and  perhaps  in  the  filamentous  species  generally,  the 
*  hoops'  appear  to  keep  the  new  frustules  united  together  for  some  time. 
This  is  at  first  the  case  also  in  Biddulpliia  and  Istlimia  (Fig.  181),  in 
which  the  continued  connection  of  the  two  frustules  by  its  means  give 
rise  to  an  appearance  of  two  complete  frustules  having  been  developed 
within  the  original  (Fig.  167,  A,  c)  \  subsequently,  however,  the  two  new 
frustules  slip  out  of  the  hoop,  which  then  becomes  completely  detached. 
The  same  thing  happens  with  many  other  Diatoms,  so  that  the  Mioops' 
are  to  be  found  in  large  numbers  in  the  settlings  of  water  in  which  these 
plants  have  long  been  growing.  But  in  some  other  cases  all  trace  of  the 
hoop  is  lost;  so  that  it  may  be  questioned  whether  it  has  ever  been  prop- 
erly silicified,  and  whether  it  does  not  become  fused  (as  it  were)  into  the 
gelatinous  envelope. — During  the  healthy  life  of  the  Diatom,  the  process 
of  self -division  is  continually  being  repeated;  and  a  very  rapid  multipli- 
cation of  frustules  thus  takes  places,  all  of  which  (as  in  the  cases  already 
cited,  §§  229,  271),  must  be  considered  to  be  repetitions  of  one  and  the 
same  individual  form.  Hence  it  may  happen  that  myriads  of  frustules 
may  be  found  in  one  locality,  uniformly  distinguished  by  some  peculiarity 
of  form,  size,  or  marking;  which  may  yet  have  had  the  same  remote 
origin  as  another  collection  of  frustules  found  in  some  different  locality, 
and  alike  distinguished  by  some  peculiarity  of  its  own.  For  there  is 
strong  reason  to  believe  that  such  differences  spring-up  among  the  pro- 
geny of  any  true  generative  act  (§  229);  and  that  when  that  progeny  is 
dispersed  by  currents  into  different  localities,  each  will  continue  to  mul- 
tiply its  own  special  type  so  long  as  the  process  of  self -division  goes  on. 

279.  It  is  uncertain  whether  the  Diatomaceae  also  multiply  by  the 
breaking-up  of  their  endochrome  into  segments,  and  by  the  liberation  of 
these,  either  in  the  active  condition  of  *  zoospores,'  or  in  the  state  of 
*stilP  or  ^resting'  spores.  Certain  observations  by  Focke,^  however, 
taken  in  connection  with  the  analogy  of  other  Protophytes,  and  with  the 
fact  that  the  zygospore-frustules  almost  certainly  thus  multiply  by  gonidia 
280),  seem  to  justify  the  conclusion  that  such  a  method  of  multiplication 
does  obtain  in  this  group.  And  it  is  not  at  all  unli  kely  that  very  considerable 
differences  in  the  size,  form,  and  markings  of  the  frustules,  such  as  many 
consider  sufficient  to  establish  a  diversity  of  species,  have  their  origin  in 


1  " Physiologisch.  Studien,"  Heftii.,  1853. 


MICKOSCOPIC  FOKMS  OF  VEGETABLE  LIFE.  28 J 

this  mode  oj  propagation.  It  seems  probable  that,  so  long  as  the  veo-e- 
tatmg  processes  are  in  full  activity,  multiplication  takes  place  in  prefer- 
ence by  self-division;  and  that  it  is  when  deficiency  of  warmth,  of 
moisture,  or  of  some  other  condition,  gives  a  check  to  these,  that  the 
formation  of  encysted  'gonidia,'  having  a  greater  power  of  resisting 
unfavorable  influences,  will  take  place;  whereby  the  species  is  maintained 
m  a  dormant  state  until  the  external  conditions  favor  a  renewal  of  active 
vegetation  (§  234). 

280.  Conjugation,  so  far  as  is  at  present  known,  takes  place  among 
the  ordinary  Diatomace^  almost  exactly  as  among  the  Desmidiacese;  ex- 
cept that  it  sometimes  results  in  the  production  of  two  'zygospores  ' 
insead  of  a  single  one.    Thus  in  Surirella  (Fig.  175),  the  valves  of  two 


Biddulphia  pulchella ;— a,  chain  of  cells 
in  different  states;  a,  full  size;  6,  elongation 
preparatory  to  subdivision;  c,  formation  of 
two  new  cells;  d,  e,  young  cells ;—b,  end-view; 
— c,  side-view  of  a  cell  more  highly  magni- 
fied. 


rio.1% 


Conjugation  of  Epithemia  turgida:—A,  front 
view  of  single  frustule ;  b,  side  view  of  the  same ; 
c,  two  frustules  with  their  concave  surfaces  in 
close  apposition ;  d,  front  view  of  one  of  the  frus- 
tules showing  the  separation  of  its  valves  along 
the  suture;  e,  f,  side  and  front  views  after  the 
formation  of  the  zygospores. 


free  and  adjacent  frustules  separate  from  each  other  at  the  sutures,  and 
the  two  endochromes  (probably  included  in  their  primordial  utricles)  are 
discharged;  these  coalesce  to  form  a  single  mass,  which  becomes  inclosed 
in  a  gelatinous  envelope;  and  in  due  time  this  ^zygospore'  shapes  itself 
into  a  frustule  resembling  that  of  its  parent,  but  of  larger  size.  But  in 
Epithemia  (Fig  168,  A,  b) — the  first  Diatom  in  which  the  conjugating 
process  was  observed  by  Mr.  Thwaites' — the  endochrome  of  each  of  the 
conjugating  frustules  (c,  d)  appears  to  divide  at  the  time  of  its  discharge 
into  two  halves;  each  half  coalesces  with  half  of  the  other  endochrome; 
and  thus  two  *  zygospores'  (e,  f)  are  formed,  which,  as  in  the  preceding 
case,  become  invested  with  a  gelatinous  envelope,  and  gradually  assume 


1  See  "  Annals  of  Natural  History,"  Ser.  1,  Vol.  xx.  (1847),  pp.  9,  343;  and 
Ser.  2,  Vol.  i.  (1848),  p.  161. 


282 


THE  MICROSCOPE  AND  ITS  REVELATIONS. 


the  form  and  markings  of  the  parent-frustules,  but  grow  to  a  very  much 
l.irger  size,  the  sporangial  masses  having  obviously  a  power  of  self-increase 
up  to  the  time  when  their  envelopes  are  consolidated.  It  seems  to  be  in 
this  way  that  the  normal  size  is  recovered,  after  the  progressive  diminu- 
tion which  is  incident  to  repeated  binary  multiplication  (§  278).  Of 
the  subsequent  history  of  the  ^zygospores'  much  remains  to  be  learned; 
and  it  may  not  be  the  same  in  all  cases.  Appearances  have  been  seen 
which  make  it  almost  certain  that  the  contents  of  each  zygospore  break-, 
up  into  a  brood  of  (joiiidiay  and  that  it  is  from  these  that  the  new  genera- 
tion originates.  These  gonidia,  if  each  be  surrounded  (as  in  many  other 
cases)  by  a  distinct  cyst,  may  remain  undeveloped  for  a  considerable 
period;  and  they  must  augment  considerably  in  size  before  they  obtain 
the  dimensions  of  the  parent  frustule. — It  is'^in  this  stage  of  the  process 
that  the  modifying  influence  of  external  agencies  is  most  likely  to  exert 
its  effects;  and  it  may  be  easily  conceived  that  (as  in  higher  Plants  and 
Animals)  this  influence  may  give  rise  to  various  diversities  among  the 

respective  individuals  of 
^iG.i60,  the  same   brood;  which 

diversities,  as  we  have 
seen,  will  be  transmitted 
to  all  the  repetitions  of 
each  that  are  produced 
by  the  self -dividing  pro- 
cess. Hence  a  very  con- 
siderable latitude  is  to  be 
allowed  to  the  limits  of 
Species,  when  the  differ- 
ent forms  of  Diatomaceae 
are  compared;  and  here, 
as  in  many  other  cases, 
a  most  important  question 
arises  to  what  are  those 
limits — a  question  which 
can  only  be  answered  by 
such  a  careful  study  of  the 
entire  life-history  of  every 
single  type,  as  may  advantageously  occupy  the  attention  of  many  a  Mi- 
croscopist  who  is  at  present  devoting  himself  to  the  resolution  of  the 
markings  on  Diatom-valves,  and  to  the  multiplication  of  reputed  species 
by  the  detection  of  minute  differences.^ 

281.  This  formation  of  what  are  termed  'auxospores' — as  serving  to 
augment  the  size  of  the  cells  which  are  to  give  origin  to  a  new  genera- 
tion— takes  place  on  a  very  different  plan  in  some  of  those  filamentous 
types,  such  as  Melosira  (Figs.  177,  178),  in  which  a  strange  inequality 
presents  itself  in  the  diameters  of  the  different  cells  of  the  same  filaments, 
the  larger  ones  being  usually  in  various  stages  of  binary  subdivision,  by 
which  they  multiply  themselves  longitudinally.  According  to  the  obser- 
vations of  Mr.  Thwaites  (loc.  cit.),  these  also  are  the  products  of  a  kind 


Self -Conjugation  (?)  of  Melosira  Italica  (Aulacoseira  cren- 
alata,  Thwaites):—!,  simple  filament;  2,  filament  developing 
auxospores;  a,  6,  c,  successive  stages  in  the  formation  of 
auxospores;  3,  auxospore-frustules,  in  successive  stages,  a,  6, 
c,  of  multiplication. 


1  See  on  this  subject  a  valuable  Paper  by  Prof.  W.  Smith  '  On  the  Determina- 
tion of  Species  in  the  Diatomacece, '  in  the  Quart.  Journ.  of  Microsc.  Science," 
Vol.  iii.  (1855),  p.  180;  a  Memoir  Prof.  W.  Gregory  *  On  Shape  of  Outline  as  a  spe- 
cific character  of  Diatomacece,'  in  Trans,  of  Microsc.  Soc,"  2d  Series,  Vol.  iii. 
(1855),  p.  10;  and  the  Author's  Presidential  Address  in  the  same  volume,  pp. 
44-50. 


MICROSCOPIC  FORMS  OF  VEGETABLE  LIFE. 


283 


of  conjugation  between  the  adjacent  cells  of  the  ordinary  diameter; 
taking  place  before  the  completion  of  their  separation.  He  describes  the 
endochrome  of  particular  frustules,  after  separating  as  if  for  the  forma- 
tion of  a  pair  of  new  cells,  as  moving  back  from  the  extremities  towards 
the  centre,  rapidly  increasing  in  quantity,  and  aggregating  into  a  zygo- 
spore (Fig.  169,  2,  a,  i,  c):  around  this  a  new  envelope  is  developed,  which 
may  or  may  not  resemble  that  of  the  ordinary  frustules,  but  which  re- 
mains in  continuity  with  them;  and  this  zygospore  soon  undergoes  binary 
subdivision  (3,  a,  b,  c),  the  cells  of  the  new  series  thus  developed  pre- 
senting the  character  of  those  of  the  original  filament  (1),  but  greatly  ex- 
ceeding them  in  size.  From  what  has  been  already  stated  (§  278),  it 
seems  probable  that  a  gradual  reversion  to  the  smaller  form  takes  place 
in  subsequent  subdivisions;  a  further  reduction  being  checked  by  a  new 
formation  of  zygospores.  Whether  this  formation  partakes  of  the  charac- 
ter of  ^conjugation'  (as  supposed  by  Mr.  Thwaites)  is  still  doubtful;  some 
later  observers  regarding  '  auxospores '  as  simply  enlarged  forms  of  single 
cells. 

282.  Most  of  the  Diatoms  which  are  not  fixed  by  a  stipes,  possess 
some  power  of  spontaneous  movement;  and  this  is  especially  seen  in 
those  whose  frustules  are  of  a  long  narrow  form,  such  as  that  of  the 
Naviculce  generally.  The  motion  is  of  a  peculiar  kind,  being  usually  a 
series  of  jerks,  which  carry  forward  the  frustule  in  the  direction  of  its 
length,  and  then  carry  it  back  through  nearly  the  same  path.  Some- 
times, however,  the  motion  is  smooth  and  equable;  and  this  is  especially 
the  case  with  the  curious  Bacillaria  paradoxa  (Fig.  171),  whose  frus- 
tules slide  over  each  other  in  one  direction  until  they  are  all  but 
detached,  and  then  slide  as  far  in  the  opposite  direction,  repeating  this 
alternate  movement  at  very  regular  intervals.^  In  either  case  the  motion 
is  obviously  quite  of  a  different  nature  from  that  of  beings  possessed  of  a 
])ower  of  self-direction.  ^^An  obstacle  in  the  path,"  says  Prof.  W. 
Smith,  '^is  not  avoided,  but  pushed  aside;  or,  if  it  be  sufficient  to  avert 
the  onward  course  of  the  frustule,  the  latter  is  detained  for  a  time  equal 
to  that  which  it  would  have  occupied  in  its  forward  progression,  and 
then  retires  from  the  impediment  as  if  it  had  accomplished  its  full 
course."  The  character  of  the  movement  is  obviously  similar  to  that  of 
those  motile  forms  of  Protophyta  which  have  been  already  described; 
but  it  has  not  yet  been  definitely  traced  to  any  organ  of  impulsion;  and 
the  cause  of  it  is  still  obscure.  By  Prof.  W.  Smith  it  is  referred  to  forces 
operating  within  the  frustule,  and  originating  in  the  vital  operations  of 
growth,  etc.,  which  may  cause  the  surrounding  fluid  to  be  drawn  in 
through  one  set  of  apei/tures,  and  expelled  through  the  other.'*  ^  "  If,"  as 
he  remarks,  ^^the  motion  be  produced  by  the  exosmose  taking  place 
alternatively  at  one  and  the  other  extremity,  while  endosmose  is  pro- 
ceeding at  the  other,  an  alternating  movement  would  be  the  result  in 
frustules  of  a  linear  form;  whilst  in  others  of  an  elliptical  or  orbicular 

1  This  curious  phenomenon  the  Author  has  himself  repeatedly  had  the  oppor- 
tunity of  witnessing. 

'-^It  has  been  objected  to  this  view,  by  the  Authors  of  the  Micrographic 
Dictionary,"  that,  if  such  were  the  case,  the  Hke  movements  would  be  frequently 
met  with  in  other  minute  unicellular  organisms.  But  there  are  no  other  such 
organisms  in  which  the  cell  is  almost  entirely  inclosed  in  an  impermeable  enve- 
lope, so  that  the  imbibition  and  expulsion  of  fluid  are  limited  to  a  small  number 
of  definite  points,  instead  of  being  allowed  to  take  place  equally  (as  in  other  uni- 
cellular organisms)  over  the  entire  surface.—See  Mereschkowski  in  Journ.  Roy. 
Microsc.  Soc,"  Ser.  2,  Vol.  i.  (,1881),  p.  102. 


284 


THE  MICROSCOPE  AND  ITS  KEVELATIONS^ 


outline,  in  which  foramina  exist  along  the  entire  line  of  suture,  the 
movements,  if  any,  must  be  irregular  or  slowly  lateral.  Such  is  precisely 
the  case.  The  backward  and  forward  movements  of  the  Naviculce  have 
been  already  described;  in  Surirella  (Pig.  175)  and  Campy lo discus  (Pig. 
176),  the  motion  never  proceeds  further  than  a  languid  roll  from  one 
side  to  the  other;  and  in  Gomphonema  (Pig.  187),  in  which  a  foramen 
fulfilling  the  nutritive  office  is  found  at  the  larger  extremity  only,  the 
movement  (which  is  only  seen  when  the  frustule  is  separated  from  its 
stipes)  is  hardly  a  perceptible  advance  in  intermitted  jerks  in  the  direc- 
tion of  the  narrow  end.'' 

283.  The  principles  upon  which  this  interesting  group  should  be 
classified,  cannot  be  properly  determined,  until  the  history  of  the  Gen- 
erative process — of  which  nothing  whatever  is  yet  known  in  a  large  pro- 
portion of  Diatoms,  and  very  little  in  any  of  them — shall  have  been 
thoroughly  followed  out.  The  observations  of  Pocke*  render  it  highly 
probable  that  many  of  the  forms  at  present  considered  as  distinct  from 
each  other,  would  prove  to  be  but  dilferent  states  of  the  same,  if  their 
history  were  ascertained.  On  the  other  hand,  it  is  by  no  means 
impossible  that  some  which  appear  to  be  nearly  related  in  the  structure 
of  their  frustules  and  in  their  mode  of  growth,  may  prove  to  have  quite 
different  modes  of  reproduction.  At  present,  therefore,  miy  classification 
must  be  merely  provisional;  and  in  the  notice  now  to  be  taken  of  some 
of  the  most  interesting  forms  of  DiatomacecB,  the  method  of  Prof.  Kiit- 
zing,  which  is  based  upon  the  characters  of  the  individual  frustules,  is 
followed  in  preference  to  that  of  Prof.  W.  Smith,  which  was  founded  on 
the  degree  of  connection  remaining  between  the  several  frustules  after 
self-division.^ — In  each  Pamily  the  frustules  may  exist  under  four  con- 
ditions, {a)  free,  the  self-division  being  entire,  so  that  the  frustules  sep- 
arate as  soon  as  the  process  has  been  completed;  {b)  stipitate,  the 
frustules  being  implanted  upon  a  common  stem  (Pig.  172),  which  keeps 
them  in  mutual  connection  after  they  have  themselves  undergone  a  com- 
plete self-division;  {c)  united  in  a  filament,  which  will  be  continuous 
(Pig.  177)  if  the  cohesion  extend  to  the  entire  surfaces  of  the  sides  of  the 
frustules,  but  may  be  a  mere  zigzag  chain  (Pig.  173)  if  the  cohesion  be 
limited  to  their  angles;  {d)  aggregated  into  a  frond  (Pig.  188),  which 
consists  of  numerous  frustules  more  or  less  regularly  inclosed  in  a 
gelatinous  investment.  It  is  not  in  every  family,  however,  that  these 
four  conditions  are  at  present  known  to  exist;  but  they  have  been 
noticed  in  so  many,  that  they  may  be  fairly  presumed  to  be  capable  of 
occurring  in  all. — Excluding  the  family  Actiniscem  (of  whose  silicified 
skeletons  we  have  examples  in  Pig.  191,  c,  d),  which  seem  to  have  no 
adequate  title  to  rank  among  Diatoms  (their  true  alliance  being  apjDar- 
ently  with  the  Polycystina)^  the  entire  group  may  be  divided  into  two 


^According  to  this  observer  (**  Ann.  of  Nat.  Hist.,"  2d  Ser.,  Vol.  xv.,  1855,  p. 
237),  Navicula  hifrons  forms,  by  the  spontaneous  fission  of  its  internal  substance, 
spherical  bodies  which,  like  gemmules,  give  rise  to  Surirella  microcora.  These 
by  conjugation  produce  N,  splendida,  vrhich  gives  rise  to  N,  hifrons  by  the  same 
process.  He  is  only  able  to  speak  positively,  however,  as  to  the  production  of  N, 
hifrons  from  N.  splendida  ;  that  of  Surirella  microcora  from  N.  hifrons,  and  that 
of  N.  splendida  from  Surirella  microcora,  being  matters  of  inference  from  the 
phenomena  witnessed  by  him. 

*^The  method  of  Ktitzing  is  the  one  followed,  with  some  modification,  by  Mr. 
Ralfs  in  his  revision  of  the  group  for  the  4th  Edition  of  Pritchard's  Infusoria;" 
and  to  his  systematic  arrangement  the  Author  would  refer  such  as  desire  more 
detailed  information. 


MICROSCOPIC  FORMS  OF  VEGETABLE  LIFE. 


285 


prihcipal  Sections;  one  (B)  containing  those  forms  in  which  the  valves 
possess  a  true  central  nodule  and  median  longitudinal  line  (as  Pleuro- 
sigma.  Fig.  165,  and  Goinphonema,  Fig.  186,  a);  and  the  other  (A) 
including  all  those  in  which  the  valves  are  destitute  of  a  central  nodule 
(as  Surirellay  Fig.  175,  a).  Among  the  latter,  however,  we  find  some 
{!))  in  which  there  is  an  umbilicus  or  pseudo-nodule  with  radiating  lines 
or  cellules,  whilst  there  are  others  (a)  which  have  no  central  marking 
whatever. 

284.  Commencing  with  the  last-named  division  (A),  the  first  Family 
is  that  of  EunotiecB,  of  which  we  have  already  seen  a  characteristic  ex- 
ample in  Epithemia  turgida  (Fig.  168).  Tiie  essential  characters  of  this 
family  consist  in  the  more  or  less  lunate  form  of  the  frustules  in  the 
lateral  view  (Fig.  168,  B),  and  in  the  striae  being  continuous  across  the 
valves  without  any  interruption  by  a  longitudinal  line.  In  the  genus 
Eunotia  the  frustules  are  free;  in  Epithemia  they  are  very  commonly 
adherent  by  the  flat  or  concave  surface  of  the  connecting  zone;  and  in 
Himantidium        are  usually  united  into  ribbon-like  filaments. — In  the 


Fig.  170. — Meridion  circulare.  Fig.  171. — Bacillaria  paradoxa. 

Family  Meridiece  we  find  a  similar  union  of  the  transversely-striated  in- 
dividual frustules;  but  these  are  narrower  at  one  end  than  at  the  other, 
so  as  to  have  a  cuneate  or  wedge-like  form;  and  are  regularly  disposed 
with  their  corresponding  extremities  always  pointing  in  the  same  direc- 
tion, so  that  the  filament  is  curved  instead  of  straight,  as  in  the  beautiful 
Meridion  circulare  (Fig.  170).  Although  this  plant,  when  gathered  and 
placed  under  the  microscope,  presents  the  appearance  of  circles  overlying 
one  another,  it  really  grows  in  a  helical  (screw-like)  form,  making  several 
continuous  turns.  This  Diatom  abounds  in  many  localities  in  this 
country;  but  there  is  none  in  which  it  presents  itself  in  such  rich  luxuri- 
ance as  in  the  mountain-brooks  about  West  Point  in  the  United  States, 
the  bottoms  of  which,  according  to  Prof.  Bailey,  ^^are  literally  covered 
in  the  first  warm  days  of  spring  with  a  ferruginous-colored  mucous  mat- 
ter, about  a  quarter  of  an  inch  thick,  which,  on  examination  by  the 
microscope,  proves  to  be  filled  with  millions  and  millions  of  these  ex- 
quisitely-beautiful silicious  bodies.  Every  submerged  stone,  twig,  and 
spear  of  grass  is  enveloped  by  them;  and  the  waving  plume-like  appear- 


286 


THE  MICROSCOPE  AND  ITS  REVELATIONS. 


aiice  of  a  filamentous  body  covered  in  this  way  is  often  very  elegant." 
The  frustules  of  Meridion  are  attached  when  young  to  a  gelatinous 
cushion;  but  this  disappears  with  the  advance  of  age. — In  the  Family 
LicmophorecB  also  the  frustules  are  wedge-shaped;  in  some  genera  they 
have  transverse  markings,  whilst  in  others  these  are  deficient;  but  in 
most  instances  there  are  to  be  observed  two  longitudinal  suture-like  lines 
on  each  valve  (which  have  received  the  special  designation  of  vittce)  con  v 
necting  the  puncta  at  their  two  extremities.  The  newly-formed  part  of 
the  stipes  in  the  genus  LicmopJiora^  instead  of  itself  becoming  double 
with  each  act  of  self-division  of  the  frustule,  increases  in  breadth,  while 
the  frustules  themselves  remain  coherent;  so  that  a  beautiful  fan-like 
arrangement  is  produced  (Fig.  172).  A  splitting-away  of  a  few  frustules 
seems  occasionally  to  take  place,  from  one  side  or  the  other,  before  the 
elongation  of  the  stipes;  so  that  the  entire  j)lant  jiresents  us  with  a  more 


Licmophora  flabellata. 


Fig.  173.— Diatoma  vulgar e  :—a,  side  view 
of  frustule;  6,  frustule  undergoing  self-division. 

Fig.  174. — Orammatophora  serpentina:— a^ 
front  and  side  views  of  single  frustule;  6,  />, 
front  and  end  views  of  divided  frustule;  c,  frus- 
tule about  to  undergo  self-division ;  frustule 
completely  divided. 


or  less  Qom^lQiQ  flahella  or  fan  upon  the  summit  of  the  branches,  with 
imperfect  flabellsB  or  single  frustules  irregularly  scattered  throughout  the 
entire  length  of  the  footstalk.  This  beautiful  plant  is  marine,  and  is 
parasitic  upon  Seaweeds  and  Zoophytes. 

285.  In  the  next  Family,  that  of  Fragillariece,  the  frustules  are  of  the 
same  breadth  at  each  end,  so  that,  if  they  unite  into  a  filament,  they 
form  a  straight  band.  In  some  genera  they  are  smooth,  in  others  trans- 
versely striated,  with  a  central  nodule;  when  striae  are  present,  they  run 
across  the  valves  without  interruption. — To  this  family  belongs  the  genus 
Diatomay  which  gives  its  name  to  the  entire  group;  that  name  (which 
means  cutting  through)  being  suggested  by  the  curious  habit  of  the 
genus,  in  which  the  frustules  after  self-division  separate  from  each  other 
along  their  lines  of  junction,  but  remain  connected  at  their  angles,  so  as 
to  form  zigzag  chains  (Fig.  173).    The  valves  of  Diatomay  when  turned 


MICROSCOPIC  FORMS  OF  VEGETABLE  LIFE. 


287 


sideways  (a),  are  seen  to  be  strongly  marked  by  transverse  striae,  which 
extend  into  the  front  view.  The  proportion  between  the  length  and  the 
breadth  of  each  valve  is  found  to  vary  so  considerably,  that,  if  the  ex- 
treme forms  only  were  compared,  there  would  seem  adequate  ground  for 
regarding  them  as  belonging  to  different  species.  The  genus  inhabits 
fresh  water,  preferring  gently-running  streams,  in  which  it  is  sometimes 
very  abundant. — The  genus  Fragillaria  is  nearly  allied  to  Diatoma,  the 
difference  between  them  consisting  chiefly  in  the  mode  of  adhesion  of 
the  f rustules,  which  in  Fragillaria  form  long  straight  filaments  with  paral- 
lel sides:  the  filaments,  however,  as  the  name  of  the  genus  implies,  very 
readily  break-up  into  their  component  frustules,  often  separating  at  the 
slightest  touch.  Its  various  species  are  very  common  in  pools  and 
ditches. — This  family  is  connected  with  the  next  by  the  genus  Nitzschia, 
which  is  a  somewhat  aberrant  form  distinguished  by  the  presence  of  a 
prominent  keel  on  each  valve,  dividing  it  into  two  portions  which  are 
usually  unequal,  while  the  entire  valve  is  sometimes  curved,  in  N.  sig- 
moidea,  which  is  sometimes  used  as  a  Test-object,  but  is  not  suitable  for 
that  purpose  on  account  of  the  extreme  variability  of  its  striation. — 
Nearly  allied  to  this  is  the  genus  Bacillaria,  so  named  from  the  elongated 
staff-like  form  of  its  frustules;  its  valves  have  a  longitudinal  punctated 
keel,  and  their  transverse  striae  ^ 


Fro.  IlHL 


are  interrupted  in  the  median 
line.  The  principal  species  of 
this  genus  is  the  B,  paradoxa, 
whose  remarkable  movement 
has  been  already  described  (§ 
282).  Owing  to  this  displace- 
ment of  the  frustules,  its  fila- 
ments seldom  present  them- 
selves with  straight  parallel 
sides,  but  nearly  always  in 
forms  more  or  less  oblique, 
such  as  those  represented  in 
Fig.  171.  This  curious  object 
is  an  inhabitant  of  salt  or  of 
brackish  water.  Many  of  the 
species  formerly  ranked  under 
this  genus  are  now  referred 
to  the  genus  Diatoma.  The 

Genera  Nitzschia  and  Bacillaria  are  now  associated  by  Mr.  Ealfs,*  with 
some  other  genera  which  agree  with  them  in  the  bacillar  or  staff-like  form 
of  the  frustules  and  in  the  presence  of  a  longitudinal  keel,  in  the  Sub-fam- 
ily Nitzschiece,  which  ranks  as  a  section  of  the  Sttrirellece, — Another  Sub- 
family, Sijnedrece,  consists  of  the  genus  Synedra  and  its  allies,  in  which 
the  bacillar  form  is  retained  (Fig.  192,  I),  but  the  keel  is  wanting,  and 
the  valves  are  but  little  broader  than  the  front  of  the  frustule. 

285.  In  the  SurirellecB  proper,  the  frustules  are  no  longer  bacillar, 
and  the  breadth  of  the  valves  is  usually  (though  not' always)  greater  than 
the  front  view.  The  distinctive  character  of  the  genus  Surirella,  in  ad- 
dition to  the  presence  of  the  supposed  ^canaliculi'  (§  275),  is  derived 
from  the  longitudinal  line  down  the  centre  of  each  valve  (a),  and  the 

^  See  Pritchard's  Infusoria,"  4th  Ed.  p.  940.  The  genus  Nitzschia  was  in  the 
first  instance  placed  by  Mr.  Ralfs  in  the  family  Fragillariecey  and  the  genus 
Bacillaria  in  the  family  Surirellece, 


Surirella  constricta  :—a,  side  view;  b,  front  view; 
binary  subdivision. 


288 


THE  MICROSCOPE  AND  ITS  REVELATIONS. 


prolongation  of  the  margins  into  ^alae.'  Numerous  species  are  known, 
which  are  mostly  of  a  somewhat  ovate  form,  some  being  broader  and 
others  narrower  than  S.  constricta;  the  greater  part  of  them  are  inhab- 
itants of  fresh  or  brackish  water,  though  some  few  are  marine;  and 
several  occur  in  those  Infusorial  earths  which  seem  to  have  been  deposited 
at  the  bottoms  of  lakes,  such  as  that  of  the  Mourne  mountains  in  Ireland 


and  of  their  being  here  really  costm  or  internally  projecting  ribs,  no  rea- 
sonable doubt  can  remain  after  examination  of  them  under  the  Binocu- 
lar microscope,  especially  with  the  /  blackground  ^  illumination.  The 
form  of  the  valves  in  most  of  the  species  is  circular  or  nearly  so;  some  are 
nearly  flat,  whilst  in  others  the  twist  is  greater  than  in  the  species  here 
represented.  Some  of  the  species  are  marine,  whilst  others  occur  in  fresh 
Avater;  a  very  beautiful  form,  the  C,  clypeus,  exists  in  such  abundance  in 
the  Infusorial  stratum  discovered  by  Prof.  Ehrenberg  at  Soos  near  Ezer 
in  Bohemia,  that  the  earth  seems  almost  entirely  composed  of  it. 

287.  The  next  Family,  Striatellem,  forms  a  very  distinct  group,  differ- 
entiated from  every  other  by  having  longitudinal  costse  on  the  connecting 
portions  of  the  frustules;  these  costae  being  formed  by  the  inward  pro- 
jection of  annular  siliceous  plates  (which  do  not,  however,  reach  to  the 
centre),  so  as  to  form  septa  dividing  the  cavity  of  the  cell  into  imper- 
fectly-separated chambers.  In  some  instances  these  annular  septa  are 
only  formed  during  the  production  of  the  valves  in  the  act  of  self-divi- 
sion, and  on  each  repetition  of  such  production  being  thus  always  defi- 
nite in  number,  whilst  in  other  cases  the  formation  of  the  septa  is  con- 
tinued after  the  production  of  the  valves,  and  is  repeated  an  uncertain 
number  of  times  before  the  recurrence  of  a  new  valve-production,  so 
that  the  annuli  are  indefinite  in  number.  In  the  curious  Grammato- 
])liora  serpentina  (Fig.  174)  the  septa  have  several  undulations  and  in- 
curved ends,  so  as  to  form  serpentine  curves,  the  number  of  which  seems 
to  vary  with  the  length  of  the  frustule.  The  lateral  surfaces  of  the 
valves  in  Grammatophora  are  very  finely  striated;  and  some  species,  as 
G,  suMilissinia  and  G.  marina  are  used  as  Test-object  (§  161).  The 
frustules  in  most  of  the  genera  of  this  family  separate  into  zigzag  chains, 
as  in  Diatoma;  but  in  a  few  instances  they  cohere  into  a  filament,  and 
still  more  rarely  are  furnished  with  a  stipes. — The  small  Family  Terpsi- 
noece  is  separated  by  Mr.  Ralfs  from  the  Striatellese,  with  which  it  is 
nearly  allied  in  general  characters,  because  its  septa  (which  in  the  latter 
are  nearly  longitudinal  and  divide  the  central  portions  into  chambers) 
are  transverse  and  are  confined  to  the  lateral  portions  of  the  frustules, 


Campylodiscus  costatus: — a,  front  view;  b,  side  view 


(Fig.  192,  b,  c,  k)—lTi 
the  genus  Campylodiscus 
(Fig.  176)  the  valves  are 
so  greatly  increased  in 
breadth  as  to  present  al- 
most the  form  of  disks 
(a),  and  at  the  same  time 
have  more  or  less  of  a 
peculiar  twist  or  saddle- 
shaped  curvature  (b).  It 
is  in  this  genus  that  the 
supposed  ^canaliculi'  are 
most  developed,  and  it  is 
consequently  here  that 
they  may  be  best  studied; 


MICROSCOPIC  FORMS  OF   VEGETABLE  LIFE. 


289 


which  appear  in  the  front  view  as  in  BiddidpUiece  (§  292).  The  typical 
form  of  this  family  is  the  Terpsinbe  micsica,  so  named  from  the  resem- 
blance which  the  markings  of  its  costae  bear  to  musical  notes. 

288.  We  next  come  to  two  Families  in  which  the  lateral  surfaces  of 
the  frustules  are  cirmilar  ;  so  that,  according  to  the  flatness  or  convexity 
of  the  valves  and  the  breadth  of  the  intervening  hooped  band,  the  frus- 
tules may  have  the  form  either  of  thin  disks,  short  cylinders,  bi-convex 
lenses,  oblate  spheroids,  or  even  of  spheres.  Looking  at  the  structure 
of  the  individual  frustules,  the  line  of  demarcation  between  these  two 
families,  Melosirece  and  CoscinodiscecBy  is  by  no  means  distinct;  the  prin- 
cipal difference  between  them  being  that  the  valves  of  the  latter  are  com- 
monly cellulated,  whilst  those  of  the  former  are  smooth.  Another 
important  difference,  however  lies  in  this,  that  the  frustules  of  the  Cos- 
cinodiscece  are  always  free,  whilst  those  of  the  Melosirece  remain  cohe- 
rent into  filaments,  which  often  so  strongly  resemble  those  of  the  simple 
Co7ifervacece  as  to  be  readily  distinguishable  only  by  the  effect  of  heat.  Of 
these  last  the  most  important  Genus  is  Melosira  (Figs.  177,  178).  Some 
of  its  species  are  marine,  others  fresh- water;  one  of  the  latter,  the  M. 
ochracea,  seems  to  grow  best  in  boggy  pools  containing  a  ferruginous 
impregnation;  and  it  is  stated  by  Prof.  Ehrenberg  to  take  up  from  the 
water,  and  to  incorporate  with  its  own  substance,  a  considerable  quan- 
tity of  iron.  The  filaments  of  Melosira  very  commonly  fall  apart  at  the 
slightest  touch:  and  in  the  Infusorial  earths,  in  which  some  species 
abound,  the  frustules  are  always  found  detached  (Fig.  192,  a  a,  d  d). 

The  meaning  of  the  remarkable 


"FiCL  177 


difference  in  the  sizes  and  forms  of 
the  frustules  of  the  same  filaments 
(Figs.  177,  178)  has  not  yet  been 
fully  ascertained  (§  281).  The 
sides  of  the  valves  are  often  mark- 
ed with  radiating  striae  (Figs.  192, 
d  d);  and  in  some  species  they 
have  toothed  or  serrated  margins, 
by  which  the  frustules  lock-to- 
gether. To  this  family  belongs 
the  genus  Hyalodiscus,  of  which 
H.  suhtilis  was  first  brought  into 
notice  by  the  late  Prof.  Bailey 
as  a  Test-object,  its  disk  being 
marked,  like  the  engine-turned 
back  of  a  watch,  with  lines  of  ex- 
ceeding delicacy,  only  visible  by 
the  highest  magnifying  powers 
and  the  most  careful  illumina- 
tion. 

289.  The  family  CoscinodiscecB 
includes  a  large  proportion  of  the 
most  beautiful  of  those  discoidal  Diatoms,  of  which  the  valves  do  not 
present  any  considerable  convexity,  and  are  connected  by  a  narrow  zone. 
The  genus  CoscinodiscuSy  which  is  easily  distinguished  from  most  of  the 
genera  of  this  family  by  not  having  its  disk  divided  into  compartments, 
is  of  great  interest  from  the  vast  abundance  of  its  valves  in  certain  fos- 
sil deposits  (Fig.  191,  a,  a,  a),  especially  the  Infusorial  earth  of  Eich- 
mond  in  Virginia,  of  Bermuda,  and  of  Oran,  as  also  in  Guano.  Each 
19 


Melosira  suhflexilis. 


Melosira  varians. 


290 


THE  MICROSCOPE  AND  ITS  REVELATIONS. 


frustule  is  of  discoidal  shape,  being  composed  of  two  delicately  undulat- 
ing valves,  united  by  a  hoop;  so  that,  if  the  frustules  remain  in  adhe- 
sion, they  would  form  a  filament  resembling  that  of  Melosira  (Fig.  178). 
The  regularity  of  the  hexagonal  areolation  shown  by  its  valves  renders 
them  beautiful  microscopic  objects;  in  some  species  the  areolae  are  small- 
est near  the  centre,  and  gradually  increase  in  size  towards  the  margin ; 
in  others  a  few  of  the  central  areolae  are  the  largest,  and  the  rest  are  of 
nearly  uniform  size;  while  in  others,  again,  there  are  radiating  lines 
formed  by  areolae  of  a  size  different  from  the  rest.  Most  of  the  species 
are  either  marine,  or  are  inhabitants  of  brackish  water;  when  living  they 
are  most  commonly  found  adherent  to  Sea-weeds  or  Zoophytes;  but  when 
dead,  the  valves  fall  as  a  sediment  to  the  bottom  of  the  water.  In  both 
these  conditions,  they  were  found  by  Prof.  J.  Quekett  in  connection 
with  Zoophytes  which  had  been  brought  home  from  Melville  Island  by 
Sir  E.  Parry;  and  the  species  seemed  to  be  identical  with  those  of  the 
Kichmond  earth. — The  investigations  of  Mr.  J.  Stephenson,^  on 
Ooscmodiscus  oculus  Iridis  show  that  the  peculiar  eye-like  appear- 
ance in  the  centre  of  each  of  its  hexagonal  areola  arises  from  the  inter- 
mingling of  the  markings  of  two  distinct  layers,  differing  considerably 
in  structure;  the  markings  of  the  lower  layer  being  partially  seen  through 


Structure  of  siliceous  valve  of  Coscinodiscus  oculus  Iridis:—!.  Hexagonal  areola  of  inner  or 
*  eye-spot '  layer;  2.  Areola  of  outer  layer. 

those  of  the  upper.  By  fracturing  these  Diatoms,  Mr.  Stephenson  has 
succeeded  in  separating  portions  of  the  two  layers,  so  that  each  could 
be  examined  singly.  He  has  also  mounted  them  in  bisulphide  of  car- 
bon, the  refractive  power  of  which  is  very  high;  and  also  in  a  solution 
of  phosphorus  in  bisulphide  of  carbon,  which  has  a  still  higher  refrac- 
tive index.  If  we  suppose  a  Diatom  to  be  marked  with  convex  depres- 
sionSy  they  would  act  as  concave  lenses  in  air,  which  is  less  refractive 
than  their  own  silex;  but  when  such  lenses  are  immersed  in  bisulphide 
of  carbon,  or  in  the  phosphorus  solution,  they  would  be  converted  into 
convex  lenses  of  the  more  refractive  substance,  and  have  their  action  in 
air  reversed.  Analogous  but  opposite  changes  must  take  place,  when 
convex  Diatom-lenses  are  viewed  first  in  air,  and  then  in  the  more  re- 
fractive media.  Applying  these  and  other  tests  to  Coscinodiscus  oculus 
Iridis y  Mr.  Stephenson  considers  both  layers  to  be  composed  of  hexagons, 
represented  in  Fig.  179,  from  drawings  by  Mr.  Stewart.  The  upper 
layer  is  much  stronger  and  thicker  than  the  lower  one;  and  the  frame- 
work of  its  hexagons  more  readily  exhibits  its  beaded  appearance.  The 


^    Monthly  Microscopical  Journal,"  Vol.  x.  (1873),  p.  1. 


MICROSCOPIC  FORMS  OF  VEGETABLE  LIFE. 


291 


lower  layer  is  nearly  transparent,  and  little  conspicuous  when  seen  in  bi- 
sulphide of  carbon,  except,  as  shown  in  the  figure,  when  the  framework 
of  the  hexagons,  and  the  rings  in  the  midst  of  them,  appear  thickened 
and  more  refractive.  In  both  layers  the  balance  of  observations  tends 
to  the  belief  that  the  hexagons  have  no  floors,  and  are  in  fact  perforated 
by  foramina  like  those  of  minute  Polycystina.  The  cells  formed  by  the 
hexagons  of  the  upper  layer  are  of  considerable  depth;  those  of  the 
lower  layer  are  shallower.  In  both  layers  fractured  edges  show  the  hex- 
agon-frames to  be  the  strongest  parts;  and  in  neither  has  Mr.  Stephen- 
son been  able  to  detect  any  broken  remnants  of  floors,  which  might  be 
expected  to  be  visible  with  high  powers  if  they  existed  at  all. — If  further 
observations  should  confirm  Mr.  Stephenson's  belief  that  Coscinodisci 
are  perforated  by  numerous  foramina,  a  similar  structure  will  be  sough t- 
for  in  other  Diatoms,  and  the  views  of  naturalists  as  to  the  character  of 
the  group  may  be  materially  modified.  At  present  the  chief  difference 
in  minute  structure  that  has  been  recognized,  may  be  seen  by  comparing 
the  apparently  simple  beading  of  Pleiirosigma  with  the  hexagonal  for- 
mations in  CoscinodisctiSy  etc. ;  but  a  far  more  important  divergence  will 
have  to  be  considered,  if  some  Diatom- valves  have  a  multiplicity  of  fora- 
mina, and  others  either  none,  or  only  a  few  at  certain  spots.  It  is  very  de- 
sirable that  living  forms  of  Coscinodisci  should  be  carefully  examined; 
since,  if  they  really  have  foramina,  some  minute  organs  may  be  protruded 
through  them. 

290.  The  genus  Actinocyclus  ^  closely  resembles  the  preceding  in 
form,  but  differs  in  the  markings  of  its  valvular  disks,  which  are 
minutely  and  densely  punctated  or  cellulated,  and  are  divided  radially 
by  single  or  double  dotted  lines,  which,  however,  are  not  continuous  but 
interrupted  (Plate  i..  Fig.  1).  The  disks  are  generally  iridescent;  and, 
when  mounted  in  balsam,  they  present  various  shades  of  brown,  green, 
blue,  purple,  and  red:  blue  or  purple,  however,  being  the  most  frequent. 
An  immense  number  of  species  have  been  erected  by  Prof.  Ehrenberg  on 
minute  differences  presented  by  the  rays  as  to  number  and  distribution; 
but  since  scarcely  two  specimens  can  be  found  in  which  there  is  a  perfect 
identity  as  to  these  particulars,  it  is  evident  that  such  minute  differences 
between  organisms  otherwise  similar  are  not  of  sufficient  account  to  serve 
for  the  separation  of  species.  This  form  is  very  common  in  guano  from 
Ichaboe. — Allied  to  the  preceding  are  the  two  genera  Asterolampra  and 
Aster omphalus,  both  of  which  have  circular  disks  of  which  the  marginal 
portion  is  minutely  areolated,  whilst  the  central  area  is  smooth  and  per- 
fectly hyaline  in  appearance,  but  is  divided  by  lines  into  radial  comparii- 
ments  which  extend  from  the  central  umbilicus  towards  the  periphery. 
The  difference  between  them  simply  consists  in  this;  that  in  Asterolampra 
all  the  compartments  are  similar  and  equidistant,  and  the  rays  equal 
(Plate  I.,  Fig.  2);  whilst  in  Asteromplialus  two  of  the  compartments  are 
closer  together  than  the  rest,  and  the  inclosed  hyaline  ray  (which  is  dis- 
tinguished as  the  median  or  basal  ray)  differs  in  form  from  the  others, 
and  is  sometimes  specially  continuous  with  the  umbilicus  (Plate  i..  Fig. 
4).  The  eccentricity  thus  produced  in  the  other  rays  has  been  made  the 
basis  of  another  generic  designation,  Spatangidiim ;  but  it  may  be 


^  The  Author  concurs  with  Mr.  Ralf s  in  thinking  it  preferable  to  limit  the 
genus  Actinocyclus  to  the  forms  originally  included  in  it  by  Ehrenberg,  and  to 
restore  the  genus  Actinoptychus  of  Ehrenberg,  which  had  been  improperly  united 
with  Actinocyclus  by  Profs.  Kiitzing  and  W.  Smith. 


292 


THE  MICROSCOPE  AND  ITS  REVELATIONS. 


doubted  whether  this  is  founded  on  a  valid  distinction.'  These  beautiful 
disks  are  for  the  most  part  obtainable  from  guano,  and  from  soundings 
in  tropical  and  antarctic  seas. — From  these  we  pass  on  to  the  genus  Acti- 
noptychus  (Fig.  180),  of  which  also  the  frustules  are  discoidal  in  form, 

but  of  which  each  valve,  instead  of  be- 
ing  flat,  has  an  undulating  surface,  as  is 
^^"^  seen  in  front  view  (b);  giving  to  the 

side  view  (a)  the  appearance  of  being 
marked  by  radiating  bands.  Owing  to 
this  peculiarity  of  shape,  the  whole  sur- 
face cannot  be  brought  into  focus  at 
once  except  with  a  low  power;  and  the 
difference  of  aspect  which  the  different 
radial  divisions  present  in  Fig.  180,  is 
simply  due  to  the  fact  that  one  set  is 

Actinoptychusun^^at^^^^  OUt  of  foCUS  whilst  the  othcr  is  in  it, 

since  the  appearances  are  reversed  by 
merely  altering  the  focal  adjustment.  The  numler  of  radial  divisions 
has  been  considered  a  character  of  sufficient  importance  to  serve  for  the 
distinction  of  species;  but  this  is  probably  subject  to  variation;  since  we 
not  unfrequently  meet  with  disks,  of  which  one  has  (say)  8  and  another 
10  such  divisions,  but  which  are  precisely  alike  in  every  other  particular. 
The  valves  of  this  genus  also  are  very  abundant  in  the  Infusorial  earth  of 
Eichmond,  Bermuda,  and  Oran  (Fig.  191,  Z>,  5,  Z>);  and  many  of  the 
same  species  have  been  found  recently  in  guano,  and  in  the  seas  of  vari- 
ous parts  of  the  world.  The  frustules  in  their  living  state  appear  to  be 
generally  attached  to  Seaweeds  or  Zoophytes. 

29h  The  Bermuda  earth  also  contains  the  very  beautiful  form  (Plate 
I.,  fig.  3),  which,  though  scarcely  separable  from  Actinoptychus  except 
by  its  marginal  spines,  has  received  from  Prof.  Ehrenberg  the  distinctive 
appellation  of  Heliopelta  (sun-shield).  The  object  is  represented  as  seen 
on  its  internal  aspect  by  the  Parabolic  Illuminator  (§  105),  which  brings 
into  view  certain  features  that  can  scarcely  be  seen  by  ordinary  trans- 
mitted light.  Five  of  the  radial  divisions  are  seen  to  be  marked  out  into 
circular  areolae;  but  in  the  five  which  alternate  with  them,  a  minute  ' 
beaded  structure  is  observable.  This  may  be  shown  by  careful  adjust- 
ment of  the  focus  to  exist  over  the  whole  interior  of  the  valve,  even  on 
the  divisions  in  which  the  circular  areolation  is  here  displayed;  and  it 
hence  appears  that  this  marking  belongs  to  the  internal  layer  ^  (§  289), 
and  that  the  circular  areolation  exists  in  the  outer  layer  of  the  silicified 
lorica.  In  the  alternating  divisions  whose  surface  is  here  displayed,  the 
areolation  of  the  outer  layer,  when  brought  into  view  by  focussing  down 
to  it,  is  seen  to  be  formed  by  equilateral  triangles;  it  is  not,  however, 
nearly  so  well  marked  as  the  circular  areolation  of  the  first-mentioned 
divisions.  The  dark  spots  seen  at  the  ends  of  the  rays,  like  the  dark 
centre,  appear  to  be  solid  tubercles  of  silex  not  traversed  by  markings, 

^  See  Greville  in  Quart.  Journ.  of  Microsc.  Science,"  Vol.  vii.  (1859),  p.  158, 
and  in  Transact,  of  Microsc.  Soc,"  Vol.  viii.,  N.S.  (1860),  p.  102,  and  Vol.  x. 
(1862),  p.  41;  also  Wallich  in  the  same  Transactions,  Vol.  viii.  (1860),  p.  44. 

2  It  is  stated  by  Mr.  Stodder  (''Quart.  Journ.  of  Microsc.  Science,"  Vol.  iii., 
N.S.,  1863,  p.  215),  that  not  only  has  he  seen,  in  broken  specimens,  the  inner 
granulated  plate  projecting  beyond  the  outer,  but  that  he  has  found  the  inner 
plate  altogether  separated  from  the  outer.  The  Author  is  indebted  to  this 
gentleman  for  pointing  out  that  his  Figure  represents  the  inner  surface  of  the 
valve. 


MICROSCOPIC  FOKMS  OF  VEGETABLE  LIFE. 


293 


as  in  many  other  Diatoms;  most  assuredly  they  are  not  orifices,  as  sup- 
posed by  Prof.  Ehrenberg.  Of  this  type,  again,  specimens  are  found 
presenting  6,  8,  10,  or  12  radial  divisions,  but  in  other  respects  exactly 
similar;  on  the  other  hand,  two  specimens  agreeing  in  their  number  of 
divisions  may  exhibit  minute  differences  of  other  kinds;  in  fact,  it  is 
rare  to  find  two  that  are  precisely  alike.  It  seems  probable,  then,  that 
we  must  allow  a  considerable  latitude  of  variation  in  these  forms,  before 
attempting  to  separate  any  of  them  as  distinct  species. — Another  very 
beautiful  discoidal  Diatom,  which  occurs  in  Guano,  and  is  also  found 
attached  to  Sea-weeds  from  different  parts  of  the  world  (especially  to  a 
species  employed  by  the  Japanese  in  making  soup),  is  the  Arac/moidis- 
cus  (Plate  XI.),  so  named  from  the  resemblance  which  the  beautiful 
markings  on  its  disk  cause  it  to  bear  to  a  Spider's  web.  According  to 
Mr.  Shadbolt,^  who  first  carefully  examined  its  structure,  each  valve  con- 
sists of  two  layers;  the  outer  one,  a  thin  flexible  horny  membrane,  inde 
structible  by  boiling  in  nitric  acid;  the  inner  one,  siliceous.  It  is  the 
former  which  has  upon  it  the  peculiar  spider's  web-like  markings:  whilst 
it  is  the  latter  that  forms  the  supporting  frame- work,  which  bears  a  very 
strong  resemblance  to  that  of  a  circular  Gothic  window.  The  two  can 
occasionally  be  separated  entire,  by  first  boiling  the  disks  for  a  consider- 
able time  in  nitric  acid,  and  then  carefully  washing  them  in  distilled 
water.  Even  without  such  separation,  however,  the  distinctness  of  the 
two  layers  can  be  made  out  by  focussing  for  each  separately  under  a  l-4th  or 
1-5 th  inch  Objective;  or  by  looking  at  a  valve  as  an  opaque  object  (either 
by  the  Parabolic  Illuminator,  or  by  the  Lieberkiihn,  or  by  a  side  light) 
with  a  4-lOths  inch  Objective,  first  from  one  side,  and  then  from  the 
other.  — This  family  is  connected  with  the  succeeding  by  the  small 
group  of  EupodiscecB,  the  members  of  which  agree  with  the  Coscino- 
discece  in  the  general  character  of  their  discoid  frustules,  and  with  the 
BiddulpliiecB  in  having  tubercular  processes  on  their  lateral  surfaces.  In 
the  beautiful  Aulacodiscus  (Plate  i..  Fig.  5)  these  tubercles  are  situated 
near  the  margin,  and  are  connected  with  bands  radiating  from  the 
centre;  the  surface  also  is  frequently  inflated  in  a  manner  that  reminds 
us  of  Actinoptychus.  These  forms  are  for  the  most  part  obtained  from 
Guano. 

292.  The  members  of  the  next  Family  Biddidpliiem  differ  greatly  in 
their  general  form  from  the  preceding;  being  remarkable  for  the  great 
development  of  the  lateral  valves,  which,  instead  of  being  nearly  flat  or 
discoidal,  so  as  only  to  present  a  thin  edge  in  front  view,  are  so  convex 
or  inflated  as  always  to  enter  largely  into  the  front  view,  causing  the  cen- 
tral zone  to  appear  like  a  band  between  them.  This  band  is  very  narrow 
when  the  new  frustules  are  first  produced  by  self-division  (§  278);  but  it 
increases  gradually  in  breadth,  until  the  newfrustule  is  fully  formed  and 
is  itself  undergoing  the  same  dujDlicative  change.  In  Biddulplda  (Fig. 
167)  the  frustules  have  a  quadrilateral  form,  and  remain  coherent  by 
their  alternate  angles  (which  are  elongated  into  toothlike  projections),  so 
as  to  form  a  zigzag  chain.  They  are  marked  externally  by  ribbings 
which  seem  to  be  indicative  of  internal  costce  partially  subdividing  the 
cavity.  Nearly  allied  to  this  is  the  beautiful  genus  Isthmia  (Fig.  181), 
in  which  the  frustules  have  a  trapezoidal  form  owing  to  the  oblique  pro- 


^  "  Transact,  of  Microsc.  Society,"  First  Series,  Vol.  iii.,  p.  49. 
2  These  valves  afford  admirable  objects  for  showing  the  *  conversion  of  relief ' 
in  Nachet's  Stereo-Pseudoscopic  Microscope  (§  38). 


PLATE  XI. 


MICROSCOPIC  FORMS  OF  VEGETABLE  LIFE. 


295 


longation  of  the  valves;  the  lower  angle  of  each  frustule  is  coherent  to 
the  middle  of  the  next  one  beneath,  and  from  the  basal  frustule  proceeds 
a  stipes  by  which  the  filament  is  attached.  Like  the  preceding,  this 
genus  is  marine,  and  is  found  attached  to  the  Algce  of  our  own  shores. 
Theareolated  structure  of  its  surface  (Pig.  163)  is  very  conspicuous  both 
in  the  valves  and  in  the  connecting  '  hoop;'  and  this  hoop,  being  silici- 
fied,  not  only  connects  the  two  new  frustules 
(as  at  b,  Fig.  181),  until  they  have  separated 
from  each  other,  but,  after  such  separation,  re- 
mains for  a  time  round  one  of  the  frustules,  so  as 
to  give  it  a  truncated  appearance  {a,  c). 

293.  The  Family  AnguliferecBy  distinguished 
by  the  angular  form  of  its  valves  in  their  lateral 
aspect,  is  in  many  respects  closely  allied  to  the 
preceding;  but  in  the  comparative  flattening  of 
their  valves,  its  members  more  resemble  the 
CoBcinodiscecB  and  Eupodiscem.  Of  this  family 
we  have  a  characteristic  example  in  the  genus 
Triceratium;  of  which  striking  form  a  consider- 
able number  of  species  are  met  with  in  the  Ber- 
muda and  other  Infusorial  earths,  while  others 
are  inhabitants  of  the  existing  ocean  and  of  tidal 
rivers.  The  T.  favus  (Fig.  164),  which  is  one 
of  the  largest  and  most  regularly-marked  on  any 
of  these,  occurs  in  the  mud  of  the  Thames  and 
in  various  other  estuaries  on  our  own  coast;  it 
has  been  found,  also,  on  the  surface  of  large 
sea-shells  from  various  parts  of  the  world,  such 
as  those  of  Hippopus  and  HaliotiSy  before  they 
have  been  cleaned;  and  it  presents  itself  like- 
wise in  the  Infusorial  earth  of  Petersburg  (U. 
S.).  The  projections  at  the  angles  which  are  shown  in  that  species, 
are  prolonged  in  some  other  species  into  '  horns,'  whilst  in  others, 
again,  they  are  mere  tubercular  elevations.  Although  the  triangular 
form  of  the  frustule,  when  looked  at  sideways,  is  that  which  is  character- 
istic of  the  genus,  yet  in  some  of  the  species  there  seems  a  tendency  to 
produce  quadrangular  and  even  pentagonal  forms;  these  being  marked 
as  varieties  by  their  exact  correspondence  in  sculpture,  color,  etc.,  with 
the  normal  triangular  forms.  ^  This  departure  is  extremely  remarkable, 
since  it  breaks  down  what  seems  at  first  to  be  the  most  distinctive  char- 
acter of  the  genus;  and  its  occurrence  is  an  indication  of  the  degree  of 
latitude  which  we  ought  to  allow  in  other  cases.  It  is  difficult,  in  fact, 
to  distinguish  the  square  forms  of  Triceratium  from  those  included  in 
the  genus  Ampliitetras^  which  is  chiefly  characterized  by  the  cubiform 
shape  of  its  frustules.  In  the  latter,  the  frustules  cohere  at  their  angles 
so  as  to  form  zigzag  filaments,  whilst  in  the  former  the  frustules  are  usu- 
ally free,  though  they  have  occasionally  been  found  catenated. — Another 
group  that  seems  allied  to  the  Biddulphiece  is  the  curious  assemblage  of 
forms  brought  together  in  the  Family  ChcstocerecB,  some  of  the  filament- 
ous types  of  which  seem  also  allied  to  the  Melosirece.    The  peculiar  dis- 

*  See  Mr.  Brightwell's  excellent  Memoirs  *0n  the  genus  Triceratium,^  in 
Quart.  Journ.  of  Microsc.  Science,"  Vol.  i.  (1853),  p.  245,  Vol.  iv.  (1856),  p.  272, 
Vol.  vi.  (1858),  p.  153;  also  Wallich  in  the  same  Journal,  Vol.  iv.  (1858),  p.  242; 
and  Greville  in  ''Transact,  of  Microsc.  Soc,"  N.  S.,  Vol.  ix.  (1861),  pp.  43,  69. 


Isthmia  nervosa. 


296 


THE  MICROSCOPE  AND  ITS  REVELATIONS. 


tinction  of  this  group  consists  in  the  presence  of  tubular  ^  awns/  fre- 
quently proceeding  from  the  connecting  hoop,  sometimes  spinous  and 
serrated,  and  often  of  great  length  (Fig.  182);  by  the  interlacing  of  which 
the  frusfcules  are  united  into  filaments,  whose  continuity,  howeyer,  is 
easily  broken.  In  the  genus  Bacteriastrum  (Fig.  183)  there  are  some- 
times as  many  as  twelve  of  these  awns,  radiating  from  each  frustule  like 
the  spokes  of  a  wheel,  and  in  some  instances  regularly  bifurcating. 
With  this  group  is  associated  the  genus  Rhizosolenia,  of  which  several 
species  are  distinguished  by  the  extraordinary  length  of  the  frustule 
(which  may  be  from  6  to  20  times  its  breadth),  giving  it  the  aspect  of  a 
filament  (Fig.  184),  by  a  transverse  annulation  that  imparts  to  this  fila- 
ment a  jointed  appearance,  and  by  the  termination  of  the  frustule. at  each 
end  in  a  cone,  from  the  apex  of  which  a  straight  awn  proceeds.  It  is  not 
a  little  remarkable  that  the  greater  number  of  the  examples  of  this  curi- 
ous family  are  obtained  from  the  stomachs  of  Ascidians,  Salpae,  Holo- 
thuriae,  and  other  Marine  animals.  ^ 


Chcetoceros  Wighamii:—a,  front  view,  and  5,  side  view  of  frustule ;  c,  side  view  of  connecting 
hoop  and  awns ;  d,  entire  filament. 

294.  The  second  principal  division  (B)  of  the  Diatomaceae  consists,  it 
will  be  remembered,  of  those  in  which  the  frustules  have  a  median  longi- 
tudinal line  and  a  central  nodule.  In  the  first  of  the  Families  which  it 
includes,  that  of  CoccojieidecB,  the  central  nodule  is  obscure  or  altogether 
wanting  on  one  of  the  valves,  which  is  distinguished  as  the  inferior. 
This  family  consists  of  but  a  single  genus  Cocco7ieiSy  which  includes, 
however,  a  great  number  of  species,  some  or  other  of  them  occurring  in 
every -part  of  the  globe.  Their  form  is  usually  that  of  ellipsoidal  disks, 
with  surfaces  more  or  less  exactly  parallel,  plane,  or  slightly  curved;  and 
they  are  very  commonly  found  adherent  to  each  other.  The  frustules  in 
this  genus  are  frequently  invested  by  a  membranous  envelope  which  forms 
a  border  to  them;  but  this  seems  to  belong  to  the  immature  state,  subse- 
quently disappearing  more  or  less  completely. — Another  Family  in  which 
there  is  a  dissimilarity  in  the  two  lateral  surfaces,  is  that  of  Achnanthece; 
the  frustules  of  which  are  remarkable  for  the  bend  they  show  in  the  direc- 

^  See  Bri^htwell  in  "  Quart.  Journ.  of  Microsc.  Science,"  Vol.  iv.  (1856),  p.  105; 
Vol.  vi.  (1858),  p.  93;  Wallich  in  ''Trans,  of  Microsc.  Soc,"  N.S.,  Vol.  viii.  (1860), 
p.  48;  and  West  in  the  same,  p.  151. 


MICROSCOPIC  FORMS  OF  VEGETABLE  LIFE. 


297 


cion  of  their  length,  often  more  conspicuously  than  in  the  example  here 
represented.  This  family  contains  free,  adherent,  and  stipitate  forms; 
one  of  the  most  common  of  the  latter  being  the  Achnanthes  longipes 
(Fig.  185),  which  is  often  found  growing  on  Marine  Algae.  The  differ- 
ence between  the  markings  of  the  upper  and  lower  valves  is  here  distinctly 
seen;  for  while  both  are  traversed  by  striae,  which  are  resolvable  under  a 
sufficient  power  into  rows  of  dots,  as  well  as  by  a  longitudinal  line  which 
sometimes  has  a  nodule  at  each  end  (as  in  Navicula),  the  lower  valve  (a) 
has  also  a  transverse  line,  forming  a  sturos  or  cross,  which  is  wanting  in 


Rhizosolenia      Achnanthes  longipes:—a,  6,         Gomphonema  geminatum:  its  frus- 
imbricata.     c,  d,  e,  successive  frustules  in      tules  connected  by  a  dichotomous 
different  stages  of  self-divi-  stipes, 
sion. 

the  upper  valve  {e),  A  persistence  of  the  connecting  membrane,  so  as  to 
form  an  additional  connection  between  the  cells,  may  sometimes  be  ob- 
served in  this  genus;  thus  in  Fig.  185,  it  not  only  holds  together  the  two 
new  frustules  resulting  from  the  subdivision  of  the  lowest  cell,  a,  which 
are  not  yet  completely  separated  the  one  from  the  other,  but  it  may  be 
observed  to  invest  the  two  frustules,  b  and  c,  which  have  not  merely  sep- 
arated, but  are  themselves  beginning  to  undergo  binary  subdivision;  and 
it  may  also  be  perceived  to  invest  the  frustule  d,  from  which  the  frustule 
e,  being  the  terminal  one,  has  more  completely  freed  itself. — In  the 
Family  CymMlece,  on  the  other  hand,  both  valves  possess  the  longitu- 
dinal line  with  a  nodule  in  the  middle  of  its  length;  but  the  valves  have 
the  general  form  of  those  of  the  EimotiecB,  and  the  line  is  so  much 
nearer  one  margin  than  the  other,  that  the  nodule  is  sometimes  rather 


298 


THE  MICROSCOPE  AND  ITS  REVELATIONS. 


Gomphonema  geminatum,  more  highly 
magnified:  a,  side  view  of  frustule;  b,  front  aS 
view ;  c.  frustule  in  the  act  of  self -division.  i 

place. 


marginal  than  central,  as  we  see  in  Cocconerna  (Fig.  192). — The  Gomplio- 
nemecB,  like  the  Meridiece  and  LicmophorecBy  have  frustules  which  are 
cuneate  or  wedge-shaped  in  their  front  view  (Figs.  186,  187),  but  are  dis- 
tinguished from  those  forms  by  the  presence  of  the  longitudinal  line  and 
central  nodule.  Although  there  are  some  free  forms  in  this  family,  the 
greater  part  of  them,  included  in  the  genus  Gomphonema,  have  their 

frustules  either  affixed  at  their  bases 
or  attached  to  a  stipes.  This  stipes 
seems  to  be  formed  by  an  exudation 
from  the  frustule,  which  is  secreted 
only  during  the  process  of  self-divi- 
sion :  hence  when  this  process  has  been 
completed,  the  extension  of  the  single 
filament  below  the  frustule  ceases; 
but  when  it  recommences,  a  sort  of 
joint  or  articulation  is  formed,  from 
which  a  new  filament  begins  to  sprout 
for  each  of  the  half-frustules;  and 
when  these  separate,  they  carry  apart 
the  peduncles  which  support  them, 
far  as  their  divergence  can  take 
It  is  in  this  manner  that  the 
dichotomous  character  is  given  to  the  entire  stipes  (Fig.  186).  The  spe- 
cies of  Gomphonema  are,  with  scarcely  an  exception,  inhabitants  of  fresh 
water,  and  are  among  the  commonest  forms  of  Diatomaceae. 

295.  Lastly,  we  come  to  the  large  'Fdiinilj  Naviculem,  the  members  of 
which  are  distinguished  by  the  symmetry  of  their  frustules  as  well  in  the 
lateral  as  in  the  front  view,  and  by  the  presence  of  a  median  longitudinal 
line  and  central  nodule  in  both  valves.  In  the  genus  Navicula  and  its 
allies,  the  frustules  are  free  or  simply  adherent  to  each  other;  whilst  in 
another  large  section  they  are  included  within  a  gelatinous  envelope,  or 
are  inclosed  in  a  definite  tubular  or  gelatinous  frond.  Of  the  genus  Na- 
vicula  an  immense  number  of  species  have  been  described,  the  grounds 
of  separation  being  often  extremely  trivial.  Those  which  have  a  lateral 
sigmoid  curvature  (Fig.  165)  have  been  separated  by  Prof.  W.  Smith 
under  the  designation  Pleiirosigma,  which  is  now  generally  adopted;  but 
his  separation  of  another  set  of  species  under  the  name  Pinnularia 
(which  had  been  previously  applied  by  Ehrenberg  to  designate  the  stri- 
ated species),  on  the  ground  that  its  striae  (costse)  are  not  resolvable  into 
dots,  was  not  considered  valid  by  Mr.  Ralfs,  on  the  ground  that  in  many 
of  the  more  minute  species  it  is  impossible  to  distinguish  with  cer- 
tainty between  striae  and  costae.  Mr.  Slack  has  since  given  an  account 
of  the  resolution  of  the  so-called  costae  of  twelve  species  of  Pinnularice 
into  beaded  structures.  *  The  beautiful  genus  Stauroneis,  which  belongs 
to  the  same  group,  differs  from  all  the  preceding  forms  in  having  the 
central  nodule  of  each  valve  dilated  laterally  into  a  band  free  from  striae, 
which  forms  a  cross  with  the  longitudinal  band. 

296.  The  multitudinous  species  of  the  genus  Navicula  are  for  the  most 
part  inhabitants  of  fresh  water;  and  they  constitute  a  large  part  of  most 
of  the  so-called  ^  infusorial  earths  ^  which  were  deposited  at  the  bottoms  of 
lakes.    Among  the  most  remarkable  of  such  deposits  are  the  substances 


1    Monthly  Microscopical  Journal,"  Vol.  vi.  (1871),  p.  71. 


MICROSCOPIC  FOKMS  OF  VEGETABLE  LIFE 


299 


largely  used  in  the  arts  for  the  polishing  of  metals,  under  the  names  of 
Tripoli  and  rotten-stone;  these  consist  in  great  part  of  the  frustules  of 
Naviculae  and  Pinnulariae.  The  Polierscliiefer  or  ^polishing  slate'  of 
Bilin  in  Bohemia,  the  powder  of  which  is  largely  used  in  Germany  for 
the  same  purpose,  and  which  also  furnishes  the  fine  sand  used  for  the 
most  delicate  castings  in  iron,  occurs  in  a  series  of  beds  averaging  four- 
teen feet  in  thickness;  and  these  present  appearances  which  indicate  that 
they  have  been  at  some  time  exposed  to  a  high  temperature.  The  well- 
known  ^Turkey  stone,'  so  generally  employed  for  the  sharpening  of 
edge-tools,  seems  to  be  essentially  composed  of  a  similar  aggregation  of 
frustules  of  Naviculae,  etc.,  which  has  been  consolidated  by  heat. — The 
species  of  Pleurosigma,  on  the  other  hand,  are  for  the  most  part  either 
marine  or  are  inhabitants  of  brackish  water;  and  they  comparatively 
seldom  present  themselves  in  a  fossilized  state.  Of  StaiironeiSy  some 
species  inhabit  fresh  water,  while  others  are  marine;  and  the  former  pre- 
sent themselves  frequently  in  certain  ^  infusorial  earths.' 


Schizonema  GrevilUi ;— a,  natural  size;  b,  portion  magnified  five  diameters;  c,  filament  magni- 
fied 100  diameters ;  d,  single  f  rustule. 

297.  Of  the  members  of  the  Suo-family  SchizonemecBy  consisting  of 
those  Naviculem  in  which  the  frustules  are  united  by  a  gelatinous  enve- 
lope, some  are  remarkable  for  the  great  external  resemblance  they  bear  to 
acknowledged  Alg83.  This  is  especially  the  case  with  the  genus  Scliizo- 
nema;  in  which  the  gelatinous  envelope  forms  a  regular  tubular  frond, 
more  or  less  branched,  and  of  nearly  equal  diameter  throughout,  within 
which  the  frustules  lie  either  in  single  file  or  without  any  definite 
arrangement  (Fig.  188);  all  these  frustules  having  arisen  from  the  self- 
division  of  one  individual.  In  the  genus  Mastogloia,  which  is  specially 
distinguished  by  having  the  annulus  furnished  with  internal  costse 
projecting  into  the  cavity  of  the  frustule,  each  frustule  is  separately  sup- 


1 


300  THE  MICROSCOPE  AND  ITS  REVELATIONS. 

ported  on  a  gelatinous  cushion  (Fig.  189,  b),  which  may  itself  be  either 
borne  on  a  branching  stipes  (a),  or  may  be  aggregated  with  others  into 
an  indefinite  mass  (Fig.  190). — The  careful  study  of  these  composite 
forms  IS  a  matter  of  great  importance;  since  it  enables  us  to  bring  into 
comparison  with  each  other  great  numbers  of  frustules  which  have 
unquestionably  a  common  descent,  and  which  must  therefore  be  ac- 
counted as  of  the  same  species;  and  thus  to  obtain  an  idea  of  the  range 
of  variation  prevailing  in  this  group,  without  a  knowledge  of  which 
specific  definition  is  altogether  unsafe.  Of  the  very  strongly  marked 
varieties  which  may  occur  within  the  limits  of  a  single  species,  we  have 
an  example  in  the  valves  c,  D,  e,  f  (Fig.  189),  which  would  scarcely  have 
been  supposed  to  belong  to  the  same  specific  type,  did  they  not  occur 
upon  the  same  stipes.  The  careful  study  of  these  varieties  in  every 
instance  in  which  any  disposition  to  variation  shows  itself,  so  as  to 


Fig.  189. 


Fig.  189.  Mastogloia  Smithii  .—A,  entire  stipes ;  b,  f rustule  in  the  gelatinous  envelope ;  c— f,  dif- 
ferent forms  of  f rustule  as  seen  in  side  view;  g,  front  view;  h,  f rustule  undergoing  subdivision. 
Fig.  190.  Mastogloia  lanceolata, 

reduce  the  enormous  number  of  species  with  which  our  systematic 
treatises  are  loaded,  is  a  pursuit  of  far  greater  real  value  than  the  multi- 
^plication  of  species  by  the  detection  of  such  minute  differences  as  may  be 
presented  by  forms  discovered  in  newly  explored  localities;  such  differ- 
ences, as  already  pointed  out,  being,  probably,  in  a  large  proportion  of 
cases,  the  result  of  the  multiplication  of  some  one  form,  which,  under 
modifying  influences  that  we  do  not  yet  understand,  has  departed  from 
the  ordinary  type.  The  more  faithfully  and  comprehensively  this  study 
is  carried  out  in  any  department  of  Natural  History,  the  more  does  it 
prove  that  the  range  of  variation  is  far  more  extensive  than  had  been 
previously  imagined;  and  this  is  especially  likely  to  be  the  case  with  such 
humble  organisms  as  those  we  have  been  considering,  since  they  are 


MICROSCOPIC  FORMS  OF  VEGETABLE  LIFE. 


301 


obviously  more  influenced  than  those  of  higher  types  by  the  conditions 
under  which  they  are  developed,  whilst,  from  the  very  wide  Geographical 
range  through  which  the  same  forms  are  diffused,  they  are  subject  to 
very  great  diversities  of  such  conditions. 

298.  The  general  habits  of  this  most  interesting  group  cannot  be 
better  stated  than  in  the  words  of  Prof.  W.  Smith.  ^^The  Diatomaceae 
inhabit  the  sea  or  fresh  water;  but  the  species  peculiar  to  the  one  are 
never  found  in  a  living  state  in  any  other  locality;  though  there  are  some 
which  prefer  a  medium  of  a  mixed  nature,  and  are  only  to  be  met  with 
in  water  more  or  less  brackish.  The  latter  are  often  found  in  great 
abundance  and  variety  in  districts  occasionally  subject  to  marine  influ- 
ences, such  as  marshes,  in  the  neighborhood  of  the  sea,  or  the  deltas  of 
rivers,  where,  on  the  occurrence  of  high  tides,  the  freshness  of  the  water 
is  affected  by  percolation  from  the  adjoining  stream,  or  more  directly  by 
the  occasional  overflow  of  its  banks.  Other  favorite  habitats  of  the 
Diatomaceae  are  stones  of  mountain  streams  or  waterfalls,  and  the 
shallow  pools  left  by  the  retiring  tide  at  the  mouths  of  our  larger  rivers. 
They  are  not,  however,  confined  to  the  localities  I  have  mentioned — they 
are,  in  fact,  most  ubiquitous,  and  there  is  hardly  a  roadside  ditch,  water 
trough,  or  cistern,  which  will  not  reward  a  search,  and  furnish  specimens 
of  the  tribe. Such  is  their  abundance  in  some  rivers  and  estuaries, 
that  their  multiplication  is  affirmed  by  Prof.  Ehrenberg  to  have  exercised 
an  important  influence  in  blocking  up  harbors  and  diminishing  the 
depth  of  channels !  Of  their  extraordinary  abundance  in  certain  parts 
of  the  Ocean,  the  best  evidence  is  afforded  by  the  observations  of  Sir  J. 
D.  Hooker  upon  the  Diatomaceae  of  the  southern  seas;  for  within  the 
Antarctic  Circle  they  are  rendered  peculiarly  conspicuous  by  becoming 
inclosed  in  the  newly  formed  ice,  and  by  being  washed  up  in  myriads  by 
the  sea  on  to  the  ^pack^  and  ^  bergs,'  everywhere  staining  the  white  ice 
and  snow  of  a  pale  ochreous  brown.  A  deposit  of  mud,  chiefly  consisting 
of  the  siliceous  loricae  of  Diatomaceae,  not  less  than  400  miles  long  and 
120  miles  broad,  was  found  at  a  depth  of  between  200  and  400  feet,  on 
the  flanks  of  Victoria  Land  in  70°  South  latitude.  Of  the  thickness  of 
this  deposit  no  conjecture  could  be  formed;  but  that  it  must  be  contin- 
ually increasing  is  evident,  the  silex  of  which  it  is  in  a  great  measure 
composed  being  indestructible.  A  fact  of  peculiar  interest  in  connection 
with  this  deposit,  is  its  extension  over  the  submarine  flanks  of  Mount 
Erebus,  an  active  Volcano  of  12,400  feet  elevation;  since  a  communica- 
tion between  the  ocean  waters  and  the  bowels  of  a  volcano,  such  as  there 
are  other  reasons  for  believing  to  be  occasionally  formed,  would  account 
for  the  presence  of  Diatomaceae  in  volcanic  ashes  and  pumice,  which  was 
discovered  by  Prof.  Ehrenberg.  It  is  remarked  by  Sir  J.  D.  Hooker, 
that  the  universal  presence  of  this  invisible  vegetation  throughout  the 
South  Polar  Ocean  is  a  most  important  feature,  since  there  is  a  marked 
deficiency  in  this  region  of  higher  forms  of  vegetation;  and  were  it  not 
for  them,  there  would  neither  be  food  for  aquatic  Animals,  nor  (if  it 
were  possible  for  these  to  mamtain  themselves  by  preying  on  one  another) 
could  the  Ocean  waters  be  purified  of  the  carbonic  acid  which  animal 
respiration  and  decomposition  would  be  continually  imparting  to  them. 
It  is  interesting  to  observe  that  some  species  of  marine  Diatoms  are 
found  through  every  degree  of  latitude  between  Spitzbergen  and  Vic- 
toria Land,  whilst  others  seem  limited  to  particular  regions.  One  of  the 
most  singular  instances  of  the  preservation  of  Diatomaceous  forms,  is 
"their  existence  in  Guano;  into  which  they  must  have  passed  from  the 


302 


THE  MICROSCOPE  AND  ITS  REVELATIONS. 


intestinal  canals  of  the  Birds  of  whose  accumulated  excrement  that  sub- 
stances is  composed;  those  birds  having  received  them,  it  is  probable, 
from  Shell-fish,  to  which  these  minute  organisms  serve  as  ordinary  food 
(§  300). 

299.  The  indestructible  nature  of  the  silicified  casings  of  Diatomacece 
has  also  served  to  perpetuate  their  presence  in  numerous  localities  from 
which  their  living  forms  have  long  since  disappeared;  for  the  accumula- 
tion of  sediment  formed  by  their  successive  production  and  death,  even 
on  the  bed  of  the  Ocean,  or  on  the  bottoms  of  fresh-water  Lakes,  give^- 
rise  to  deposits  which  may  attain  considerable  thickness,  and  which,  by 
subsequent  changes  of  level,  may  come  to  form  part  of  the  dry  land. 
Thus  very  extensive  Siliceous  strata,  consisting  almost  entirely  of  marine 
Diatomacece,  are  found  to  alternate,  in  the  neighborhood  of  the  Mediter- 
ranean, with  Calcareous  strata  chiefly  formed  of  Foraminifera  (Chap. 
XII.);  the  whole  series  being  the  representative  of  the  Chalk  formation 
of  Northern  Europe,  in  which  the  silex  that  was  probably  deposited  at 
first  in  this  form  has  undergone  conversion  into  flmt^  by  agencies  here- 
after to  be  considered  (Chaps,  xii.,  xxi.).  Of  the  Diatomaceous  compo- 
sition of  these  strata  we  have  a  characteristic  example  in  Fig.  191,  which 
represents  the  Fossil  Diatomacese  of  Oran  in  Algeria.  The  so-called 
^  infusorial  earth  ^  of  Eichmond  in  Virginia,  and  that  of  Bermuda,  also 
Marine  deposits,  are  very  celebrated  among  Microscopists  for  the  number 
and  beauty  of  the  forms  they  have  yielded;  the  former  constitutes  a 
stratum  of  18  feet  in  thickness,  underlying  the  whole  city,  and  extending 
over  an  area  whose  limits  are  not  known.  Several  deposits  of  more  lim- 
ited extent,  and  apparently  of  fresh-water  origin,  have  been  found  in  our 
own  islands;  as  for  instance  at  Dolgelly  in  North  Wales,  at  South  Mourne 
in  Ireland  (Fig.  192),  and  in  the  island  of  Mull  in  Scotland.  Similar 
deposits  in  Sweden  and  Norway  are  known  under  the  name  of  ierg-mehl 
or  mountain-fiour;  and  in  times  of  scarcity  the  inhabitants  of  those 
countries  are  accustomed  to  mix  these  substances  with  their  dough  in 
making  bread.  This  has  been  supposed  merely  to  have  the  effect  of 
giving  increased  bulk  to  their  loaves,  so  as  to  render  the  really  nutritive 
portion  more  satisfying;  but  as  the  berg-mehl  has  been  found  to  lose 
from  a  quarter  to  a  third  of  its  weight  by  exposure  to  a  red-heat,  there 
seems  a  strong  probability  that  it  contains  Organic  matter  enough  to 
render  it  nutritious  in  itself.  When  thus  occurring  in  strata  of  a  fossil 
or  sub-fossil  character,  the  Diatomaceous  deposits  are  generally  distin- 
guishable as  white  or  cream-colored  powders  of  extreme  fineness. 

300.  For  collecting  fresh  Diatomacece,  those  general  methods  are  to 
be  had  recourse  to  which  have  been  already  described  (§  269).  Their 
living  masses,'^  says  Prof.  W.  Smith,  present  themselves  as  colored 
fringes  attached  to  larger  plants,  or  forming  a  covering  to  stones  or  rocks 
in  cushion-like  tufts — or  spread  over  their  surface  as  delicate  velvet — or 
depositing  themselves  as  a  filmy  stratum  on  the  mud,  or  intermixed  with 
the  scum  of  living  or  decayed  vegetation  floating  on  the  surface  of  the 
water.  Their  color  is  usually  a  yellowish-brown  of  a  greater  or  less  in- 
tensity, varying  from  a  light  chestnut,,  in  individual  specimens,  to  a  shade 
almost  approaching  black  in  the  aggregated  masses.  Their  presence  may 
often  be  detected  without  the  aid  of  a  microscope,  by  the  absence,  in  many 
species,  of  the  fibrous  tenacity  which  distinguishes  other  plants:  when 
removed  from  their  natural  position  they  become  distributed  through  the 
water,  and  are  held  in  suspension  by  it,  only  subsiding  after  some  little 
time  has  elapsed.     Notwithstanding  every  care,  the  collected  specimens 


MICROSCOPIC  FORMS  OF  VEGETABLE  LIFE. 


303 


are  liable  to  be  mixed  with  much  foreign  matter:  this  may  be  partly  got 
rid  of  by  repeated  washings  in  pure  water,  and  by  taking  advantage,  at 
the  same  time,  of  the  different  specific  gravities  of  the  Diatoms  and  of 
the  intermixed  substances,  to  secure  their  separation.  Sand,  being  the 
heaviest,  will  subside  first;  fine  particles  of  mud  on  the  other  hand,  will 
float  after  the  Diatoms  have  subsided.  The  tendency  of  living  Dia- 
toms to  make  their  way  towards  the  light,  will  afford  much  assistance  in 
procuring  the  free  forms  in  a  tolerably  clean  state;  for  if  the  gathering 
which  contains  them  be  left  undisturbed  for  a  sufficient  length  of  time  in 
a  shallow  vessel  exposed  to  the  sunlight,  they  may  be  skimmed  from  the 
surface.  Marine  forms  must  be  looked-for  upon  Sea- weeds,  and  in  the 
fine  mud  or  sand  of  soundings  or  dredgings;  they  are  frequently  found 
also,  in  considerable  numbers,  in  the  stomachs  of  Holothurise,  Ascidians, 


Fossil  Diatomaceoe,  etc.,  from  Gran:— a,  a,  a,  Coscinodiscus;  6,  6,  h,  Actinocyclus;  c,  Dictyochya 
fibula;  d,  Lithasteriscus  radiatus;  e,  Spongolithis  acicularis ;  /, /,  Grammatophora  parallela  (si(\e 
view) ;        Grammatophora  angulosa  (front  view). 

and  Salpae,  in  those  of  the  oyster,  scallop,  whelk,  and  other  testaceous 
MoUusks,  in  those  of  the  crab  and  lobster,  and  other  Crustacea,  and 
oven  in  those  of  the  sole,  turbot,  and  other  Flat-fish.  In  fact  the  Dia- 
tom-coUector  will  do  well  to  examine  the  digestive  cavity  of  any  small 
aquatic  animals  that  may  fall  in  his  way;  rare  and  beautiful  forms  having 
been  obtained  from  the  interior  of  Noctiluca  (Fig.  297).  The  separation 
of  the  Diatoms  from  the  other  contents  of  these  stomachs  must  be  accom- 
plished by  the  same  process  as  that  by  which  they  are  obtained  from 
Guano  or  the  calcareous  ^infusorial  earths';  of  this,  the  following  are 
the  most  essential  particulars: — The  guano  or  earth  is  first  to  be  washed 
several  times  in  pure  water,  which  should  be  well  stirred,  and  the  sedi- 
ment then  allowed  to  subside  for  some  hours  before  the  water  is  poured 
off,  since,  if  it  be  decanted  too  soon,  it  may  carry  the  lighter  forms  away 
with  it.  Some  kinds  of  earth  have  so  little  impurity  that  one  washing 
suffices;  but  in  any  case  it  is  to  be  continued  so  long  as  the  water  remains 


304: 


THE   MICROSCOPE  AND  ITS  REVELATIONS. 


colored.  The  deposit  is  then  to  be  treated,  in  a  flask  or  test-tube,  with 
Hydrochloric  (muriatic)  acid;  and  after  the  first  efEervescence  is  over,  a 
gentle  heat  may  be  applied.  As  soon  as  the  action  has  ceased,  and  time 
has  been  given  for  the  sediment  to  subside,  the  acid  should  be  poured  off, 
and  another  portion  added;  and  this  should  be  repeated  as  often  as  any 
effect  is  produced.  When  hydrochloric  acid  ceases  to  act,  strong  IsTiti'lc 
acid  should  be  substituted;  and  after  the  first  effervescence  is  over,  a 
continued  heat  of  about  200°  should  be  applied  for  some  hours.  When 
sufficient  time  has  been  given  for  subsidence,  the  acid  may  be  poured  off 
and  the  sediment  treated  with  another  portion;  and  this  is  to  be  repeated 
until  no  further  action  takes  place.  The  sediment  is  then  to  be  washed 
until  all  trace  of  the  acid  is  removed:  and,  if  there  have  been  no  admix- 
ture of  siliceous  sand  in  the  earth  or  guano,  this  sediment  will  consist 
almost  entirely  of  Diatomacece,  with  the  addition,  perhaps,  of  Sponge- 
spicules.    The  separation  of  siliceous  sand,  and  the  subdivision  of  the 

Eia  132 


Fossil  Dtatomaceoe,  etc.,  from  Mourne  Mountain,  Ireland:— a,  a,  a,  Gaillonella  (Melosira)  pro- 
cera,  and  G.  granulata;  d,  d,  d,  G.  biseriata  (side  view);  6,  6,  Surirella  plicata;  c,  S.  craticula;  fc, 
S.  caledonica;  e,  Gomphonema  gracile;/,  Coccouema  f usidium ;  g,  Tabellaria  vulgaris;  h,  Pinnu- 
laria  dactylus;  i,  P.  nobilis;  I,  Synedra  ulna. 

entire  aggregate  of  Diatoms  into  the  larger  and  the  finer  kinds,  may  be 
accomplished  by  stirring  the  sediment  in  a  tall  jar  of  water,  and  then, 
while  it  is  still  in  motion,  pouring  off  the  supernatant  fluid  as  soon  as  the 
coarser  particles  have  subsided;  this  fluid  should  be  set  aside,  and,  as 
soon  as  a  finer  sediment  has  subsided,  it  should  again  be  poured  off;  and 
this  process  may  be  repeated  three  or  four  times  at  increasing  intervals, 
until  no  further  sediment  subsides  after  the  lapse  of  half  an  hour.  The 
first  sediment  will  probably  contain  all  the  sandy  particles,  with,  perhaps, 
some  of  the  largest  Diatoms,  which  may  be  picked  out  from  among  them; 
and  the  subsequent  sediments  will  consist  almost  exclusively  of  Diatoms, 
the  sizes  of  which  will  be  so  graduated,  that  the  earliest  sediments  may 
be  examined  with  the  lower  powers,  the  next  with  medium  powers,  while 


MICROSCOPIC  FORMS  OF  VEGETABLE  LIFE. 


305 


the  latest  will  require  the  higher  powers — a  separation  which  is  attended 
with  great  convenience/  It  sometimes  happens  that  fossilized  Diatoms 
are  so  strongly  united  to  each  other  by  Siliceous  cement,  as  not  to  be 
separable  by  ordinary  methods;  in  this  case,  small  lumps  of  the  deposit 
should  be  boiled  for  a  short  time  in  a  weak  Alkaline  solution,  which  will 
act  upon  this  cement  more  readily  than  on  the  siliceous  frustules;  and  as 
soon  as  they  are  softened  so  as  to  crumble  to  mud,  this  must  be  immedi- 
ately Avashed  in  a  large  quantity  of  water,  and  then  treated  in  the  usual 
way.  If  a  verj;  weak  alkaline  solution  does  not  answer  the  purpose,  a 
stronger  one  may  then  be  tried.  This  method,  devised  by  Prof.  Bailey, 
has  been  practised  by  him  with  much  success  in  various  cases.  ^ 

301.  The  mode  of  mounting  spcimens  of  Diatomacem  will  depend 
upon  the  purpose  which  they  are  intended  to  serve.  If  they  can  be  ob- 
tained quite  fresh,  and  if  it  be  desired  that  they  should  exhibit,  as  closely 
as  possible  the  appearance  presented  by  the  living  plants,  they  should  be 
put-up  in  aqueous  media  (§  ^306)  within  Cement-cells  (§  211) f  but  if  they 
are  not  thus  mounted  within  a  short  time  after  they  have  been  gathered, 
about  a  tenth-part  of  alcohol  should  be  added  to  the  water.  If  it  be  de- 
sired to  exhibit  the  stipitate  forms  in  their  natural  parasitism  upon  other 
aquatic  plants,  the  entire  mass  may  be  mounted  in  Deane's  Medium  or 
in  Glycerine  jelly  (§  206  7^),  in  a  deeper  cell;  and  such  a  preparation  is 
a  very  beautiful  object  for  the  back-ground  illumination.  If,  on  the 
other  hand,  the  minute  structure  of  the  siliceous  envelope  is  the  feature 
to  be  brought  into  view,  the  fresh  Diatoms  must  be  boiled  in  nitric  or 
hydrochloric  acid,  which  must  then  be  poured  oti  (sufficient  time  being 
allowed  for  the  deposit  of  the  residue);  and  the  sediment,  after  being 
washed,  should  be  boiled  in  water  with  a  small  piece  of  soap,  whereby 
the  Diatoms  will  be  cleansed  from  the  flocculent  matter  which  they  often 
obstinately  retain.^  After  a  further  washing  in  pure  water,  they  are  to 
be  either  mounted  in  Balsam  in  the  ordinary  manner  (§  210),  or  to  be 
set-up  ^dry'  on  a  very  thin  slide  (§§  165,  1C9).  In  order  to  obtain  a 
satisfactory  view  of  their  markings,  Objectives  of  very  Avide  angular 
aperture  are  required,  and  all  the  refinements  which  have  recently  been 
introduced  into  tlie  methods  of  Illumination  need  to  be  put  in  practice. 
(Chaps.  III.,  lY.) — It  will  often  be  convenient  to  mount  certain  particular 
forms  of  DiatomacecB  separately  from  the  general  aggregate;  but  on 
account  of  their  minuteness,  they  cannot  be  selected  and  removed  by  the 
usual  means.  The  larger  forms,  which  may  be  readily  distinguished 
under  a  Simple  Microscope,  may  be  taken-up  by  a  camel-hair  pencil 
which  has  been  so  trimmed  as  to  leave  two  or  three  hairs  projecting  beyond 
the  rest.    But  the  smaller  can  only  be  dealt  with  by  a  single  fine  bristle 


1  A  somewhat  more  complicated  method  of  applying  the  same  principle  is  de- 
scribed by  Mr.  Okeden,  in  the  *•  Quart.  Journ.  of  Microsc.  Science,"  Vol.  iii. 
(1855),  p.  158.  The  Author  believes,  however,  that  the  method  above  described 
will  answer  every  purpose. 

2  For  other  methods  of  cleaning  and  preparing  Diatoms,  see  Quart.  Journ.  of 
Microsc.  Science,"  Vol.  vii.  (1859),  p.  167,  and  Vol.  i.,  N.S.  (1861),  p.  143;  and 

Trans,  of  Microsc.  Soc,"  Vol.  xi.,  N.S.  (1863),  p.  4.— A  little  book  entitled 
Practical  Directions  for  collecting,  preserving,  transporting,  preparing,  and 
mounting  Diatoms"  (New  York,  1877),  containing  Papers  by  Professors  A.  Mead 
Edwards,  Christopher  Johnson,  and  Hamilton  L.  Smith,  will  be  found  to  contain 
much  useful  information. 

'See  Prof.  H.  L.  Smith  in  Amer.  Journ.  of  Microscopy,"  Vol.  v.  (1880),  p. 
257.— It  is  important  that  the  soap  should  be  free  from  kaolin,  silica,or  any  other 
insoluble  matter. 

20 


306 


THE  MICROSCOPE  AND  ITS  REVELATIONS. 


or  stont  sable-hair,  Avhich  may  be  inserted  into  the  cleft-end  of  a  slender 
wooden-handle;  and  if  the  bristle  or  hair  should  be  split  at  its  extremity 
in  a  brush-like  manner,  it  will  be  particularly  useful.  (Such  split-hairs 
may  always  be  found  in  a  Shaving-brush  which  has  been  for  some  time 
in  use;  those  should  be  selected  which  have  their  split-portions  so  closely 
*  in  contact,  that  they  appear  single  until  touched  at  their  ends.)  When 
the  split  extremity  of  such  a  hair  touches  the  glass-slide,  its  parts  sep- 
arate from  each  other  to  an  amount  proportionate  to  the  pressure;  and, 
on  being  brought  up  to  the  object,  first  pushed  to  the  edge  of  the  fluid 
on  the  slide,  may  generally  be  made  to  seize  it.  A  very  experienced 
American  Diatomist,  Prof.  Hamilton  Smith,  strongly  recommends  a 
thread  of  glass  drawn-out  to  capillary  fineness  and  flexibility,  by  which 
(he  says)  the  most  delicate  Diatom  may  be  safely  taken  uj),  and  deposited 
upon  a  slide  damped  by  the  breadth. — For  the  selection  and  transference 
of  Diatoms  under  the  Compound  Microscope,  recourse  may  be  had  to 
some  of  the  forms  of  ^  mechanical  finger '  which  have  been  recently  de- 
vised by  American  Diatomists.^ 


^  For  a  description  of  those  of  Prof.  Hamilton  Smith  and  Dr.  Rezner,  see 
''Journ.of  Roy.  Microsc.  Soc,"  Vol.  ii.  (1879),  p.  951;  and  that  of  Mr.  Veeder, 
Vol.  iii.  (1880),  p.  700,  of  the  same  Journal. 


PEOTOPHYTIO   AND  OTHER  FUNGI. 


307 


CHAPTER  VII. 
PROTOPHYTIC  AND  OTHER  FUNGI.— LICHENS. 

302.  Ik  the  lowest  forms  of  the  group  of  Fungi,  we  return  to  tlie 
simplest  type  of  Vegetation — the  single  cell;  and  such  forms,  equally 
with  the  lowest  AlgcB,  rank  as  Protophytes.  Their  essential  difference 
from  the  Protophytic  Algae  seems  to  lie  in  their  incapacity  for  the  for- 
mation of  Chlorophyll  and  of  carbon-compounds,  under  the  influence  of 
Light,  out  of  the  simple  binary  compounds — Water,  Carbonic  Acid,  and 
Ammonia — supplied  by  the  Inorganic  world;  and  in  their  dependence 
(like  Animds,  §  220)  upon  those  more  complex  combinations  which  the 
Organic  world  alone  supplies.  There  seems,  however,  to  be  this  general 
difference  between  the  nutrition  of  Fungi  and  that  of  ordinary  Ani- 
mals: that  the  former  not  only  live,  but  thrive  best,  in  the  midst  of 
decomposing  or  decomposable  Organic  matter,  apparently  utilizing  the 
products  of  such  decomposition;  whilst  the  latter  directly  convert  into 
their  own  substance  the  nitrogenous  compounds  prepared  for  them  by 
Plants.  There  are,  however,  cases  in  which  this  distinction,  also,  seems 
to  fail;  and  in  which  it  is  impossible,  in  the  present  state  of  our  knowledge, 
to  draw  a  definite  line  of  division  between  Fungi  and  Protozoa  (§  322). 

303.  Among  the  Protophytic  Fungi,  there  are  none  of  which  the 
study  is  more  practically  important,  than  the  group  of  ScUizomycetes; 
consisting  of  a  series  of  very  minute  organisms,  known  as  Bacteria,  Vi- 
triojies,  etc.,  which  were  formerly  ranked  by  Ehrenberg  and  Dujardin 
among  Animalcules.  They  are  all  aquatic  in  their  habit,  and  are  in  that 
respect  allied  to  Algce ;  but  they  cannot  live  in  pure  water,  thriving  best 
in  liquids  that  contain  decomposing  or  decomposable  organic  matter; 
whilst  many  of  them  also  grow  and  reproduce  themselves  in  solutions  in 
which  ammonia-salts  of  the  vegetable  acids  (acetates,  tartrates,  or  citrates) 
are  combined  with  purely  inorganic  ash-salts,  so  that  they  may  be  ^culti- 
vated such  liquids  for  the  purposes  of  study.  ^  Thus  Scldzomy- 
cetes  resemble  ordinary  Plants  in  forming  JSTitrogenized  compounds  out 
of  ammonia,  which  Animals  cannot  do;  while  they  differ  from  green 
Plants  in  their  inability  to  form  Carbon-compounds  by  the  decomposi- 
tion of  carbonic  acid,  requiring  for  their  support  the  carbo-hydrates  or 
their  derivatives. — They  all  consist  of  minute  cells,  which  multiply  rap- 
idly by  subdivision;  and  most  of  them,  at  some  stage  of  their  existence, 
have  the  power  of  moving  more  or  less  quickly  through  the  liquid  they 
inhabit,  by  the  action  of  flagella.  Although  usually  colorless,  or  nearly 
so,  they  sometimes  form  reddish,  bluish,  or  brownish  coloring  matters; 


*  Cohn's  solution  is  composed  of  1  part  of  Potassium  Phosphate,  1  part  of 
Magnesium  Sulphate,  2  parts  of  Ammonium  Tartrate,  and  0.1  part  of  Calcium 
Chloride,  dissolved  in  200  parts  of  distilled  water.— See  also  p.  123  note. 


308 


THE  MICROSCOPE  AND  ITS  REVELATIONS. 


and  thus,  when  they  multiply  to  a  sufficient  extent,  make  their  presence 
apparent  to  the  unaided  eye,  either  as  colored  films  on  the  sides  of  the 
glass  jars  holding  the  solutions,  or  as  (in  the  cases  of  blood-colored  milk 
and  blue-green  pus)  imparting  their  color  to  the  whole  liquid.  Liquids 
in  which  any  of  these  ScMzomycetes  are  actively  developing  themselves, 
usually  bear  on  their  surface  a  gelatinous  scum,  which  is  termed  by  Prof. 
Cohn  (who  first  drew  attention  to  it)  the  Zooglcea.  This  scum,  when  ex- 
amined microscopically,  is  found  to  contain  Schizomycetes,  sometimes  of 
several  different  kinds,  and  in  different  stages  of  development,  often 
mingled  with  true  Infusoria^  matted  together  into  a  mass;  at  the  edges 
of  which  they  present  themselves  in  a  more  separated  condition,  and 
seem  escaping  to  disperse  themselves  freely  through  the  liquid  beneath. 
By  Prof.  Cohn,*  who  has  made  a  special  study  of  this  group,  it  is  con- 
sidered to  include  a  large  number  of  generic  and  specific  types,  whose 
distinctness  is  always  preserved;  but  other  observers,  who  have  devoted 
themselves  to  the  more  prolonged  and  complete  study  (by  ^  cultivation ') 
of  a  small  number  of  forms,  seem  to  have  made  it  clear  that  there  is — 
at  least  in  certain  types — a  wide  range  of  variation;  so  that  when  the 
entire  life-history  of  any  one  type  shall  be  completely  known,  a  number 
of  supposed  species  will  be  merged  in  it,  either  m  transitory  phases  of 
its  existence,  or  as  varieties  resulting  from  differences  in  the  media  in 
which  they  develop  themselves.^  There  are,  however,  five  well-marked 
types,  of  each  of  which  it  will  be  desirable  to  give  a  separate  account; 
namely — 1.  Micrococcus;  2.  Bacterium;  3.  Bacillus;  4.  Vibrio;  and 
5.  Spirillum. 

304.  The  Micrococci  are  darkish  or  colored  granules,  so  minute  as  not 
to  be  measurable  with  certainty,  and  destitute  of  any  power  of  move- 
ment; which  may  occur  either  solitarily,  or  forming  small  groups  or 
beaded  chains,  such  as  would  be  produced  by  cell-division;  but  which 
may  also  accumulate  in  irregular  aggregations.  The  Monas  prodigisa 
of  Ehrenberg,  which  is  sometimes  found  imparting  to  the  surface  of 
mouldy  bread  a  blood-red  tinge  (attributed  by  the  superstitious  to  a  mi- 
raculous exudation  of  blood),  is  regarded  by  Cohn  as  a  Micrococcus. 
There  is  considerable  doubt  whether  any  of  these  Micrococci  are  independ- 
ent organisms;  as  it  is  certain  that  some  of  them  are  nothing  else  than 
sporules  of  Bacteria  or  Bacilli  (Plate  xii.,  figs.  1-3).  But  as  some  of 
them  do  not,  under  cultivation,  develop  themselves  into  any  higher 
form,  continuing  to  multiply  as  isolated  cells  by  binary  subdivision,  they 
must  for  the  present  be  ranked  as  distinct.^ 

305.  Bacteria  are  minute  oblong  cells,  which  are  usually  seen  at- 
tached in  pairs  end  to  end  (Fig.  193,  a,  c),  but  not  unfrequently  pre- 
sent themselves  singly  (b,  d),  the  pairs  being  produced  by  the  self-divi- 
sion of  solitary  cells.  They  are  usually  seen  in  *  vacillating'  movement, 
produced  by  the  action  of  their  jlagella,  of  which,  in  their  paired  state, 
each  cell  bears  one  at  its  free  extremity,  whilst  the  solitary  cells  bear  a 
flagellum  at  each  extremity.  The  formation  of  the  second  flagellum 
seems  to  take  place  by  the  drawing-out  of  a  filament  of  protoplasm  be- 


1  Beitrage  zur  Biologie  der  Pflanzen,"  Band  i.,  Heft  ii.  (1872),  and  Heft  iii. 
(1875) ;  Band,  ii.,  Heft  ii.  (1870). 

2  See  especially  Prof.  E.  Ray  Lankester's  account  of  '  A  Peach-colored  ^acfc- 
rmm,' in  Quart.  Journ.  Microsc.  Science,"  Vol.  xiii.  (1873),  p.  408;  and  Mr.  J. 
C.  Ewart  *0n  the  Life-history  of  Bacillus  anthracis/  in  Vol.  xviii.  (1878),  p.  161,, 
of  the  same  Journal. 

3  See  Ewart  in    Proceedings  of  Royal  Society,"  June  20th,  1878. 


PROTOPHYTIC  AND  OTHER  FUNGI. 


309 


tween  two  cells  that  are  separating  from  each  other  (as  in  Pig.  196,  a, 
i),  the  rupture  of  which  gives  a  new  flagelluin  to  each.  Two  species  of 
this  type,  differing  considerably  in  size,  have  been  especially  studied. 
The  cells  of  Bacteriicm  termo  (Fig.  193,  a),  which  seem  to  be  the  'fer- 
ment'of  ordinary  putrefactive  change,  have  a  diameter  of  about  1-20- 
000th  of  an  inch,  and  are  somewhat  longer  than  they  are  broad.  Their 
flagella  are  so  minute  as  to  be  among  the  most  'difficult'  of  all  Micro- 
scopic objects  (p.  162);  their  diameter  being  estimated  by  Mr.  Dallinger 
at  no  more  than  l-200,000th  of  an  inch. '    Although  this  species  does  not 


0-  37 

A,  Bacterium  termo,  each  cell  furnished  with  a  single  flagellum:  Magnified  4  000  diameter*? 
?'n&»5'-  ^"^f^***^"*  lineola,  each  cell  when  separated  having  a  tlagellum  at  either  end.  Magnified 
OjOOU  diameters. 

ordinarily  multiply  in  any  other  way  than  by  transverse  subdivision,  yet, 
under  'cultivation'  at  a  temperature  of  86^  Fahr.,  its  cells  have  been 
seen^  to  elongate  themselves  into  motionless  rods,  resembling  those  of 
Bacilli  {Vl^iQXU,),  whose  endoplasm  breaks  up  into  separate  particles 
that  are  set  free  as  small,  bright,  almost  spherical  spores,  which  some- 
times congregate  so  as  to  form  a  zooglma-Glm.  These  germinate  into 
short,  slender  rods,  which  are  at  first  motionless,  but  soon  undergo  trans- 
verse fission,  and  then  acquire  flagella.'— The  B.  lineola,  which  is  the 


A,  Bacillus  subtilis ;  each  cell,  when  separated,  biflagellate.   Magnified  4,000  dia:neters.  b 
Bacillus  ulna,  each  cell'biflagellate.   Magnifiei  3,000  diameters. 

special  'ferment'  that  turns  milk  sour,  occasioning  the  conversion  of  its 
sugar  into  lactic  acid,  has  about  three  times  the  length  and  diameter  of 
the  preceding,  and  exhibits  much  stronger  to-and-fro  movements. 

306.  The  special  peculiarity  of  Bacillus  consists  in  the  extension  of 
its  cells  into  straight  rods,  sometimes  of  considerable  length,  which 
break-up  by  transverse  subdivision  into  separate  cells,  each  of  which  has 
a  flagellum  at  either  end,  though,  when  the  cells  are  paired  (like  those 
of  Bacteria),  each  carries  a  flagellum  at  its  free  end  alone.  The  B.  sub- 
tilts  ( Vibrio  subtilis  of  Ehrenberg),  found  in  stale  boiled  milk  that  is 
undergoing  the  butyric  fermentation,  is  a  slender  supple  thread  (Fig. 
•  194,  a),  whose  cells  average  about  l-5,000th  of  an  inch  in  length,  mov- 
ing in  a  ^pausing'  manner,  ''like  a  fish  forcing  its  way  through  reeds.'' 
The  B.  ulna  (Fig.  194,  b),  found  by  Cohn  in  a  stale  infusion  of  boiled  egg, 

»    Joum.  of  Roy.  Microsc.  Soc.    Vol.  i.  (1878),  p.  175. 
*  Ewart,  loc,  cit. 


310 


THE  MICROSCOPE  AND  ITS  REVELATIONS. 


LIFE-HISTORY  OP  BACILLUS  ANTHRAcis  (after  Ewart). 
Fig.  1.  spores  which  have  escaped  from  the  filaments.  2.  Division  of  spore  into  four  sporules. 
3.  Sporules  forming  a  zooglaea.  4,  5.  Sporules  developing  into  a  rod,  which  at  a  divides  into 
two  segments.  6.  A  rod  undergoing  segmentation,  and  the  segments  showing  flagella.  7.  Rods 
with  corpuscles  (vacuoles  or  nuclei?).  8.  A  newly-developed  filament.  9.  Filament  in  which  the 
endoplasm  has  divided  into  somewhat  long  segments.    10.  Further  segmentation  of  a  filament. 

11.  First  appearance  of  spores  as  minute  specks  in  the  endoplasm  near  the  ends  of  the  segments.  ^ 

12.  Fully  developed  spores  formed  by  contraction  of  the  endoplasm.  13.  Granular  matters  in 
spaces  between  spores,  indicative  of  disintegration  of  filament.  14.  Almost  complete  disappear- 
ance of  filament.  15.  Filament  from  which  spores  have  escaped.  16.  Filament  broken  into  short 
segments,  of  which  some  still  contain  spores.  17.  Filament  still  more  disintegrated,  with  one  c  f 
the  spores,  a,  in  process  of  division.  19.  Rods  forming  a  zooglaea.  20.  Rod  undergoing  segmenta- 
tion. 21.  Rod  lengthening  into  filament.  22.  Filament  containing  spores  becoming  granular  at 
one  end,  with  transverse  lines  between  spores.  23.  Spore-bearing  filaments  forming  rope-work. 
24.  Part  of  filament  containing  a  spore  in  process  of  division.  25.  Different  stages  of  development 
of  spA-e  into  rod.   26.  Short  filaments  containing  spores. 


PROTOPHYTIC  AND  OTHER  FUNGI. 


311 


is  distinguished  from  the  preceding  by  the  greater  thickness  of  its  fila- 
ments and  by  its  rigidity.  The  B,  antliracis,  which  is  found  in  the 
blood  and  tissues  of  animals  affected  with  carbuncle  and  splenic  fever, 
usually  presents  itself  in  straight  slender  rods,  of  from  1.2,000th  to  1-10- 
OOOths  of  an  inch  in  length  (Fig.  195);  these,  so  long  as  they  are  im- 
bedded in  living  tissues,  seem  to  multiply  indefinitely  by  transverse  divi- 
sion (Plate  XII.,  5,  6),  thus  continuing  to  produce  short  motile  filaments, 
furnished  with  flagella,  without  extending  themselves  into  longer  fila- 
ments, or  giving  origin  to  spores.  When,  however,  these  are  '  cultivated  ^ 
at  about  the  temperature  of  90°,  they  lengthen-out  (after  alternations 
of  rest  and  motion)  into  very  long  filaments  (22),  whose  endoplasm 
divides  into  numerous  segments  (9),  which  may  again  divide  (10,  11), 
and  then  rapidly  contract  to  form  spores  (12,  13).  These  spores,  escap- 
ing by  the  disintegration  of  the  filaments  (14,  17),  and  presenting  them- 
selves (1)  as  Micrococcus-forms,  may  either  multiply  as  round  or  oval 
cells  by  binary  subdivision  (2),  sometimes  aggregating  into  a  zooglaea  (3); 
or  they  may  at  once  develop  themselves  (4,  5)  into  the  straight  rods 
characteristic  of  the  type.    The  sporuliferous  filaments  (20,  21)  are  often 


Matted  Rods  of  Bacillus  anthraciSt  extending  in  rcws  between  connective  tissue-fibres  of  sub- 
cutaneous tissue. 

very  much  smaller  in  diameter  than  the  ordinary  rods;  and  are  disposed 
to  break  up  and  aggregate  themselves  either  into  an  ordinary  zooglaea 
(19),  or  into  a  double  spiral  rope-work  (23).'  It  appears  from  Mr. 
Ewart's  later  observations''  on  a  Bacillus  from  sea-water  resembling  B. 
anthracis  in  size  and  form,  that  by  the  continued  subdivision  and  aggre- 
gation of  the  spores  (or,  possibly,  by  the  emission  of  their  contents), 
granular  masses  of  considerable  size  are  produced;  the  rupture  of  which 
by  pressure  diffuses  over  the  field  their  component  granules,  every  one 
of  which  seems  capable,  when  placed  in  a  drop  of  sea-water,  of  germin- 
ating into  a  rod.  If,  as  seems  probable,  similar  ^minimization^  and 
multiplication  of  the  reproductive  germs  takes  place  in  Bacteria  also  (as 
it  will  be  shown  to  do  in  the  true  Monads  (§  417),  the  idea  of  the  uni- 
versal diffusion  of  such  germs  through  the  atmosphere,  which  seems  ne- 
cessary to  account  for  the  phenomena  of  putrefaction  (§  310),  should  not 
be  found  difficult  of  acceptance. 

307.  The  Vihriones,  although  very  long  known,  have  not  been 
studied  with  the  same  completeness  as  other  Schizomycetes.  They  re- 
semble Bacilla  in  the  slenderness  of  their  forms;  but  instead  of  being 
straight  and  rod-like,  are  flexible,  with  more  or  less  of  S-shaped  curva- 

*  See  Ewart,  loc,  cit, 

2    Proceedings  of  Royal  Society,"  June  20th,  1878. 


312 


THE  MICROSCOPE  AND  ITS  REVELATIONS. 


ture.  They  present  themselves  abundantly  in  infusions  of  decomposing 
organic  matter,  in  combination  with  other  Bacteria  forms,  from  which 
they  are  distinguishable  by  their  wavy  serpentine  movement.  The 
length  of  one  of  the  commonest  species,  F.  rugula  (Fig.  196),  is  usually 
from  1-1, 200th  to  1-2, 500th  of  an  inch. 


Via,  lorr 


Four  individuals  of  Vibrio  rugula,  each  showing  flagellum  at  one  or  both  ends;  two  other 
individuals,  a  and  6,  separating  from  each  other,  and  drawing  out  protoplasmic  filament  to  their 
flagella.   Magnified  2,000  diameter. 

308.  Spirilla,  which  are  the  largest  of  the  whole  group,  are  charac- 
terized by  the  spiral  coiling  of  their  cells  (Fig.  197),  and  by  their  cork- 
screw-like movement;  and  are  found  not  so  much  in  newly  decomposing 
infusions  of  organic  matter,  as  in  stale  liquids  which  have  passed  through 
the  active  stages  of  putrescence.  Nothing  has  been  certainly  known, 
until  recently,  of  their  life-history;  but  the  observations  of  Messrs. 
Geddes  and  Ewart  *  seem  to  render  it  clear  that  they  pass  through  a 


A 


A,  Spirillum  undula,  showing  flagellum  at  each  end.  Magnified  3,000  diameters,  b.  Spirillum 
.  volutans.   Magnified  2,000  diameters. 

series  of  stages  closely  corresponding  to  those  of  Bacillus.  The  ^zooglaea' 
film  formed  by  the  aggregation  of  Spirilla  has  a  brownish  tint;  some  of 
the  organisms  of  which  it  is  composed  are  at  rest,  and  others  in  rapid 
movement.  The  resting  Spirilla  are  sometimes  nearly  straight,  with  a 
slight  curvature  at  one  or  both  ends,  the  curve  increasing  until  the 
characteristic  spiral  of  the  motile  form  is  attained;  and  the  change  from 
the  still  to  the  motile  state  may  take  place  very  rapidly,  often  with  a 
passage  through  a  transition  Vibrio  stage.  But  Spirillumy  like  the 
forms  already  described,  may  lengthen  out  into  long  filaments,  which 
lose  their  characteristic  twist  and  their  motile  powers;  and  their  endo- 
plasm  breaks  up  into  spores,  which,  after  their  escape  from  the  filaments, 
form  a  distinct  capsular  investment,  which  holds  them  together  in  groups 
while  multiplying  by  subdivision.    Sometimes,  again,  a  mere  cellulose 


»  "  Proceedings  of  Royal  Society,"  June  20th,  1878. 


PROTOPHYTIC  AND  OTHER  FUNGI. 


313 


envelope  is  formed,  in  which  the  spores  lie  irregularly  imbedded,  con- 
tinuing to  multiply  by  subdivision,  so  as  to  form  large  irregular  masses. 
The  development  of  the  spore  into  a  filament  commences  by  the  putting 
forth  of  a  small  curved  prolongation,  which  gives  it  the  shape  of  a 
comma;  and  as  every  possible  gradation  in  size  and  form  is  seen  between 
the  smallest  comma  and  the  largest  filament,  there  can  be  no  reasonable 
doubt  of  the  development  of  the  former  into  the  latter.  Granular 
spheres  are  also  seen,  which  may  consist,  like  those  of  Bacillus,  of 
minute  particles  emitted  from  the  spores,  and  capable  of  development 
into  a  new  generation  of  Spirilla. 

309.  Of  the  whole  of  this  group  it  may  be  remarked  that,  so  far  as  is 
yet  known,  they  multiply  either  by  transverse  cell-division,  or  by  the 
breaking  up  of  their  endoplasm  into  spores,  the  production  of  which  is 
entirely  non-sexual.  Nothing  like  'conjugation,'  or  any  other  form  of 
sexual  Generation,  has  yet  been  witnessed  in  any  of  them;  and  until  such 
shall  have  been  discovered,  no  confidence  can  be  felt  that  we  know  the 
entire  life  history  of  any  one  type. —It  is  a  fact  of  great  importance  in 
the  physiology  of  the  Scliizomycetes,  that,  in  certain  stages  of  their  lives, 
they  can  resist  both  very  high  and  very  low  temperatures  without  the 
loss  of  their  vitality.  In  the  active  state  of  Bacteria,  etc.,  they  appear 
from  the  experiments  of  Dr.  Eidam  (which  were  conducted  under  the 
superintendence  of  Prof.  Cohn)  to  be  killed  by  continuous  exposure  to  a 
temperature  of  124°  for  three  hours,  or  to  115°  for  thirteen  or  fourteen 
hours,  although  capable  of  sustaining  a  temperature  of  120°  for  a  short 
time  wi  thout  losing  their  vitality.  But  in  the  Micrococcus-^i^ig^,  although 
killed  by  being  loiled  for  a  few  minutes,  they  can  sustain  exposure  to  a 
dry  heat  of  230°  Falir.,  but  are  killed  by  being  heated  to  248°.'  And 
this  is  probably  the  explanation  of  the  fact,  that  Prof.  Tyndall  found 
that  he  could  not  sterilize  an  infusion  of  old  hay  (the  Bacteria  germs 
contained  in  which  may  be  supposed  to  have  had  peculiarly  dry  hard 
envelopes)  without  boiling  it  continuously  for  several  hours;  though 
repeated  short  boilings  with  intervals  of  cooling  would  effectually  destroy 
their  power  of  germinating.^  Even  severe  cold  does  not  destroy  the 
vitality  of  Bacteria  and  Bacilli  in  their  ordinary  condition,  although  it 
suspends  their  activity;  for  Bacteria  have  been  found  to  recover  them- 
selves completely  after  exposure  to  a  temperature  of  0°  Fahr. ;  and  the 
spores  of  Bacillus  anthracis  have  recovered  their  germinal  power  after 
exposure  for  several  hours  to  a  temperature  averaging  8°  below  the  zero 
of  Fahrenheit. 

310.  When  these  facts  are  allowed  their  due  weight,  no  difiiculty 
ought  to  be  felt  in  admitting  the  action  of  Bacteria,  etc.,  in  producing 
decomposition,  under  conditions  which  might  at  first  view  be  fairly  sup- 
posed to  preclude  the  possibility  of  their  presence.  This  action  is 
altogether  analogous  to  that  of  the  Yeast-plant  (§  311)  in  producing 
saccharine  fermentation;  and  the  careful  and  exact  experiments  of 
Pasteur,*  repeated  and  verified  in  a  great  variety  of  modes  by  Professors 

^  As  it  seems  unquestionable  that  among  the  higher  Fungi  '  conjugation '  often 
takes  place  at  a  very  early  stage  of  growth,  it  seems  a  not  improbable  surmise 
that  the  *  granular  spheres '  observed  by  Mr.  E wart  in  Bacillus  and  Spirillum, 
which  seem  to  correspond  with  the  '  microplasts '  observed  by  Prof.  E.  Ray 
Lankester  in  his  Bacterium  ruhescens,  may  be  a  product  of  conjugation  in  the 
Micrococcus-stage  of  these  organisms. 

2  Beitrage  zur  Biologie  der  Pflanzen,"  Heft  3  (1875). 

3  Philosophical  Transactions,"  1877,  p.  183. 

^  The  experiments  which  have  always  seemed  to  the  Author  most  satisfactory, 


314 


THE  MICROSCOPE  AND  ITS  REVELATIONS. 


Lister,  Tyndall,  and  others,  seem  to  the  writer  to  leave  no  reasonable 
doubt  on  these  two  points — (I)  that  putrefactive  fermentation  does  not 
take  place,  even  in  liquids  which  are  peculiarly  disposed  to  pass  into  it, 
except  in  the  presence  of  Bacteria-germs;  and  (2)  that  neither  these 
germs,  nor  any  others,  arise  in  such  liquids  de  novo,  but  that  they  are  all 
conveyed  into  them  by  the  air,  when  not  otherwise  introduced.  Whether 
there  are  different  species  of  Bacteria^  Bacillus,  etc.,  which  (as  main- 
tained by  some)  excite  distinct  forms  of  disease  respectively  peculiar  to 
them,  in  the  bodies  of  animals  into  which  they  find  their  way,  is  a  ques- 
tion which  he  thinks  is  scarcely  yet  ripe  for  decision.  Strong  evidence 
in  its  favor  is  afforded  by  the  facts  now  accumulated  in  regard  to  the 
transmission  of  special  forms  of  disease  by  inoculation,  in  some  instances 
with  Bacillus  germs,  and  in  others  with  very  minute  germinal  particles 
termed  microzymes,  whose  nature  is  still  unknown.  Thus  '  splenic  fever ' 
is  producible  by  the  inoculation  of  Bacillus  anthracis,  and  the  typhoid 
fever  of  the  pig  by  inoculation  with  another  species  ot  Bacillus  ;^  the 
plants  having  been  in  both  cases  ^  cultivated,^  so  as  to  be  free  from  other 
contaminating  matter.  Again,  it  has  been  ascertained  by  careful  micro- 
scopical examination  of  the  fluid  of  the  Vaccine  vesicle,  that  it  is  charged 
with  a  multitude  of  minute  granules  not  above  l-20,000th  of  an  inch  in 
diameter;  and  it  has  been  further  determined  that  these,  rather  than  the 
fluid  in  which  they  are  suspended,  are  the  active  agents  in  the  produc- 
tion of  a  similar  vesicle  in  the  skin  into  which  they  are  inserted.  This 
vesicle  must  contain  hundreds  or  thousands  of  ^microzymes^  for  every 
one  originally  introduced;  and  it  is  obvious  that  their  multiplication  has 
so  strong  an  analogy  to  that  of  Bacteria,  as  to  suggest  the  idea  that  it 
must  take  place  by  a  like  process  of  cell-development.  Similar  observa- 
tions have  been  made  upon  Glanders,  Sheep-pox,  and  Cattle  plague;  so 
that  an  animal  suffering  under  either  of  these  terrible  diseases  is  a  focus 
of  infection  to  others,  for  precisely  the  same  reason  that  a  tub  of  ferment- 


are  those  in  which  the  careful  filtration  of  the  air  (as  by  simply  plugging  the 
mouth  of  the  tube  or  flask  with  cotton-wool),  or  its  purification  by  the  subsi- 
dence of  the  minute  particles  it  ordinarily  holds  in  suspension  (as  demonstrated 
by  Prof.  Tyndall's  optical  test),  prevents  any  putrefactive  change  from  taking 
place  in  organic  liquids  exposed  to  it.  He  would  refer  such  as  wish  to  study  this 
important  question,  to  the  following  papers  in  particular;— Prof.  Huxley's  Presi- 
dential Address  to  the  British  Association  in  1870,  reprinted  in  his  Critiques 
and  Addresses;"  Prof.  Lister  on  *  Bacteria  and  the  Germ-theory,' in  "  Quart. 
Journ.  of  Microsc.  Science,"  Vol.  xiii.,  p.  380,  on  *  The  Nature  of  Fermentation' 
in  vol.  xviii.  of  the  same  Journal,  p.  177,  and  his  Address  to  the  British  Medical 
Association  at  Cambridge,  in  *  Brit.  Med.  Journ.,'  1880,  p.  363;  and  Prof.  Tyndall 
on  '  The  Optical  Deportment  of  the  Atmosphere  in  Relation  to  the  Phenomena  of 
Putrefaction  and  Infection,'  in  **Philos.  Transact.,"  1876,  p.  27,  and  *  Further 
Researches'  in  **Philos.  Trans.,"  1877,  p.  149;  also,  for  an  account  of  recent 
Pathological  researches  by  Pasteur  and  others,  *  Journ.  of  Roy.  Microsc.  Soc.,' 
vol.  iii.  (1880)  pp.  1006-1020.  The  doctrines  advanced  on  the  other  side  in  Dr.  H. 
Charlton  Bastian's  Beginnings  of  Life,"  do  not,  in  his  judgment,  stand  the  test 
of  rigid  scrutiny. 

^  The  dried  blood  of  horses  that  had  died  in  India  of  *Loodiana  fever,'  having 
been  sent  to  the  Brown  Institution,  a  crop  of  Bacillus  anthracis  was  grown  from 
it,  which  reproduced  the  disease  in  healthy  animals  — The  very  important  fact 
has  been  discovered  at  the  same  institution,  that  the  '  brewers' grains '  largely 
used  as  food  for  cattle,  afford  a  soil  which  is  peculiarly  favorable  for  the  growth 
and  development  of  the  spore-filaments  of  Bacillus;  and  thus  an  obvious  explana- 
tion was  given  of  an  epidemic  of  anthrax  in  a  previously  uninfected  district, 
destroying  a  large  number  of  animals,  all  of  which  had  been  fed  with  *  grains ' 
obtained  from  a  particular  brewery. 


PROTOPHYTIO  AND  OTHER  FUNGI. 


315 


ingbeer  is  capable  of  propagating  its  fermentation  to  fresh  wort. '—A 
most  notable  instance  of  such  propagation  is  afforded  by  the  spread  of 
the  disease  termed  ^pebrine'  among  the  Silkworms  of  the  south  of 
France;  the  mortahty  caused  by  it  being  estimated  to  produce  a  money- 
loss  of  from  three  to  four  millions  sterling  annually,  for  several  years 
following  1853,  when  it  first  broke  out  with  violence.  It  has  been  shown 
by  microscopic  investigation,  that  in  silkworms  strongly  affected  with 
this  disease,  every  tissue  and  organ  in  the  body  is  swarming  with  minute 
cylindrical  corpuscles  about  l-6,000th  of  an  inch  long;  and  tluit  these 
even  pass  into  the  undeveloped  eggs  of  the  female  moth,  so  that  the  dis- 
ease is  hereditarily  transmitted.  And  it  has  been  further  ascertained  by 
the  researches  of  Pasteur,  that  these  corpuscles  are  the  active  agents  in 
the  production  of  the  disease,  which  is  engendered  in  healthy  silkworms 
by  their  reception  into  their  bodies;  whilst,  if  due  precautions  be  taken 
against  their  transmission,  the  malady  may  be  completely  exterminated. 

311.  Nearly  allied  to  the  Scldzomycetes  in  the  simplicity  of  its  charac- 
ter and  in  its  '  zymotic  '  action,  is  the  Snccliaromyces  {Torula)  cerevisice; 
the  presence  of  which  in  Yeast  gives  to  it  the  power  of  exciting  the  alco- 
holic fermentation  in  saccharine  liquids.  When  a  small  drop  of  yeast  is 
placed  under  a  magnifying  p3wer  of  400  or  5^0  diameters,  it  is  seen  to 
contain  a  large  number  of  globular  or  ovoid  cells,  averaging  about 
1-3, 000th  of  an  inch  in  diameter,  for  the  most  part  isolated,  but  some- 


Torula  cerivisice,  or  Yeast-plant,  as  developed  during  the  process  of  fermentation:— a,  6,  c,  cf, 
successive  stages  of  cell-multiplication. 

times  connected  in  short  series;  and  each  cell  is  filled  with  a  nearly  color- 
less ^endoplasm,^  usually  exhibiting  one  or  more  vacuoles,  but  never 
showing  a  nucleus.  When  placed  in  a  fermentible  fluid  containing  some 
form  of  nitrogenous  matter  in  addition  to  sugar, they  vegetate,  in  the 
manner  represented  in  Fig.  198.  Each  cell  puts  forth  one  or  two  projec- 
tions, which  seem  to  be  young  cells  developed  as  buds  or  offsets  from  their 
predecessors;  these,  in  the  course  of  a  short  time,  become  complete  cells, 
and  again  perform  the  same  process;  and  in  this  manner  the  single  cells 
of  yeast  develop  themselves,  in  the  course  of  a  few  hours,  into  rows  of 
four,  five,  or  six,  which  remain  in  continuity  with  each  other  whilst  the 
plant  is  still  growing,  but  which  separate  if  the  fermenting  process  be 
checked,  and  return  to  the  isolated  condition  of  those  which  originally 
constituted  the  yeast.    Thus  it  is  that  the  quantity  of  yeast  first  intro- 


^  See  Prof.  Burdon  Sanderson  '  On  the  Intimate  Pathology  of  Contagion '  in 
the  Privy  Council    Reports  on  the  Public  Health  "  for  1870. 

^  It  appears  from  the  researches  of  Pasteur,  that  although  the  presence  of  Al- 
buminous matter  (such  as  is  contained  in  a  saccharine  wort,  or  in  the  juices  of 
fruits)  favors  the  growth  and  reproduction  of  Yeast,  yet  that  it  can  live  and  mul- 
ply  in  a  solution  of  pure  Sugar,  containing  ammonium-tartrate  with  small  quan- 
tities of  Mineral  salts;  the  decomposition  of  the  ammonia-salt  affording  it  the  ni- 
trogen it  requires  for  the  production  of  protoplasm,  while  the  sugar  and  water 
supply  the  carbon,  oxygen,  and  hydrogen. 


316 


THE  MICROSCOPE  AND  ITS  REVELATIONS. 


duced  into  the  fermentible  fluid,  is  multiplied  six  times  or  more  during 
the  changes  in  which  it  takes  part.  Under  certain  conditions  not  yet 
determined,  the  Yeast-cells  multiply  in  another  mode — namely,  by  the 
breaking-up  of  the  endoplasm  into  segments,  usually  f oar  in  number, 
around  each  of  which  a  new  '  cell-wall^  forms  itself;  and  these  endogoni- 
dia  (which  correspond  with  the  ^  zoospores^  of  Algae,  save  in  having  no 
motile  power)  being  set  free  by  the  dissolution  of  the  wall  of  the  parent- 
cell,  soon  enlarge  and  comport  themselves  as  ordinary  Yeast-cells.  No 
conjugation  or  other  form  of  sexual  action  has  yet  been  observed  in 
Torula;  and  there  are  various  reasons  for  surmising  that  we  do  not  yet 
know  its  whole  life-history. — Many  other  Fungi  of  like  simplicity  have 
the  power  to  act  as  ^ferments:'  thus  in  wine-making,  the  fermentation  of 
the  juices  of  the  grapes  or  other  fruit  employed,  is  set  going  by  the  de- 
velopment of  minute  fungi  whose  germs  have  settled  on  their  skins;  these 
germs  not  being  injured  by  desiccation,  and  being  readily  transported  by 
the  atmosphere  in  the  dried-up  state.  There  is  reason  to  believe,  more- 
over, that  a  similar  '  zymotic  ^  action  may  be  excited  by  Fungi  of  a  higher 
grade  in  the  earlier  stages  of  their  growth;  the  alcoholic  fermentation  be- 
ing set-up  in  a  suitable  liquid  (such  as  an  aqueous  solution  of  cane-sugar, 
with  a  little  fruit-juice)  by  sowing  in  it  the  sporules  of  any  one  of  the 
ordinary  '  moulds,^  such  as  Peiiicillium  glaiicum,  Mucor,  or  Aspergillus, 
provided  the  temperature  be  kept  up  to  blood-heat;  and  this  even  though 
the  solution  has  been  previously  heated  to  284''  Fahr.,  a  temperature 
which  must  kill  any  germs  ;it  may  itself  contain.  * 

312.  The  Sarcifia  ventriculi  (Fig. 
199)  is  another  Protophyte  which 
seems  related  both  to  AlgcB  and 
Fungi;  corresponding  with  the  former 
in  its  aquatic  habit  and  mode  of 
growth,  and  with  the  latter  in  requir- 
ing organic  matter  of  some  kind  for  its 
sustenance.  This  Plant  is  most  fre- 
quently found  in  the  matters  vomited 
by  persons  sufferiug  under  disorder 
of  the  stomach,  but  has  also  been 
met  with  in  other  diseased  parts  of 
the  body.  It  has  been  detected  in 
the  contents  of  the  stomach,  however, 
under  circumstances  which  seem  to 
indicate  that  it  is  not  an  uncommon 
tenant  of  that  organ  even  in  health,  and  that  it  may  accumulate  there 
to  a  considerable  amount  without  producing  any  inconvenience.  It 
seems  probable,  therefore,  that  its  presence  in  disease  is  rather  to  be  con- 
sidered as  favored  by  the  changed  state  of  the  fluids  which  the  disease 
induces  (either  an  acid  or  a  fermentible  state  of  the  contents  of  the 
stomach  having  been  generally  found  to  exist  in  the  cases  in  which  the 
plant  has  been  most  abundant),  than  to  be  itself  the  occasion  of  the  dis- 
ease, as  some  have  supposed.  The  Sarcina  presents  itself  in  the  form  of 
clusters  of  adherent  cells  arranged  in  squares,  each  square  containing 
from  4  to  64,  and  the  number  of  cells  being  obviously  multiplied  by  du- 
plicative subdivision  in  directions  transverse  to  each  other.    In  fact,  its 

1  See  the  observations  of  Mad.  Liiders,  in  Sohulze's  ^' Archiv  fiir  Mikroscopi- 
sche  Anatomie,*'  Band  iii.,  abstracted  in  Quart.  Journ.  Microsc.  Sci.,"  N.S., 
vol.  xiii.  (1868),  p.  35. 


Sarcina  ventriculi. 


PROTOPHYTIC  AND  OTHER  FUNGI. 


31T 


general  mode  of  growth  would  indicate  a  near  relation  to  Gonium,  one  of 
the  VolvocinecB,  which  presents  itself  in  similar  quadripartite  aggrega- 
tions; but  there  can  be  little  doubt  that,  as  no  fructification  has  yet  been 
seen  in  it,  only  its  earlier  and  simpler  condition  is  yet  known  to  us;  and 
its  true  place  cannot  be  determined  until  its  whole  life-history  shall  have 
been  followed  out. 

313.  Another  form  of  Fungous  vegetation  that  develops  itself  within 
the  living  body,  and  which  is  of  great  economic  importance  as  well  as  of 
scientific  interest,  is  the  Botrytis  bassiana  (Fig.  200),  a  kind  of  'mould,' 
the  growth  of  which  is  the  real  source  of  the  disease  termed  muscardine, 

Fig.  200 


Botrytis  bctssiana A,  the  fungus  as  it  first  appears  at  the  orifices  of  the  stigmata;  b,  tubular 
filaments,  bearing  short  branches,  as  seen  two  days  afterwards;  e,  magnified  view  of  the  same;  c. 
D,  appearance  of  filaments  on  the  fourth  and  sixth  days;  p,  masses  of  mature  spores  falling  off 
the  branches,  with  filaments  proceeding  from  them. 

which  formerly  carried  off  Silk-worms  in  large  numbers,  just  when  they 
were  about  to  enter  the  chrysalis  state,  to  the  great  injury  of  their 
breeders.  The  plant  presents  itself  under  a  considerable  variety  of  forms 
(a-f),  all  of  which,  however,  are  of  extremely  simple  structure,  consist- 
ing of  elongated  or  rounded  cells,  connected  in  necklace-like  filaments, 
very  nearly  as  in  the  ordinary  '  bead-moulds.'  The  sporules  of  this  fun- 
gus, floating  in  the  air,  enter  the  breathing-pores  (Fig.  433)  which  open 
into  the  tracheal  system  of  the  Silkworm:  they  first  develop  themselves 
within  the  air-tubes,  which  are  soon  blocked  up  by  their  growth;  and 
they  then  extend  themselves  through  the  fatty  mass  beneath  the  skin, 


318 


THE  MICROSCOPE  AND  ITIS  BEVELATI0N8. 


occasioning  the  destruction  of  this  tissue,  which  is  yery  important  as  a 
reservoir  of  nutriment  to  the  animal  when  it  is  about  to  pass  into  its 
chrysalis  condition.  The  disease  invariably  occasions  the  death  of  the 
worm  which  it  attacks;  but  it  seldom  shows  itself  externally  until  after- 
wards,  when  it  rapidly  shoots-fortli  from  beneath  the  skin,  especially  at  the 
junction  of  the  rings  of  the  body.  Although  it  spontaneously  attacks 
only  the  larva,  yet  it  may  be  communicated  by  inoculation  to  the  chry- 
salis and  the  moth,  as  well  as  to  the  worm;  and  it  has  also  been  observed 
to  attack  other  Lepidopterous  Insects.  A  careful  investigation  of  the 
circumstances  which  favor  the  development  of  this  disease  was  made  by 
Audouin,  who  first  discovered  its  real  nature;  and  he  showed  that  its 
spread  was  favored  by  the  overcrowding  of  the  worms  in  the  breeding 
establishments,  and  particularly  by  the  practice  of  throwing  the  bodies 
of  such  as  died  into  a  heap  into  the  immediate  neighborhood  of  the  living 
worms:  for  this  heap  speedily  became  covered  with  this  kind  of  '  mould,' 
which  found  upon  it  a  most  congenial  soil;  and  it  kept  up  a  continual 
supply  of  sporules,  which,  being  diffused  through  the  atmosphere  of  the 
neighborhood,  were  drawn  into  the  breathing  pores  of  individuals  previ- 
ously healthy.  The  precautions  obviously  suggested  by  the  knowledge 
of  the  nature  of  the  disease,  thus  afforded  by  the  Microscope,  having 
been  duly  put  in  force,  its  extension  was  successfully  kept  down. 

314.  An  example  of  the  like  kind  is  frequently  presented  in  the 
destruction  of  the  common  House-fly  by  a  minute  fungus  termed  Eyn- 
pusa  musccB,  In  its  fully  developed  condition,  the  spore-bearing  filaments 
of  this  plant  stand  out  from  the  body  of  the  fly  like  the  '  pile  ^  of  velvet; 
and  the  spores  thrown  off  from  these  in  all  directions  form  a  white  circle 
round  it,  as  it  rests  motionless  on  a  window-pane.  The  filaments  which 
show  themselves  externally  are  the  fructification  of  the  fungus  which 
occupies  the  interior  of  the  Fly's  body;  and  this  originates  in  minute 
corpuscles  which  find  their  way  into  the  circulating  fluid  from  without. 
A  healthy  fly  shut  np  with  a  diseased  one,  takes  the  disease  from  it  by 
the  deposit  of  a  sporule  on  some  part  of  its  surface;  for  this,  beginning 
to  germinate,  sends  out  a  process  which  finds  its  way  into  the  interior, 
cither  through  the  breathing-pores,  or  between  the  rings  of  the  body; 
and  having  reached  the  interior  cavity,  it  gives  off  the  minute  corpuscles 
which  constitute  the  earliest  stage  of  the  Empiisa, — Again  it  is  not  all 
uncommon  in  the  West  Indies,  to  see  individuals  of  a  species  of  Polistes 
(the  representative  of  the  Wasp  of  our  own  country)  flying  about  with 
plants  of  their  own  length  projecting  from  some  part  of  their  surface,  the 
germs  of  winch  have  been  probably  introduced  (as  in  the  preceding  case) 
through  the  breathing-pores  at  their  sides,  and  have  taken  root  in  their 
substance,  so  as  to  produce  a  luxuriant  vegetation.  In  time,  however, 
this  fungous  growth  spreads  through  the  body,  and  destroys  the  life  of 
the  insect;  it  then  seems  to  grow  more  rapidly,  the  decomposing  tissue 
of  the  dead  body  being  still  more  adapted  than  the  living  structure  to 
afford  it  nutriment. — A  similar  growth  of  different  species  of  the  genus 
Splicer ia  takes  place  in  the  bodies  of  certain  caterpillars,  in  New  Zealand, 
Australia,  and  China;  and  being  thus  completely  pervaded  by  a  dense 
substance,  which,  when  dried,  has  almost  the  solidity  of  wood,  these 
caterpillars  come  to  present  the  appearance  of  twigs,  with  long  slender 
stalks  that  are  formed  by  the  growth  of  the  fungus  itsel The  Chinese 
species  is  valued  as  a  medicinal  drug. 

315.  The  stomachs  and  intestines  of  many  Worms  and  Insects  are 
infested  with  parasiticFungi,  which  grow  there  with  great  luxuriance.  In 


PROTOPHYTIC  AND  OTHER  FUNGI. 


319 


the 
forms 


accompanying  illustrations  (Figs.  201,  202)  are  shown  some  of  the 
IS  of  the  Enterolrym,'  which  has  been  found  by  Dr.  Leidy'  to  be  so 
constantly  present  m  the  stomach  of  certain  species  of  hilus  (gaily  worm) 
that  It  IS  extremely  rare  to  meet  with  individuals  whose  stomachs  do  not 


Growth  of  Enterohryus  spiralis  from 
mucous  membrane  of  stomach  of  lulus: 
— a,  epithelial  cells  of  mucous  membrane; 
6,  spiral  filament  of  enterobryus;  c,  pri- 
mary cell ;  d,  secondary  cell. 


contain  it.  The  Enterobryus  originally  consists  of  a  single  long  tubular 
cell,  which  sometimes  grows  in  a  spiral 
mode  (Fig.  201);  sometimes  straight  and 
tapering  (Fig.  202,  a).  In  its  young 
state  the  cell  contains  a  transparent  pro- 
toplasm, with  granules  and  globules  of 
various  sizes;  but  in  its  more  advanced 
condition  the  tube  of  the  filament  is  oc- 
cupied by  cells  in  various  stages  of  de- 
velopment; these  distend  the  terminal 
part  of  the  cell  (Fig.  202,  b),  and  press 
so  much  against  each  other  that  their 
walls  become  flattened;  whilst  nearer 
the  middle  of  the  same  filament  (c)  we 
find  them  retaining  their  rounded  form, 
and  merely  lying  in  contact  with  each 
other;  and  at  the  base  (d),  they  lie  de- 
tached in  the  midst  of  the  granular 
protoplasm.  In  E,  spiralis  the  pri- 
mary cells  (Fig.  201,  b,  c)  very  com- 
monly have  secondary  and  even  ternary  cells  {d)  developed  at  their  ex- 
tremities; but  this  is  rarely  seen  in  E.  attemiatus  (Fig.  202).  It  may  be 
considered  as  next  to  certain  that  the  tubular  filaments  rupture,  when 
the  contained  cells  have  arrived  at  maturity,  and  give  them  exit;  and  that 
these  cells  are  developed,  under  favorable  circumstances,  into  tubular 
filaments  like  those  from  which  they  sprang;  but  the  process  has  not  yet 
been  thoroughly  made  out.  This  is  obviously  not  the  true  Generation  of 
the  plant,  but  is  analogous  to  the  develojDment  of  zoospores  in  Aclilya 
(§  250). — It  is  not  a  little  curious,  moreover,  that  the  Entozoa  or  para- 
sitic Worms  infesting  the  alimentary  canal  of  these  animals  should  be 
often  clothed  externally  with  an  abundant  growth  of  such  plants;  in  one 
instance.  Dr.  Leidy  found  an  Ascaris  bearing  twenty-three  filaments  of 
Enterobryus,  which  appeared  to  cause  no  inconvenience  to  the  animal, 
as  it  moved  and  wriggled  about  with  all  the  ordinary  activity  of  the  species." 
The  presence  of  this  kind  of  vegetation  seems  to  be  related  to  the  pecu- 
liar food  of  the  animals  in  whose  stomachs  it  is  found;  for  Dr.  Leidy 
could  not  discover  traces  of  these  or  any  other  parasitic  plants  in  the  ali- 
mentary canal  of  the  carnivorous  Myriapods  which  he  examined;  whilst 
he  met  with  a  constant  and  most  extraordinary  profusion  of  vegetation 
in  the  stomach  of  a  herbivorous  Beetle,  the  Passulus  cornutus,  which 
lives  like  the  luli,  in  stumps  of  old  trees,  and  feeds  as  they  do  on  decay- 
ing wood. 

316.  There  are  various  diseased  conditions  of  the  Human  skin  and 
mucous  membranes,  in  which  there  is  a  combination  of  fungoid  Vegeta- 


^  This  plant,  also,  has  much  aflanity  to  Algse  in  its  general  type  of  structure,  and 
is  referred  to  that  group  by  many  botanists;  but  the  conditions  of  its  growth,  as 
in  the  case  of  Sarcina,  seem  rather  to  indicate  its  affinity  to  the  Fungi;  and  until 
its  proper  fructification  shall  have  been  made  out,  its  true  place  in  the  scale  must 
be  considered  as  undetermined. 

*    Smithsonian  Contributions  to  Knowledge,"  Vol.  v. 


320 


THE  MICBOSCOPE  AND  ITS  REVELATIONS. 


tion  and  morbid  growth  of  the  Animal  tissues:  this  is  the  case^  for  exam- 
ple, with  the  Tinea  favosa,  a  disease  of  the  scalp,  in  which  yellow  crusts 
are  formed  that  consist  almost  entirely  of  the  mycelium,  receptacles,  and 
sporules  of  a  fungus;  and  the  like  is  true  also  of  those  white  patches 
(Aphthce)  on  the  lining  membrane  of  the  mouth  of  infants,  which  are 
known  as  thrush,  and  of  the  exudations  of  '  false  membrane  ^  in  the  dis- 
ease termed  diphtheria.  In  these  and  similar  cases,  two  opinions  are  en- 
tertained as  to  the  relation  of  the  Fungi  to  the  diseases  in  which  they  pre- 
sent themselves:  some  maintaining  that  their  presence  is  the  essential 
condition  of  these  diseases,  which  originate  in  the  introduction  of  the  veg- 
etable germs;  and  others  considering  their  presence  to  be  secondary  to 
some  morbid  alteration  of  the  parts  wherein  tlie  fungi  appear,  which 
alteration  favors  their  development.  The  first  of  these  doctrines  derives 
a  strong  support  from  the  fact,  that  the  diseases  in  question  may  be  com- 
municated to  healthy  individuals,  through  the  introduction  of  the  germs 
of  the  Fungi  by  inoculation;  whilst  the  second  is  rather  consistent  with 
general  analogy,  and  especially  with  what  is  known  of  the  conditions 


structure  of  Enterobryus.—A,  growth  of  E.  attenuatus,  from  mucous  membrane  of  stomach  of 
Passulus;  b,  dilated  extremity  of  primary  cell  of  E.  elegans,  filed  with  secondary  cells,  which, 
near  its  termination,  become  mutually  flattened  by  pressure;  c,  lower  portion  of  the  same  fila- 
ment, containing  cells  mingled  with  granules;  d,  base  of  the  same  filament,  containing  globules 
interposed  among  granules. 

under  which  the  various  kinds  of  fungoid  ^blights '  develop  themselves  in 
or  upon  growing  Plants  (§  320). — It  is  not  a  little  remarkable  that  even 
Corals,  Shells,  Fish-scales,  and  other  hard  tissues  of  Animals,  are  not  un- 
frequently  penetrated  by  fungous  Vegetation,  which  usually  presents 
itself  in  the  form  of  simple  tubes  more  or  less  regularly  disposed  (Fig. 
203),  and  closely  resembling  those  of  an  ordiusiYy  7nycelium  (compare  Fig. 
207,  a),  but  occasionally  exhibits  a  distinct  fructification  that  enables  its 
true  charav^ter  to  be  recognized.^ 


^  See  Professor  KoUiker  *  On  the  Frequent  Occurrence  of  Vegetable  Parasites 
in  the  Hard  Tissues  of  Animals,' in  *' Quart.  Journ.  of  Microsc.  Science,"  Vol. 
viii.  (1860),  p.  171— Previously  to  the  publication  of  his  friend  Prof.  K.'s  paper, 
the  Author  had  himself  arrived  at  a  similar  conclusion  in  regard  to  the  parasitic 
nature  of  many  of  the  tubular  structures  which  had  been  originally  regarded  not 
merely  by  himself,  but  by  Prof.  Kolliker,  as  proper  to  the  Shells  in  which  they 
occur. — Prof.  Duncan  has  recognized  like  parasitic  growths,  apparently  allied  to 
Achlya,  §  250  (which  is  now  ranked  bv  many  among  Fungi),  both  in  recent  and 
Fossil  Corals.  See  '  Proceed,  of  Roy.' Soc,"  Vol.  xxv,  (1879),  p.  238;  and"  Quart. 
Journ.  Geolog.  Soc,"  Vol.  xxxii.,  p.  205. 


PROTOPHYTIC  AND  OTHER  FUNGI.  321 

317.  There  arc  scarcely  any  Microscopic  objects  more  beautiful  than 
some  of  those  forms  of  '  mould'  or  ' mildew/ which  are  commonly  found 
growing  upon  the  surface  of  jams  and  other  preserves;  especially  wlieu 
they  are  viewed  with  a  low  magnifying  power,  by  reflected  light.  For 
they  present  themselves  as  a  forest  of  stems  and  branches,  of  extremely 
varied  and  elegant  forms  (Fig.  204),  loaded  with  fruit  of  a  singular  deli- 
cacy of  conformation,  all  glistening  brightly  on  a  dark  ground.  In  remov- 
ing a  portion  of  the  '  mould'  from  the  surface  whereon  it  grows,  for  the 
purpose  of  microscopic  examination,  it  is  desirable  to  disturb  it  no  more 
than  can  be  helped,  in  order  that  it  may  be  seen  as  nearly  as  possible  in 
its  natural  condition;  and  it  is  therefore  preferable  to  take  up  a  portion 
of  the  membrane-like  substance  whereon  it  usually  rests,  which  is,  in  fact, 
a  mycelium  composed  of  interlacing  filaments  of  the  vegetative  part  of  the 
])lant,  the  stems  and  branches  being  its  reproductive  portion  (§  321). — 
The  universality  of  the  appearance  of  these  simple  forms  of  Fungi  upon 
all  spots  favorable  to  their  development,  has  given  rise  to  the  belief  that 
they  are  spontaneously  produced  by  decaying  substances:  but  there  is  no 


Shell  of  Anomia  penetrated  by  Stysanus  coLput-medusoe . 

Parasitic  Fungus. 


occasion  for  this  mode  of  accounting  for  it;  since  the  extraordinary 
means  adopted  by  Nature  for  the  production  and  diffusion  of  the  germs 
of  these  plants  adequately  suffice  to  explain  the  facts  of  the  case.  The 
number  of  sporules  which  any  one  Fungus  may  develop  is  almost  incal- 
culable; a  single  individual  of  the  puff-ball  tribe  has  been  computed  to 
send  forth  no  fewer  than  ten  millions.  And  their  minuteness  is  such  that 
they  are  scattered  through  the  air  in  the  condition  of  the  finest  possible 
dust;  so  that  it  is  difficult  to  conceive  of  a  place  from  which  they  should 
be  excluded.  This  universal  diffusion  was  clearly  proved  several  years 
ago  by  an  experiment  made  by  Dr.  Brittan,  of  Bristol;  who  caused  air  to 
be  pumped  for  several  hours  together  through  an  inverted  siphon,  the 
bend  of  which  was  immersed  in  a  freezing  mixture,  so  as  to  condense  the 
aqueous  vapor  of  the  atmosphere.  The  water  at  last  came  to  be  tinged  of 
a  deep  brown  hue;  and  was  found,  when  microscopically  examined,  to  be 
charged  with  multitudes  of  sporules  of  Fungi.  More  recently.  Prof. 
Tyndall  has  shown,  by  a  peculiar  application  of  electric  light,  that  all 
ordinary  air  contains  a  multitude  of  excessively  minute  solid  particles 
suspended  in  it;  that  these,  being  for  the  most  part  destructible  by  heat, 
are  chiefly  organic;  and  that  they  may  be  either  strained  off,  so  as  to  ren- 
21 


322 


THE  MICROSCOPE  AND  ITS  REVELATIONS. 


der  the  filtered  air  optically  pure,"  by  passing  it  through  cotton  wool, 
or  may  be  got  rid  of  by  allowing  them  time  to  subside  in  a  closed  cham- 
ber whose  bottom  is  smeared  with  glycerine,  so  that  they  are  held  down 
when  once  they  have  settled  on  it. 

318.  This  mode  of  explanation  has  received  further  confirmation  from 
the  facts  recently  ascertained,  in  regard  to  the  great  number  of  forms 
under  which  a  single  germ  may  develop  itself.  For  it  has  been  ascer- 
tained with  regard  to  the  Fungi  generally,  that  different  individuals  of 
the  same  species  may  not  only  develop  themselves  in  very  dissimilar 
modes,  but  may  even  bear  dissimilar  types  of  fructification;  and  further, 
that  even  the  same  individual  may  put  forth,  at  different  periods  of  its 
life,  those  two  kinds  of  fructification — the  Basidio-sporoiis,  in  which 
spores  are  developed  by  outgrowth  from  free  points  (basidia),  and  the 
Ascomycetous,  in  which  they  are  developed  in  the  interior  of  cases  (tliem 
or  asci^  Fig.  205) — which  had  been  previously  considered  as  separately 
characterizing  the  two  principal  groups  into  which  the  Class  was  prima- 
rily divided.  But  the  spores  produced  from  the  ostensible  ^fructifica- 
tion ^  in  this  Class  are  all  non-sexual  or  gonidial  (§  228).  In  a  large  pro- 
portion of  it,  nothing  whatever  is  known  of  the  true  Generative  process; 
and  wherever  it  has  been  detected,  it  is  performed  in  a  manner  that  car- 
ries us  back  to  the  simplicity  of  the  lower  Algal  types. — Thus  the  7nyce' 
liwn  of  the  common  Mucor  which  forms  the  '  brown  mould '  of  bread, 
preserves,  etc.,  consists  of  a  single  cell,  which  first  sends  forth  wide- 
spreading  branches  that  extend  over  the  surface  on  which  it  grows,  and 
then  develops  a  vertical  pin-like  stem,  enlarging  at  its  top  a  little  globu- 
lar '  head,^  the  cavity  of  which  is  cut  off  from  that  of  the  stem  by  a  par- 
tition, so  as  to  form  a  separate  '  sporangial  ^  cell,  whose  endoplasm  breaks 
up  into  a  number  of  '  micro-gonidia;'  and  every  one  of  these,  when  set 
free  by  the  bursting  of  the  sporangium,  can  give  origin  to  a  new  myce- 
lium. But  the  Generative  act  is  performed  in  the  mycelium  itself;  two 
branches  of  which,  coming  into  contact  with  each  other  at  their  free  ex- 
tremities, there  form  separate  terminal  cells,  the  fusion  of  which  unites 
their  two  endoplasms  into  one  (just  as  in  the  conjugation  of  Mesocarpus, 
§  235);  and  this,  surrounding  itself  with  a  thick  cell-wall,  becomes  an 
^oospore/  which  may  remain  along  time  in  the  dry  state  without  germi- 
nating. It  is  by  the  formation  of  gonidiaih^t  a  ^  mould  ^  whose  germ 
has  fallen  upon  a  fruitful  soil  rapidly  extends  itself  over  a  large  surface; 
whilst  the  carrying  of  the  oospores  by  currents  of  air  forms  the  chief 
means  of  its  transmission  to  a  distance. — The  Penicillium,  or  ^  green 
mould,'  on  the  other  hand,  sends-up  from  its  mycelium  a  branching 
stem,  the  ramifications  of  which  subdivide  into  a  brush-like  tuft  of  fila- 
ments, each  of  which  bears  at  its  extremity  a  succession  of  minute 
*  beads '  termed  conidia.  These,  detaching  themselves  and  falling  on  a 
suitable  soil,  forthwith  germinate  into  new  mycelia;  or,  drying  up,  are 
disseminated  by  atmospheric  currents,  without  loss  of  their  vitality. 
Here,  again,  the  Generative  act  is  performed  in  the  mycelium:  bnt  by  a 
somewhat  more  complex  apparatus  than  in  Mucor.  One  of  its  branches 
elongates,  and  coils  spirally  upon  itself  into  a  corkscrew-like  body,  the 
ascogo7iium,  which  constitutes  the  female  organ;  whilst  another  branch 
acts  as  the  male  organ,  the  pollinodiuyn,  which  extends  itself  over  the 
spire,  and  communicates  to  its  endoplasm  some  fertilizing  material  from 
its  own.  The  germ  thus  formed  becomes  inclosed  in  a  mass  of  sterile  tis- 
sue; and  within  this  it  develops  itself  into  a  cluster  of  asci,  each  contain- 
ing numerous  spores,  whose  liberation  gives  origin  to  a  ^new  generation.' 


PROTOPHTTIC   AND  OTHEK  FUNGI. 


323 


319.  The  ^  entophytic '  Fungi  which  infest  some  of  the  Vegetables 
most  important  to  Man  as  furnishing  his  staple  articles  of  food,  constitute 
a  group  of  special  interest  to  the  Microscopist;  of  which  a  few  of  the  chief 
examples  may  be  here  noticed.  The  mildeio  which  is  often  found  attack- 
ing the  straw  of  Wheat,  shows  itself  externally  in  the  form  of  circular 
clusters  of  pear-shaped  asci  or  spore-cases  (Fig.  205),  each  containing 
two  conipartments  filled  with  sporules;  these  (known  as  the  Puccinm 
graminis)  arise  from  a  filamentous  tissue  constituting  its  inyceliumy  the 
threads  of  which  interweave  themselves  with  the  tissue  of  the  straw;  and 
they  generally  make  their  way  to  the  surface  through  the  ^stomata'  or 
breathing-pores  of  its  epidermis.    The  rust,  which 

makes  its  appearance  on  the  leaves  and  chaff-scales  37na^2oa 
of  Wheat,  has  a  fructification  that  seems  essenti- 
ally distinct  from  that  just  described,  consisting 
of  oval  spore-cases,  which  grow  without  any  regu- 
larity of  arrangement  from  the  threads  of  the 
mycelium,  and  hence  it  has  been  considered  to 
belong  to  a  different  genus  and  species,  Uredo 
rubigo.  But  from  the  observations  of  Prof.  Hen- 
slow,  it  seems  certain  that  ^rust '  is  only  an  earlier 
form  of  ^mildew;'  the  one  form  being  capable 
of  development  into  the  other,  and  the  fructifica- 
tion characteristic  of  the  two  supposed  genera  hav- 
ing been  evolved  on  one  and  the  same  individual. 
— It  is  another  reputed  species  of  Uredo  (the  U. 
segetum),  which,  when  it  attacks  the  flower  of  the 
wheat,  reducing  the  ears  to  black  masses  of  sooty 
powder,  is  known  as  smut  or  dust-brand.  The 
corn-grains  are  entirely  replaced  by  aggregations  of  ^  raminis '  r 
spores;  and  these,  being  of  extreme  minuteness,  ^^^^^iSew!*^^^' 
are  very  easily  and  very  extensively  diffused.  The 

hunt  or  stinking  rust  is  another  species  of  Uredo  (the  TJ.  fcetida),  which 
is  chiefly  distinguished  by  its  disgusting  odor. 

320.  The  prevalence  of  these  Blights  to  any  considerable  extent  seems 
generally  traceable  to  some  seasonal  influences  unfavorable  to  the 
healthy  development  of  the  cereal;  but  they  often  make  their  appear- 
ance in  particular  localities  through  careless  cultivation,  or  want  of  due 
precaution  in  the  selection  of  seed.  It  maybe  considered  as  certain  that 
an  admixture  of  the  spores  of  any  of  these  Fungi  with  the  corn  grains 
will  endanger  the  plant  raised  from  them;  but  it  is  equally  certain 
that  the  fungi  have  little  tendency  to  develop  themselves  in  plants 
that  are  vegetating  with  perfect  healthf ulness.  The  wide  prevalence  of 
such  blights  in  bad  seasons  is  not  difficult  to  account  for,  if  it  be  true  (as 
the  observations  of  Mr.  John  Marshall  several  years  since  rendered  prob- 
able) that  there  are  really  very  few  wheat-grains,  near  the  points  of 
which  one  or  two  sporules  of  Fungi  may  not  be  found,  entangled  among 
their  minute  hairs;  and  it  may  be  fairly  surmised  that  these  germs  remain 
dormant,  unless  an  unfavorable  season  should  favor  their  development  by 
inducing  an  unhealthy  condition  of  the  wheat-plant. — The  same  general 
doctrine  probably  applies  to  the  Peronospora,  which  has  a  large  share  in 
the  production  of  the  "  Potato-disease;  "  and  to  the  Oidium,  which  has 
a  like  relation  to  the  Vine-disease  ^'  that  was  prevalent  for  some  years 
through  the  south  of  Europe.  There  seems  no  doubt,  that  in  the  fully 
developed  disearse,  the  Fungus  is  always  present;  and  that  its  growth  and 


324: 


THE  MICROSCOPE  AND  ITS  REVELATIONS. 


multiplication  have  a  large  share  in  the  increase  and  extension  of  the 
disorder,  just  as  the  growth  of  the  Yeast-plant  excites  and  accelerates 
^  fermentation;  while  its  reproduction  enables  this  action  to  be  indefinitely 
extended  through  its  instrumentality.  But  just  as  the  Yeast-plant 
will  not  vegetate  save  in  afermentible  fluid — that  is,  in  a  solution  which, 
in  addition  to  sugar,  contains  some  decomposable  nitrogenous  matter — 
so  does  it  seem  probable,  on  consideration  of  all  the  phenomena  of  the 
Potato  and  Vine  diseases,  that  neither  the  Peronospora  of  the  one  nor 
the  Oidium  of  the  other  will  vegetate  in  perfectly  healthy  plants;  but 


JEcidium  tussilaginis a,  portion  of  the  plant  magnified;  b,  section  of  one  of  the  concepta- 
cles  with  its  spores. 

that  a  disordered  condition,  induced  either  by  forcing  and  therefore 
unnatural  systems  of  cultivation,  or  by  unfavorable  seasons,  or  by  a 
combination  of  both,  is  necessary  as  a  ^predisposing^  condition.  This 
condition,  in  the  case  of  the  Potato-disease,  is  said  by  Prof.  De  Bary  to 
consist  in  an  undue  thinness  of  the  cuticle,  accompanied  by  excessive 
humidity;  whereby  the  sporules  of  the  fungus  will  germinate  on  the 


Clavaria  crispula     a,  portion  of  the  mycelium  magnified. 

surface  of  the  plant,  sending  out  processes  which  penetrate  to  its  inte- 
rior, though  otherwise  germinating  only  on  cut  surfaces. 

321.  In  those  lower  forms  of  this  Class  which  have  been  now  described, 
there  is  not  usually  any  complete  separation  between  the  Nutritive  or 
vegetative,  and  the  Eeproductive  portions  of  the  fabric.  But  such  a  sep- 
aration makes  itself  apparent  in  the  higher;  and  this  in  a  very  curious 
mode.  For  the  ostensible  Fungi,  known  as  Mushrooms,  Toadstools, 
Puff-balls,  etc.,  consist,  in  fact,  of  nothing  else  than  the  organs  of 
goiiidial  fructification  (Fig.  360),  inclosing  an  enormous  mass  of  non- 


PROTOPHYTIC  AND  OTHER  FUNGI. 


325 


sexual  spores;  while  the  nutritive  apparatus  of  these  plants  is  composed 
of  an  indefinite  mycelium,  which  is  a  filamentous  expansion  (Fig.  207, 
a),  composed  of  elongated  branching  cells  interlacing  amongst  each 
other,  but  having  no  intimate  connection;  and  this  has  sucli  an  indefinite- 
ness  of  form,  and  varies  so  little  in  the  different  tribes  of  Fungi,  that  no 
determination  of  species,  genus,  or  even  family,  could  be  certainly  made 
from  it  alone.  A  true  Generative  process  has  not  hitherto  been  detected 
with  certainty  in  these  higher  Fungi,  although  it  has  been  supposed  by 
some  observers  to  be  carried  on  in  the  myceliuDi.  And  their  Life-history 
needs  now  to  be  carefully  restudied,  with  all  the  assistance  derivable 


Development  of  Myxomycetes  :^a,  plasmotiium  of  Didynium  serpula  ;~-b,  successive  stages, 
a,  a',  6,  of  sporosacs  of  Arcijria  flava  c,  ripe  spore  of  Physarum  album  ;  d,  its  contents  escap- 
ing; E,  F,  G,  the  swarm-spore  first  becoming  flagellated,  and  then  amoeboid;  h,  conjugation  of  two 
amoeboids,  which  at  i  have  fused  together,  and  at  j  are  beginning  to  put  out  extensions  and  ingest 
nutriment,  of  which  two  pellets  are  seen  in  its  interior. 

from  our  increased  knowledge  of  the  simpler  types  of  the  group,  and  with 
the  skill  which  can  only  be  acquired  by  considerable  practice  in  Micro- 
scopical investigation. — The  subject,  however,  is  one  of  such  peculiar 
speciality,  that  it  cannot  be  advantageously  pursued  further  in  a 
Treatise  like  the  present. 

/  322.  Many  eminent  Botanists  still  rank  in  the  Fungal  series  of  Pro- 
tophytes  a  very  peculiar  group,  the  Myxomycetes,  the  members  of  which 
pass  a  large  part  of  their  lives  in  a  state  of  what  can  scarcely  be  other- 
wise described  than  as  one  of  Animal  existence.    They  grow  parasitically 


326 


THE  MICROSCOPE  AND  ITS  REVELATIONS. 


upon  decayed  wood,  bark,  heaps  of  decaying  leaves,  tan -beds,  etc.; 
spreading  over  damp  surfaces  as  a  plasmodium,  or  network  of  naked 
protoplasmic  filaments  (Fig.  208,  a),  of  a  soft  creamy  consistence,  and 
usually  of  a  yellowish  color.  The  filaments  of  this  network  exhibit 
active  undulatory  movements,  which  in  the  larger  ones  are  visible  under 
an  ordinary  lens,  or  even  to  the  naked  eye,  but  which  it  requires  micro- 
scopic power  to  discern  in  the  smaller.  With  sufficiently  high  amplifica- 
tion, a  constant  movement  of  granules  may  be  seen  flowing  along  the 
threads,  and  streaming  from  branch  to  branch.  Here  and  there  off- 
shoots of  the  protoplasm  are  projected,  and  again  withdrawn,  in  the 
manner  of  the  pseudopodia  of  an  Amceba ;  while  the  whole  organism  may 
be  occasionally  seen  to  abandon  the  support  over  which  it  had  grown, 
and  to  creep  over  neighboring  surfaces,  thus  far  resembling  in  all 
respects  a  collosal  ramified  amceba.  It  is  also  curiously  sensitive  to  light, 
and  may  sometimes  be  found  to  have  retreated  during  the  day  to  the 
dark  side  of  the  leaves  or  into  the  recesses  of  the  tan  over  which  it  had 
been  growing,  and  again  to  creep  out  on  the  approach  of  night. — After  a 
time  there  arise  from  the  surface  of  this  plasmodium  oval  capsules  or 
sporangia  (b,  a,  a\  5),  within  which  the  reproductive  bodies  or  ^spores' 
are  developed,  and  which  burst  when  mature  to  give  them  exit.  Each 
^  spore  ^  is  a  spherical  cell  (c)  inclosed  in  a  delicate  membranous  wall; 
and  when  it  falls  into  water  this  wall  undergoes  rupture  (d)  and  an 
Amoeba-like  body  (e)  escapes  from  it,  consisting  of  a  little  mass  of  pro- 
toplasm, with  a  round  central  nucleus  inclosing  a  nucleolus,  and  a  con- 
tractile vesicle.  This  soon  elongates  (f),  and  becomes  pointed  at  one 
end,  whence  a  \o\\gjlagellum  is  put  forth,  the  lashing  action  of  which 
gives  motion  to  the  body.  After  a  time,  the  flagellum  disappears,  and 
the  active  movements  of  the  spore  cease;  but  it  now  begins  to  put  forth 
and  to  withdraw  finger-like  pseudopodia,  by  means  of  which  it  creeps 
about  like  an  Amceha,  and  feeds,  like  that  Rhizopod,  upon  solid  particles 
which  it  engulfs  within  its  soft  protoplasm.  A  ^conjugation'  then 
takes  place  between  two  of  these  Myxaynoehce  (h),  their  substance  under- 
going a  complete  fusion  into  one  body  (i),  from  which  extensions  are  put 
forth  (k),  that  constitute  the  beginning  of  a  new  plasmodium.  This 
continues  to  grow  by  the  ingestion  and  assimilation  of  the  solid  nutri- 
ment which  it  takes  into  its  substance;  and,  by  the  ramification  and 
inosculations  of  these  extensions,  a  network  is  formed  resembling  that 
from'  which  it  originated,  to  bear  sporangia  in  its  turn,  from  which  a 
new  cycle  will  commence. 

323.  Under  certain  conditions  not  yet  perfectly  understood,  the 
Myxomycetes  have  been  observed  to  pass  from  the  active  into  the  ^rest- 
ing' state;  and  this  may  occur  both  in  the  amoeboid  spores  and  in  the 
Plasmodium.  The  former  return  to  the  spherical  form,  and  surround 
themselves  with  a  firm  envelope;  and  in  this  ^encysted'  condition  they 
may  be  dried-up  so  as  to  be  carried  about  as  dust,  resuming  their  origi- 
nal activity  when  again  placed  in  water.  When  the  plasmodium  is 
about  to  pass  into  the  ^resting'  state,  it  withdraws  its  finer  branches, 
and  expels  such  solid  ingesta  as  may  be  included  in  it;  and  its  motions 
then  gradually  cease.  It  then  either  breaks  up  into  a  multitude  of  poly- 
hedral cells  (?),  which,  however,  remain  connected  in  one  body,  that 
dries  into  a  horny  brittle  mass  termed  ^sclerotium;'  or  separates  into  a 
number  of  fragments  of  unequal  size,  which  take  a  spherical  form  and 
become  '  encysted '  in  a  double  envelope.  Both  these  ^  resting '  forms 
may  undergo  desiccation  without  the  loss  of  their  vitality.    When,  after 


PROTOPHYTIC  AND  OTHER  FUNGI. 


327 


many  months,  the  dry  sclerotium  is  placed  in  water,  it  swells  np,  and  its 
cells  (?)  again  flow  together  into  a  protoplasmic  mass,  which  soon  re- 
sumes its  former  life  as  2i  plasmodiiim.  So  when  the  thick- walled  cysts, 
after  being  long  desiccated,  are  placed  in  water,  their  cysts  rupture,  and 
their  protoplasmic  bodies  issue  forth,  to  lead  the  life  of  Ammbce,  and  to 
form  fresh  plasmodia,  either  by  themselves,  or  by  fusion  with  other  like 
bodies/ 

3:^4.  Another  most  interesting  connecting  link  between  the  Vegetable 
and  Animal  kingdoms,  is  an  organism  discovered  by  Mr.  W.  Archer  — 
sometimes  within  the  leaf-cells  of  Sphagnum  (§  329),  and  sometimes  at- 
tached to  the  surface  of  its  leaves — to  which  he  has  given  the  name  of 
Chlamidomyxis  labyrmthuloides  (Fig.  209).  In  its  early  condition, 
whilst  still  inhabiting  the  Sphagnum-cells,  this  parasite  resembles  a  large 
thick-walled  Vegetable  cell,  with  either  green  or  red  cell-contents;  and  is 
found  to  consist  of  a  firm  many-layered  envelope  which  shows  a  distinctly- 
cellulose  reaction,  inclosing  a  colorless  hyaline  substance,  through  which 
a  great  multitude  of  granules  are  dispersed,  some  of  them  of  a  bright  red, 
and  others  of  a  yellowish-green  color, — the  numbers  of  the  two  bearing 
so  constant  an  inverse  proportion  to  each  other,  as  to  make  it  likely  that 
the  red  are  produced  by  a  color-change  in  the  green.  If  this  state 
alone  were  known  to  us,  we  should  have  no  hesitation  in  regarding  the 
organism  as  a  Vegetable  cell,  the  '  endoplasm'  of  which  consists  of  proto- 
plasm with  chlorophyll-granules  dispersed  through  it.  But  as  it  aug- 
ments in  size,  it  produces  a  bulging  of  the  wall  of  the  Sphagnum-cell,  by 
the  rupture  of  which  it  makes  its  way  to  the  surface;  and  a  new  stage  in 
its  history  then  commences.  Though  the  many-layered  cellulose  wall  is 
so  firm  as  to  resis^  a  considerable  amount  of  external  pressure,  it  bursts 
open  from  within,  and  the  endoplasm  then  streams  forth,  carrying  with 
it  its  imbedded  granules.  The  protoplasmic  trunk,  almost  directly  that 
it  leaves  the  cell-cavity,  begins  to  subdivide  into  branches,  from  which 
others  are  put  forth;  and  by  this  continued  ramification  and  the  inoscu- 
lation of  the  offshoots,  an  extended  network  is  formed,  consisting  of 
threads  of  extreme  tenuity.  In  constant  motion  along  these  are  seen 
minute  fusiform  particles  of  a  bluish-green  color,  which  are  obviously 
identical  with  the  round  granules  of  the  central  mass,  these  changing 
their  shape  as  they  go  forth  to  wander  along  the  filaments.  Sometimes 
the  protoplasm  accumulates  in  particular  spots,  forming  ^islands'  {a,  a), 
each  of  which  may  become  a  centre  of  fresh  radiation  for  hyaline 
threads.  These  accumulations  frequently  take  place  round  Diatoms, 
Desmids,  or  other  minute  Vegetable  organisms  (5);  which,  being  thus 
imbedded  in  the  extensions  of  the  protoplasmic  body,  are  drawn  towards 
it  by  their  retraction,  and  at  last  engulfed  within  it.  It  would  appear 
that  the  whole  of  the  protruded  endoplasm  may  be  retracted  into  the 
original  cell-cavity,  and  that  this  may  be  closed  up  again  by  the  forma- 


^  The  very  peculiar  history  of  the  Myxomycetes  (previously  known  as  Myxo- 
gastric  Fungi)  was  first  investigated  by  De  Bary,  who  was  disposed  to  regard 
them  as  Animals  (*Die  Mycetozoen,'  in  '*Zeitschr.  f.  w.  ZooL,"  Bd.  x.,  1860). 
The  subject  w^as  taken  up  by  Cienkowski,  the  results  of  whose  careful  study  of  it 
will  be  found  in  his  admirable  Memoirs,  *Zur  Entwickelungsgeschichte  der 
Myxomyceten,'  in  Pringsheim's  Jahrbiicher,"  Bd.  iii.  (1863),  pp.  325,  400,  and 
*  Ueber  einige  Rhizopoden  und  verwandte  Organismen,'  in  Archiv  f.  Mikr. 
Anat.,"  Bd.  xii.  (1876)  p.  15;  and  he  also  is  disposed  to  rank  this  group  in  the 
Animal  kingdom.  On  the  other  hand,  Prof.  Sachs  and  other  high  Botanical  au- 
thorities continue  to  rank  it  among  Fungi. 

2    Quart.  Journ.  of  Microsc.  Science,"  Vol.  xv.  (1875),  p.  107. 


328 


THE  MICROSCOPE  AND  ITS  REVELATIONS. 


tion  of  a  new  layer  of  cellulose  within  the  old;  for  the  indigestible 
parts  of  yarious  organisms,  that  must  have  been  introduced  in  the  man- 
ner just  described,  are  often  distinguishable  through  the  walls  of  com- 
pletely closed-in  specimens.  Mr.  Archer  has  been  unable  to  detect  a 
^nucleus/  either  in  the  body  of  his  Chlamido^nyxis,  or  in  any  of  its  ex- 
tensions; but  ^  contractile  vacuoles/  executing  pretty  regular  rhythmical 
movements  are  to  be  seen,  not  only  in  the  body  and  primaiy  stem  (in 
which  they  are  usually  very  numerous),  but  also  in  the  branches,  and  not 


iPiG.  20D. 


Chlamidomyxis  labyrinthuloides  .'—showing  the  protoplasmic  mass  extending  itself  from  the 
ruptured  cellulose  envelope,  and  forming  a  network  whose  threads  are  traversed  by  fusiform  par- 
ticles; a,  a,  isolated  masses  of  protoplasm;  6,  a  captured  Navicula  about  to  be  drawn  into  the  pro- 
toplasmic mass. 

unfrequently  in  the  ^  islands '  also.  Thus  in  its  extended  condition  this 
creature  leads  a  life  which  is  essentially  Animal,  corresponding  in  every 
particular  with  that  of  the  ^  reticularian  '  RMzopods  hereafter  to  be  de- 
scribed (Chap.  X.). — Nothing  is  yet  known  of  its  Eeproduction.  Mr. 
Archer  has  met  with  large  individuals,  the  contents  of  whose  many- 
layered  cellulose  wall  had  divided  itself  into  a  number  of  smaller  orange- 


PROTOPHYTIC  AND  OTHER  FUNGI. 


329 


colored  spheres,  of  nearly  equal  size,  each  of  which  had  its  own  cellulose 
wall;  and  it  can  scarcely  be  doubted  that  on  the  escape  of  these  from  the 
parent  cyst,  each  would  lead  an  independent  life  resembling  that  of  its 
progenitor.  It  seems  probable,  moreover,  that  the  outlying  masses  of 
the  protoplasmic  extension  may  detach  themselves  and  live  independ- 
ently, each  forming  a  cellulose  envelope  for  itself. — But  until  ^conjuga- 
tion' or  some  other  kind  of  sexual  union  shall  have  been  discovered  in 
this  curious  organism,  we  cannot  be  said  to  know  its  whole  life-history; 
and  the  peculiar  interest  which  attaches  to  it  renders  the  further  study  of 
it  in  the  higliest  degree  desirable.  Ic  may  be  hoped  that  the  excellent 
observer  by  whom  it  has  been  brought  to  our  knowledge,  may  ere  long 
find  himself  able  to  supply  the  missing  link. 

325.  LiCHEi^s. — The  Microscopic  study  of  this  group  has  latterly  ac- 
quired a  new  interest  for  the  Botanist,  from  the  remarkable  discovery 
announced  in  its  complete  form  by  Schwendener  in  1869^  (and  now  ac- 
cepted by  the  highest  authorities)  that  instead  of  constituting  a  special 
type  of  Thallophytes,  parallel  to  Algce  (with  which  they  correspond  in 
their  vegetative  characters)  and  Fungi  (to  which  they  are  more  allied  in 
fructificatioii)^  they  are  really  to  be  regarded  as  composite  structure,  hav- 
ing an  Algal  base,  on  which  Ascomycetous  Fungi  (§  318)  have  sown 
themselves  and  live  parasitically.  As,  however,  they  do  not  furnish  ob- 
jects of  interest  to  the  ordinary  Microscopist  (the  peculiar  density  of  their 
structure  rendering  a  minute  examination  of  it  more  than  ordinarily 
difficult),  nothing  more  than  a  general  account  of  their  curious  organiza 
tion  will  here  be  attempted. — The  Algal  ^thallus'  of  a  Lichen  belongs  to 
the  group  of  Pahnellacem  (§  243)  or  its  allies;  and  consists  of  cells  termed 
gonidia—MmdiWy  green,  but  sometimes  red  or  bluish-green — interspersed 
among  long  cellular  filaments.  The  proportion  between  these  two  com- 
ponents of  the  thallus  varies  in  different  examples  of  the  type.  Thus,  in 
the  simplest  Wall-lichens,  the  Palmella-like  ^primordial  ceir  gives 
origin,  by  the  ordinary  process  of  cell-division,  to  a  single  layer  of  cells, 
which  spreads  itself  over  the  stony  surface  in  a  more  or  less  circular 
form;  and  the  ^thallus,'  which  increases  in  thickness  by  the  formation  of 
new  layers  upon  its  free  surface,  has  no  very  defined  limit,  and,  in  con- 
sequence of  the  slight  adhesion  of  its  components,  is  said  to  be  pulveru- 
lent.' But,  in  the  more  complex  forms  of  Lichens,  the  thallus  is  mainly 
composed  of  long  fibre-cells,  which  dip  down  into  the  superficial  layers 
of  the  bark  of  the  trees  on  which  they  grow,  and  form  by  their  inter- 
weaving a  hard  crustaceous  ^  thallus '  in  which  the  gonidia  are  imbedded, 
sometimes  irregularly,  sometimes  in  definite  layers,  covered  by  an 
envelope  of  interlacing  filaments.  It  is  from  this  Algal  portion  of  the 
structure,  that  the  soredia  of  Lichens  are  formed;  which  are  littlj  projec- 
tions of  the  surface,  composed  of  single  or  aggregate  gonidia,  invested  by 
fibre-cells,  and  falling,  when  dry,  into  a  powder,  of  which  every  particle 
is  a  bud,  capable  of  reproducing  the  plant  from  which  it  proceeded. 

326.  fructification  of  Lichens,  on  the  other  hand,  is  really  the 
production  of  their  Fungal  overgrowths,  which  are  nourished  by  the  Al- 
gal vegetation.  The  Lichen-forming  Fungi,  in  fact,  live  upon  their 
Algal  hosts,  like  the  Entophytic  Fungi  (such  as  the  ^blights'  of  corn, 
§  219)  which  infests  the  higher  forms  of  Vegetation  ;  each  of  the  former 

*  See  his  memorable  work  '  *  Ueber  die  Algentypen  der  Flechtengonidien  " 
(Basel,  1869),  which  is  said  by  Prof.  Sachs  Text-book  of  Botany,"  p.  273)  to 
have  settled  for  the  future  the  place  of  Lichens  among  the  Ascomycetes^  and  Sir 
J.  D.  Hooker's  Presidential  Address  to  the  Royal  Society,  1878. 


830 


THE  MICROSCOPE  AKD  ITS  REVELATIONS. 


choosing  its  own  Alga,  just  as  the  latter  mostly  attach  themselves  to 
])articular  victims.  ^^The  peculiarity  in  the  parasitism  of  the  Lichen- 
fungi  lies  in  the  fact  that  they  are  not  attached  to  their  host  externally 
at  any  one  particular  spot,  and  do  not  penetrate  into  its  cells,  but  weave 
themselves  round  them,  and  inclose  them  in  their  hyplial  tissue.''  (Sachs, 
loc.  cit.) — The  formation  of  sexually  produced  'spores'  takes  place  in 
asci  or  ^spore-cases,'  arranged  vertically  in  the  midst  of  straight  enlon- 
gated  sterile  cells  termed  paraphyses,  so  as  to  form  a  layer  that  lies  either 
on  the  surface  of  cup-shaped  receptacles  termed  apothecia,  or  is  completely 
inclosed  within  perithecia.  Each  of  the  asci  contains  a  definite  number 
of  spores  (usually  eight,  but  always  a  multiple  of  two),  which  are  pro- 
jected from  the  receptacles  with  some  force;  and  their  emission,  which 
seems  to  be  due  to  the  dilferent  effects  of  moisture  upon  the  several  layers 
of  the  receptacle,  is  of teii  kept  up  continuously  for  some  time.  The 
formation  of  these  asci^  as  in  the  case  of  the  ordinary  Ascomycetes 
(§  318),  is  the  result  of  a  sexual  union,  which  takes  place  between  the 
male  spermatia  and  the  female  tricliogyne.  These  spermatia  are  produced 
within  spermogoniay  which  resembles  on  a  very  minute  scale  the  male 
receptacles  of  the  Fucacece  (§  328);  being  budded  off  from  the  exterior  of 
the  cellular  filaments  that  line  those  cavities,  and,  when  mature,  escap- 
ing in  great  numbers  from  their  orifices.  Having  no  power  of  spontane- 
ous movement,  they  must  probably  be  conveyed  by  the  infiltration  of 
rain-water  to  a  trichogyne  (resembling  that  of  the  CeramiacecBy  §  330) 
which  lies  imbedded  in  the  tissue  beneath  ;  and  when  they  have  imparted 
their  fertilizing  influence  to  the  contents  of  the  ascogonhun,  at  its  base, 
these  develop  themselves  into  a  spore-bearing  apotkecium — the  whole 
mass  of  spores  which  this  contains  being  the  product  of  the  cell-division 
of  the  originally  fertilized  ^oospore.' 


MICROSCOPIC  STRUCTURE  OF  HIGHER  CRYPTOGAMIA- 


331 


CHAPTER  VIII. 
MICROSCOPIC  STRUCTURE  OF  HIGHER  CRYPTOGAMIA. 

327.  From  the  simple  Protophytes,  whose  minuteness  causes  their 
entire  fabrics  to  be  fitting  objects  for  Microscopic  examination,  we  pass 
to  those  higher  forms  of  Vegetable  life  whose  larger  dimensions  require 
that  they  should  be  analyzed  (so  to  speak)  by  the  examination  of  their 
separate  parts.  And  in  the  present  Chapter  we  shall  bring  under  notice 
some  of  the  principal  points  of  interest  to  the  Microscopist  which  are 
presented  by  the  Cryptogamic  series;  commencing  with  those  simpler 
Algae  which  scarcely  rank  higher  than  some  of  the  Protophytes  already 
described,  and  ending  with  the  ferns  and  their  allies,  which  closely 
abut  upon  the  Phanerogamia  or  Flowering  Plants.    In  ascending  this 


series,  we  shall  have  to  notice 
a  gradual  differentiation  of  or- 
gans; those  set  apart  for  Eepro- 
duction  being  in  the  first  place 
separated  from  those  appropri- 
ated to  Nutrition;  while  the 
principal  parts  of  the  Nutritive 
apparatus,  which  are  at  first  so 
blended  into  a  uniform  expan- 
sion or  tliallus  that  no  real  dis- 
tinction exists  between  root, 
stem,  and  leaf,  are  progressively 
evolved  on  types  more  and  more 
peculiar  to  each  respectively, 
and  have  their  functions  more 
and  more  limited  to  themselves 
alone.  Hence  we  find  a  dif- 
ferentiation,^ not  merely  in  the 
external  form  of  organs,  but 
also  in  their  intimate  structure; 
its  degree  bearing  a  close  cor- 
respondence to  the  degree  in 
which  their  functions  are  re- 
spectively specialized  or  limited 
t«  particular  actions.  But  this 
takes  place  by  very  slow  grada- 
tions; a  change  of  external  form 
often  showing  itself,  before 
there  is  any  decided  differenti- 
ation either  in  structure  or  function. 
144),  whatever  may  be  the  extent 


Terminal  portion  of  branch  of  Sphacelaria  cirJ 
'  '      "       Tribuloides.  the  ter- 


rhosa ;  b,  lateral  branchlet  of  S. 
minal  cell  of  which  is  emitting  antherozoids. 


Thus  m  the  simple  Ulvm  (Fig. 
of  the  thallus,  every  part  has 


332 


THE  MICROSCOPE  AND  ITS  REVELATIONS- 


exactly  the  same  structure,  and  performs  the  same  actioYis,  as  every 
other  part;  living  for  and  ly  itself  alone.  And  though,  when  wo 
pass  to  the  higher  Sea- weeds,  such  as  the  common  Fucus  and  Laminaria, 
we  observe  a  certain  foreshadowing  of  the  distinction  between  Eoot, 
Stem,  and  Leaf,  this  distinction  is  very  imperfectly  carried  out;  the  root- 
like and  stem-like  portions  serving  for  little  else  than  the  mechanical  at- 
tachment of  the  leaf-like  part  of  the  plant,  and  each  still  absorbing  and 
assimilating  its  own  nutriment,  so  that  no  transmission  of  fluid  takes 
place  from  one  portion  of  the  fabric  to  another.    There  is  not  yet  any 

departure  from  the  simply  cellular 
type  of  structure;  the  only  modi- 
fication being  that  the  several 
layers  of  cells,  where  many  exist, 
are  of  different  sizes  and  shapes, 
the  texture  being  usually  closer 
on  the  exterior  and  looser  within; 
and  that  the  texture  of  the  stem 
and  roots  is  denser  than  that  of 
the  leaf-like  expansions  or  fronds. 
The  group  of  Melanospermous  or 
olive-green  sea-weeds,  which  in 
the  family  Fucacece  exhibits  the 
highest  type  of  Algal  structure, 
presents  us  with  the  lowest  in  the 
family  Ectocarpacem  ;  which,  not- 
withstanding, contains  some  of  the 
most  elegant  fabrics  that  are  any- 
where to  be  found  in  the  group, 
the  full  beauty  of  which  can  only 
be  discerned  by  the  Microscope. 
Such  is  the  case,  for  example, 
with  the  Sphacelaria,  a  small  and 
delicate  sea-weed,  which  is  very 
commonly  found  parasitic  upon 
larger  Algae,  either  near  low-water 
mark,  or  altogether  submerged; 
its  general  form  being  remarkably 
characterized  by  a  symmetry  that 
extends  also  to  the  individual 
branches  (Fig.  210,  a),  the  ends 
of  which,  however,  have  a  decayed 
look  that  seems  to  have  suggested  the  name  of  the  genus  (from  the 
Greek  acpan^Xo^,  gangrene).  This  apparent  decay  really  consists  in  the 
resolution  of  the  endochrome  of  the  terminal  cells  in  antherozoids,  which 
when  mature,  escape  by  an  opening  with  a  long  tubular  neck,  which 
forms  itself  in  the  wall  of  the  sphacela.  The  same  happens  with  the  ter- 
minal cells  of  the  peculiar  lateral  branchlets,  which  are  known  as  propa- 
gative  buds,  as  is  shown  at  B.  The  germ-cells  have  not  been  certainly 
recognized;  but  they  are  believed  to  be  produced  in  what  have  been  con- 
mdered  as  propagative  buds  in  other  individuals. 

S  328.  In  the  FucacecB,  the  Generative  apparatus  is  contained  in  the 
bulbose  ^  receptacles,^  which  are  borne  at  the  extremities  of  the  fronds. 
In  some  species,  as  the  Fucus  platycarpus,  the  same  receptacles  contain 
both  ^sperm-cells^  and  ^ germ-cells; ^  in  others,  these  two  sexual  ele- 
ments are  disposed  in  different  receptacles  on  the  same  plant;  whilst  in  the 


Vertical  section  of  receptacle  of  Fucus  platycar- 
pus,  lined  with  filaments,  among  which  lie  the 
antheridial  cells,  and  the  oogonia  containing  octo- 
spores. 


MICHOSCOPIC  STRUCTURE  OF  HIGHER  CRYPTOGAMIA. 


333 


commonest  of  all,  F,  vesiculosus  (bladder-wrack),  they  are  limited  to 
dilferent  invividuals.  When  a  section  is  made  through  one  of  the  flat- 
tened receptacles  of  F,  platycarpus,  its  interior  is  seen  to  be  a  nearly 
globular  cavity  (Fig.  211),  lined  with  filamentous  cells,  some  of  which  are 
greatly  elongated,  so  as  to 


Fig.  212. 


Antheridia  and  antherozoids  of  Fucus  platycarpus : — a, 
branching  articulated  hairs,  detached  from  the  walls  of  the 
receptacle,  bearing  antheridia  in  different  stages  of  deve- 
lopment; B,  antherozoids,  some  of  them  free,  others  still 
included  in  their  antheridial  cells. 


project  through  the  pore 
by  which  the  cavity  opens 
on  the  surface.  Among 
these  are  to  be  distinguish- 
ed, towards  the  period  of 
their  maturity,  certain  fila- 
ments (Fig.  212,  a),  whose 
granular  contents  acquire 
an  orange  hue,  and  gradu- 
ally shape  themselves  into 
oval  bodies  (b),  each  with 
an  orange-colored  spot,  and 
two  long  thread-like  appen- 
dages, which,  when  dis- 
(jharged  hj  the  rupture  of 
the  containing  cell,  have 
for  a  time  a  rapid  undula- 
tory  motion,  whereby  these 
^  antherozoids '  are  diffused 
through  the  surrounding 
liquid.  Lying  amidst  the 
filamentous  mass,  near  the 
walls  of  the  cavity,  are  seen 

(Fig.  211)  numerous  dark  pear-shaped  bodies,  which  are  the  odgonia,  or 
parent-cells  of  the  ^ germ-cells.^  Each  of  these  oogonia  gives  origin,  by 
binary  subdivision,  to  a  cluster  of  eight  germ-cells  or  oospheres,  which  is 
thence  known  as  an  ^octospore;'  and  these  are  liberated  from  their  en- 
velopes before  the  act  of  fertilization  takes  place.  This  act  consists  in 
the  swarming  of  the  antherozoids  over  the  surface  of  the  oospheres,  to 
which  they  communicate  a  rotatory  motion  by  the  vibration  of  their  own 
filaments.  In  the  hermaphrodite  Fuci  it  takes  place  within  the  recepta- 
cles, so  that  the  oospheres  do  not  make  their  exit  from  the  cavity  until 
after  they  have  been  fecundated;  but  in  the  monoecious  and  dioecious 
species,  each  kind  of  receptacle  separately  discharges  its  contents,  which 
come  into  contact  on  their  exterior.  The  antheridial  cells  are  usually 
ejected  entire,  but  soon  rupture  so  as  to  give  exit  to  their  filaments; 
and  the  ^octospores^  separate  into  their  component  oospheres,  which, 
meeting  with  antherozoids,  are  fecundated  by  them.  The  fertilized  oo- 
spores soon  acquire  a  new  and  firmer  envelope;  and,  under  favorable  cir- 
cumstances, they  speedily  begin  to  develop  themselves  into  new  plants. 
The  first  change  is  the  projection  and  narrowing  of  one  end  into  a  kind 
of  footstalk,  by  which  the  oospore  attaches  itself,  its  form  passing  from 
the  globular  to  the  pear-shaped;  a  partition  is  speedily  observable  in  its 
interior,  its  single  cell  being  subdivided  into  two;  and  by  a  continuation 
of  a  like  process  of  duplication,  first  a  filament  and  then  a  frondose  ex- 
pansion is  produced,  which  gradually  evolves  itself  into  the  likeness  of 
the  parent  plant. 

329.  The  whole  of  this  process  may  be  watched  without  difficulty,  by 
obtaining  specimens  of  F,  vesiculosus  at  the  period  at  which  the  fructifi- 
cation is  shown  to  be  mature  by  the  recent  discharge  of  the  conte«its  of 


334 


THE  MICROSCOPE  AND  ITS  REVELATIONS. 


the  conceptacles  in  little  gelatinous  masses  on  their  orifices;  for  if  some 
of  the  spores  which  have  been  set  free  from  the  olive-green  (female) 
receptacles  be  placed  in  a  drop  of  sea-water  in  a  very  shallow  cell,  and  a 
small  quantity  of  the  mass  of  antherozoids,  set  free  from  the  orange- 
yellow  (male)  receptacles,  be  mingled  with  the  fluid,  they  will  speedily 
/be  observed,  with  the  aid  of  a  magnifying  power  of  200  or  250  diameters, 
\to  go  through  the  actions  just  described;  and  the  subsequent  pr*)cesses  of 
germination  may  be  watched  by  means  of  the  ^growing  slide/ ^  The 
winter  months,  from  December  to  March,  are  the  most  favorable  for  the 
observation  of  these  phenomena;  but  where  Fuci  abound,  some  indi- 
viduals will  usually  be  found  in  fructification  at  almost  any  period  of  the 
year. — Even  in  the  FucacecB,  according  to  recent  observations,  a  multi- 
plication by  ^  zoospores,^  like  that  of  Ulvacece  (§  245),  also  takes  place; 
these  bodies  being  produced  within  certain  of  the  cells  that  form  the 
superficial  layer  of  the  frond,  and  swimming  about  freely  for  a  time  after 
their  emission,  until  they  fix  themselves  and  begin  to  grow.    That  they 

are  to  be  considered  as  gemmm 
Pigt2M  buds),  and  not  as  genera- 

tive products,  appears  certain 
from  the  fact  that  they  will 
vegetate  without  the  assist- 
ance of  any  other  bodies; 
whereas  the  autherozoids  of 
themselves  never  come  to  any- 
thing; while  the  octospores 
undergo  no  farther  changes, 
but  decay  away  (as  M.  Thuret 
has  experimentally  ascertain- 
ed) if  not  fecundated  by  the 
antherozoids. 

330.  Among  the  Rliodo- 
spermecB,  or  red  Sea- weeds, 
also,  we  find  various  simple 
but  most  beautiful  forms, 
which  connect  this  group  with 
the  more  elevated  Proto- 
phytes,  especially  with  the 
family  ChcetophoracecB  (§  256); 
such  delicate  feathery  or  leaf- 
like  fronds  belong  for  the 
most  part  to  the  family  Cera- 
miacece,  some  members  of 
which  are  found  upon  every 
part  of  our  coasts,  attached 
either  to  rocks  or  stones  or  to  larger  Algae,  and  often  themselves  affording 
an  attachment  to  Zoophytes  and  Polyzoa.  They  chiefly  live  in  deeper 
water  than  the  other  sea- weeds;  and  their  richest  tints  are  only  exhibited 
when  they  grow  under  the  shade  of  projecting  rocks,  or  of  larger  dark-co- 
lored Algae.  Hence  in  growing  them  artificially  in  Aquaria,  it  is  requisite 
to  protect  them  from  an  excess  of  light;  since  otherwise  they  become  un- 
healthy. Various  species  of  the  genera  Ceramium,  Griffithsia,  Calli- 
thamnion,  and  Ptilota^  are  extremely  beautiful  objects  for  low  powers, 
when  mounted  in  glycerine  jelly. — The  only  mode  of  propagation  which 

*  A  shallow  cell  should  be  used,  so  as  to  keep  the  pressure  of  the  thin  glass 
from  the  minute  bodies  beneath,  whose  movements  it  will  otherwise  impede. 


Arrangement  of  Tetraspores  in  Carpocaulon  mediter- 
raneum  :—A,  entire  plant;  b,  longitudinal  section  of 
spore-bearing  branch.  (N.B.  Where  only  three  tetra- 
spores are  seen,  it  is  merely  because  the  fourth  did  not 
happen  to  be  so  placed  as  to  be  seen  at  the  same  view.) 


MICROSCOPIC  STRUCTURE  OF  HIGHER  CRYPTOGAMIA. 


335 


was  until  recently  known  to  exist  in  this  group,  is  the  production  and 
liberation  of  '  tetraspores  ^  (Fig.  213,  b),  formed  by  the  binary  subdivision 
of  the  endochromes  of  special  cells,  which  sometimes  form  part  of  the 
general  substance  of  the  frond,  but  sometimes  congregate  in  particular 
parts,  or  are  restricted  to  special  branches.  If  the  second  binary  division 
takes  place  in  the  same  direction  as  the  first,  the  spores  forming  the 
tetraspore  are  arranged  in  linear  series;  but  if  its  direction  is  trans- 
verse to  that  of  the  first,  the  four  spores  cluster  together.  These, 
when  separated  by  the  rupture  of  their  envelope,  do  not  comport 
themselves  as  '  zoospores,^  but,  being  destitute  of  propulsive  organs,  are 
passively  dispersed  by  the  motion  of  the  sea  itself.  Their  production, 
however,  taking  place  by  simple  cell-division,  and  not  being  the 
result  of  any  form  of  sexual  conjunction,  the  ^tetraspores^  of  the 
Rliodospermece  must  be  regarded,  like  the  zoospores  of  the  Ulvacece, 
as  go7iidia,  analogous  rather  to  the  huds  than  to  the  seeds  of 
higher  Plants. — It  is  now  known  that  a  true  Generative  process  takes 
place  in  this  group,  but  the  sexual  organs  are  not  usually  found  on  the 
plants  vv^hich  produce  tetraspores;  so  that  there  would  appear  to  be  an 
alternation  between  the  two  modes  of  propagation.  Antheridial  cell  are 
found,  sometimes  on  the  general  surface  of  the  frond,  more  commonly  at 
the  ends  of  branches,  and  occasionally  in  special  conceptacles.  Their 
contents,  however,  are  not  motile  ^antherozoids,'  but  minute  rounded 
particles  having  no  power  of  spontaneous  movement.  Sometimes  on  the 
same  individuals  as  the  antheridia,  and  sometimes  on  different  ones,  arc 
organs  that  curiously  prefigure  the  pistil  in  flowering  plants;  each  con- 
sisting of  a  projecting  cluster  of  cells,  from  which  arises  a  long  cell-tube 
termed  the  trichogyne.  Fertilization  is  effected  by  the  attachment  of 
one  of  the  antheridial  particles  to  the  trichogyne,  the  walls  of  which  are 
absorbed  at  that  spot,  so  that  the  fertilizing  material  passes  down  its  tube 
to  the  cluster  of  cells  at  its  base;  and  ^oospores'  are  thus  formed  either 
among  these  or  in  adjacent  cells. — In  the  true  Corallines,  which  are  Rliodo- 
sperms  whose  tissue  is  consolidated  by  calcareous  deposit,  the  tetraspores 
are  developed  within  a  ceramidium^ 
which  is  an  urn-shaped  case,  furnished 
with  a  pore  at  its  summit,  and  contain- 
ing a  tuft  of  pear-shaped  spores  arising 
from  the  base  of  its  cavity. 

331.  HepaiiccB, — Quitting  now  the 
Algal  type,  and  entering  the  series  of 
Terrestrial  Cryptogams,  we  have  first 
to  notice  the  little  group  of  Hepaticce  or 
Liverworts,  which  is  intermediate  be- 
tween Lichens  and  ordinary  Mosses; — 
agreeing  rather  with  the  Algal  thallus 
of  the  former  in  its  general  mode  of 
growth,  whilst  approaching  the  latter 
in  its  fructification.  This  group  presents 
numerous  objects  of  great  interest  to  the 
Microscopist;  and  no  species  is  richer  in  these  than  the  v^ry  common 
Marchantia  Polymorplia,  which  may  often  be  found  growing  between 
the  paving  stones  of  damp  court-yards,  but  which  particularly  luxuriates 
in  the  neighborhood  of  springs  or  waterfalls,  where  its  lobed  fronds  are 
found  covering  extensive  surfaces  of  moist  rock  or  soil,  adhering  by  the 
radical  (root)  filaments  which  arise  from  their  lower  surface.  At  the 
period  of  fructification  these  fronds  send  up  stalks;  which  carry  at  their 


.Fic?i 


Frond  of  Marchantia  polymorpha,  y[ 
gemmiparous  conceptacles,  and  lobed 
ceptacles  bearing  Archegonia. 


with 
re- 


336 


THE  MICROSCOPE  AND  ITS  REVELATIONS. 


summits  either  round  shield-like  disks,  or  radiating  bodies  that  bear 
some  resemblance  to  a  wheel  without  its  tire  (Pig.  214).  The  former 
carry  the  male  organs,  or  antheridia ;  while  the  latter  in  the  first 
instance  bear  the  female  organs,  or  archegonia,  which  afterwards  give 
place  to  the  sporangia  or  spore-cases.'  . 

332.  The  green  surface  of  the  frond  of  Marchantia  is  seen  under  a 
low  magnifying  power  to  be  divided  into  minute  diamond-shaped  spaces 
(Fig.  215,  A,  a,  a)  bounded  by  raised  bands  (<?,  c)-^  every  one  of  these 
spaces  has  in  its  centre  a  curious  brownish-colored  body  (5,  b),  with  an 


structure  of  frond  of  Marchantia  poly- 
morpha  .'—a,  portion  seen  from  above ;  a, 
a,  lozenge-shaped  divisions ;  b,  5,  stomata 
in  the  centre  of  the  lozenges ;  c,  c,  green- 
ish bands  separating  the  lozenges:— b, 
vertical  section  of  the  frond,  showing  a, 

a,  the  dense  layer  of  cellular  tissue  form-  Gemmiparous  Conceptacles  of  Mar- 

ing  the  floor  of  the  air-chamber,  d,  d  ;  the  chantia  polymorpha:—A,  conceptacle 

epidermic  layer,  b,  b,  forming  its  roof;  c,  fully  expanded,  rising  from  the  sur- 

c,  its  walls;  /,  /,  loose  cells  in  its  interior;  face  of  the  frond,  a,  a,  and  containing 

stoma  divided  perpendicularly;  7i,  rings  gonidial  disks  already  detached;— b, 

of  cells  forming  its  wall;  i,  cells  forming  first  appearance  of  conceptacle  on  the 

the  obturator-ring.  surface  of  the  frond,  showing  the  for- 

mation of  its  fringe  by  the  splitting 
of  the  cuticle. 

opening  m  its  middle,  which  allows  a  few  small  green  cells  to  be  seen 
through  it.  When  a  thin  vertical  section  is  made  of  the  frond  (b),  it  is 
seen  that  each  of  the  lozenge-shaped  divisions  of  its  surface  corresponds 
with  an  air-chamber  in  its  interior,  which  is  bounded  below  by  a  floor 
{a,  a)  of  closely-set  cells,  from  whose  under  surface  the  radical  filaments 
arise;  at  the  sides  by  walls  {c,  c)  of  similar  solid  parenchyma,  the  projec- 
tion of  whose  summits  forms  the  raised  bands  on  the  surface;  and  above 
by  an  epidermis  {b,  b)  formed  of  a  single  layer  of  cells;  whilst  its  interior 
is  occupied  by  a  loosely  arranged  parenchyma,  composed  of  branching 


1  In  some  species,  the  same  shields  bear  both  sets  of  organs ;  and  in  Marchan- 
tia androgyna  we  find  the  upper  surface  of  one  half  of  the  pelta  developmg 
antheridia,  whilst  the  under  surface  of  the  other  half  bears  archegonia. 


MICROSCOPIC  STRUCTUKE  OF  HIGHER  CRYPTOGAMIA.  337 

rows  of  cells  (f,  f)  that  seem  to  spring  from  the  floor, — these  cells  being 
what  are  seen  from  above,  when  the  observer  looks  down  through  the 
central  aperture  just  mentioned.  If  the  vertical  section  should  happen 
to  traverse  one  of  the  peculiar  bodies  which  occupies  the  centres  of  the 
divisions,  it  will  bring  into  view  a  structure  of  remarkable  complexity. 
Each  of  these  stomata  (as  they  are  termed,  from  the  Greek  arojia^ 
mouth)  forms  a  sort  of  shaft  (^),  composed  of  four  or  five  rings  (like  the 
^ courses^  of  bricks  in  a  chimney)  placed  one  upon  the  other  (A),  every 
ring  being  made  up  of  four  or  five  cells;  and  the  lowest  of  these  rings  {%) 
appears  to  regulate  the  aperture,  by  the  contraction  or  expansion  of  the 
cells  which  compose  it,  and  is  hence  termed  the  ^obturator-ring.^  In 
this  manner  each  of  the  air-chambers  of  the  frond  is  brought  into  com- 
munication with  the  external  atmosphere,  the  degree  of  that  communica- 
tion being  regulated  by  the  limitation  of  the  aperture.  We  shall  here- 
after find  (§  383)  that  the  leaves  of  the  higher  Plants  contain  inter- 
cellular spaces,  which  also  communicate  with  the  exterior  by  stomata; 
but  that  the  structure  of  these  organs  is  far  less  complex  in  them,  than 
in  this  humble  Liverwort. 

333.  The  frond  of  Marchantia  usually  bears  upon  its  surface,  as 
shown  in  Fig.  214,  a  number  of  little  open  basket-shaped  gemmiparous 
conceptacles  (Fig.  2iC),  which  may  often  be  found  in  all  stages  of  de- 
velopment, and  are  structures  of  singular  beauty.  They  contain,  when 
mature,  a  number  of  little  green  round  or  oblong  discoidal  gemmcB,  each 
composed  of  two  or  more  layers  of  cells;  and  their  wall  is  surmounted  by 
a  glistening  fringe  of  '  teeth,^  whose  edges  are  themselves  regularly  fringed 
with  minute  out-growths.  This  fringe  is  at  first  formed  by  the  splitting- 
up  of  the  epidermis,  as  seen  at  B,  at  the  time  when  the  '  conceptacle  ^  and 
its  contents  are  first  making  their  way  above  the  surface.  The  little 
disks  which  correspond  with  the  gonidia  of  Lichens  (§  325),  are  at  first 
evolved  as  single  globular  cells,  supported  upon  other  cells  which  form 
their  footstalks;  these  single  cells,  undergoing  duplicative  subdivision, 
evolve  themselves  into  the  disks;  and  these  disks,  when  mature,  sponta- 
neously detach  themselves  from  their  footstalks,  and  lie  free  within  the 
cavity  of  the  conceptacle.  Most  commonly  they  are  at  last  washed  out 
by  rain,  and  are  thus  carried  to  different  parts  of  the  neighboring  soil, 
on  which  they  grow  very  rapidly  when  well  supplied  with  moisture;  some- 
times, however,  they  may  be  found  growing  whilst  still  contained  within 
the  conceptacles,  forming  natural  grafts  (so  to  speak)  upon  the  stock 
from  which  they  have  been  developed  and  detached;  and  many  of  the  ir- 
regular lobes  which  the  frond  of  the  Marchantia  puts  forth,  seem  to  have 
this  origin. — The  very  curious  observation  was  long  ago  made  by  Mirbel, 
who  carefully  watched  the  development  of  these  gemmce,  that  stomata  are 
formed  on  the  side  which  happens  to  be  exposed  to  the  light,  and  that 
root-fibres  are  put  forth  from  the  lower  side;  it  being  apparently  a  matter 
of  indifference  which  side  of  the  little  disk  is  at  first  turned  upwards, 
since  each  has  the  power  of  developing  either  stomata  or  root-fibres 
according  to  the  influence  it  receives.  After  the  tendency  to  the  forma- 
tion of  these  organs  has  once  been  given,  however,  by  the  sufficiently 
prolonged  influence  of  light  upon  one  side  and  of  darkness  and  moisture 
on  the  other,  any  attempt  to  alter  it  is  found  to  be  vain;  for  if  the  sur- 
faces of  the  young  fronds  be  then  inverted,  a  twisting  growth  soon  re- 
stores them  to  their  original  aspect. 

334.  When  the  Marchantia  vegetates  in  damp  shady  situations  which 
are  favorable  to  the  nutritive  processes,  it  does  not  readily  produce  the 

23 


338 


THE  MICROSCOPE  AND  ITS  REVELATIONS. 


Fig.  217 


Archegonia  of  Marchantia  pi 
morpha,  in  successive  stages  of 
velopment. 


oly- 

de- 


true  Fructification,  which  is  to  be  looked-for  rather  in  plants  growing  in 
more  exposed  places.  Each  of  the  stalked  peltate  (shield-like)  disks  con- 
tains a  number  of  flask-shaped  cavities  opening  upon  its  upper  surface, 
which  are  brought  into  view  by  a  vertical  section;  and  in  each  of  these 
cavities  is  lodged  an  a^itheridium,  composed  of  a  mass  of '  sperm-cells/  with- 
in which  are  developed  ^  antherozoids '  like  those  of  Chara  (Fig.  154  h), 
and  surmounted  by  a  long  neck  that  projects  through  the  mouth  of  the 

flask-shaped  cavity.  The  wheel-like  recep- 
tacles (Fig.  214),  on  the  other  hand,  bear 
on  their  under  surface,  at  an  early  stage, 
concealed  between  membranes  that  connect 
the  origins  of  the  lobes  with  one  another,  a 
set  of  archegonia,  shaped  like  flasks  with 
elongated  necks  (Fig.  217);  each  of  these 
has  in  its  interior  an  ^oosphere^  or  ^germ- 
cell,'  to  which  a  canal  leads  down  from  the 
extremity  of  the  neck,  and  which  is  fertilized 
by  the  penetration  of  the  antherozoids 
through  this  canal  until  they  reach  it.  In- 
stead, however,  of  at  once  evolving  itself  into 
a  new  plant  resembling  its  parent,  the  fer- 
tilized oosphere  or  ^embryo-cell'  develops 
itself  into  a  mass  of  cells  inclosed  within  a 
capsule,  which  is  termed  a  sporangium; 
and  thus  the  mature  receptacle,  in  place  of  archegonia,  bears  capsules  or 
sporangia,  each  of  them  filled  with  an  aggregation  of  cells  that  constitute 
the  immediate  progeny  of  the  original  germ-cell.  These  cells,  discharged 
by  the  bursting  of  the  sporangium,  are  of  two  kinds:  namely,  spores,  or 
gonidial-cells,  inclosed  in  firm  yellow  envelopes;  and  elators,^  which  are 
ovoidal  cells,  each  containing  a  double  spiral  fibre  coiled  up  in  its  interior. 
This  fibre  is  so  elastic,  that,  when  the  surrounding  pressure  is  withdrawn 
by  the  bursting  of  the  sporangium,  the  spires  extend  themselves  (Fig. 
218),  tearing  apart  the  cell-membrane;  and  they  do  this  so  suddenly  as 
to  jerk  forth  the  spores  which  may  be  adherent  to  their  coils,  and  thus  to 
assist  in  their  dispersion.  The  spores,  when  subjected  to  moisture,  with 
a  moderate  amount  of  light  and  warmth,  develop  themselves  into  little 
collections  of  cells,  which  gradually  assume  the  form  of  flattened  fronds; 
and  thus  the  species  is  very  extensively  multiplied,  every  one  of  the  aggre- 
gate of  spores  which  is  the  product  of  a  single  germ-cell  being  capable  of 
giving  origin  to  an  independent  individual. 

335.  Musci. — There  is  not  one  of  the  tribe  of  whose  external 

organs  do  not  serve  as  beautiful  objects  when  viewed  with  low  powers  of 
the  Microscope  ;  while  their  more  concealed  wonders  are  admirably  fitted 
for  the  detailed  scrutiny  of  the  practised  observer.  Mosses  always  possess 
a  distinct  axis  of  growth,  commonly  more  or  less  erect,  on  which  the 
minute  and  delicately-formed  leaves  are  arranged  with  great  regularity. 
The  stem  shows  some  indication  of  the  separation  of  a  cortical  or  bark- 
like portion  from  the  medullary  or  pith-like,  by  the  intervention  of  a 
circle  of  bundles  of  elongated  cells,  which  seem  to  prefigure  the  woody 
portion  of  the  stem  of  higher  plants,  and  from  which  prolongations  pass 
into  the  leaves  so  as  to  afford  them  a  sort  of  midrib.  The  leaf  usually 
consists  of  either  a  single  or  a  double  layer  of  cells,  having  flattened  sides 
by  which  they  adhere  one  to  another  :  they  rarely  present  any  distinct 
epidermic  layer  ;  but  such  a  layer,  perforated  by  stomata  of  simple 


MICROSCOPIC  STRUCTURE  OF  HIGHER  CRYPTOGAMIA. 


339 


structure,  is  commonly  found  on  the  setcB  or  bristle-like  footstalks  bear- 
ing the  fructification  and  sometimes  on  the  midribs  of  the  leaves.  The 
root-fibres  of  Mosses,  like  those  of  Marchantia,  consist  of  long  tubular 
cells  of  extreme  transparence,  within  which  the  protoplasm  may  f  requenly 
be  seen  to  circulate,  as  in  the  elongated  cells  of  Chara  ;  and  according  to 
Dr.  Hicks/ it  is  notuncommonfor  portions  of  the  protoplasmic  substance 
to  pass  into  an  amoeboid  condition  resembling  that  of  the  gonidia  of 
Volvox  (§  242).  The  protoplasm  first  detaches  itself  from  contact  with 
the  cell- wall,  and  collects  itself  into  ovoid  masses  of  various  sizes ;  these 
gradually  change  their  color  to  red  or  reddish-brown,  subsequently,  how- 
ever, becoming  almost  colorless;  and  they  protrude  and  retract  processes, 


Elator  and  Spores 
of  Marchantia, 


Structure  of  Mosses a,  Plant  of  Funaria  hygrometrica^ 
showing  /  the  leaves,  u  the  urns  supported  upon  the  setae  or 
footstalks  s,  closed  by  the  operculum  o,  and  covered  by  the 
calyptra  c: — b,  Urns  of  Encalyptra  vulgaris,  one  of  them 
closed  and  covered  with  the  calyptra;  the  other  open;  u, 
the  urns;  o,  o,  the  opercula;  c,  calyptra;  p,  peristome;  s,  s, 
setae:— c,  longitudinal  section  of  very  young  urn  of  Splachnum ; 
a,  solid  tissue  forming  the  lower  part  of  the  capsule ;  c,  colu- 
mella; I,  loculus  or  space  around  it  for  the  development  of  the 
spores ;  e,  epidermic  layer  of  cell,  thickened  at  the  top  to  form 
the  operculum  o;  p,  two  intermediate  layers,  from  which  the 
peristome  will  be  formed ;  s,  inner  layer  of  cells  forming  the 
wall  of  the  loculus. 


exactly  after  the  manner  of  Amcelm,  occasionally  elongating  themselves 
into  an  almost  linear  form,  and  travelling  up  and  down  in  the  interior  of 
the  tubular  cells.  This  kind  of  movement  was  observed  by  Dr.  Hicks 
to  subside  gradually,  the  masses  of  protoplasm  then  returning  to  their 
ovoid  form;  but  their  exterior  subsequently  became  invested  with  minute 
cilia,  by  which  they  were  kept  in  constant  agitation  within  their  contain- 
ing cells.  As  to  their  subsequent  history,  we  are  at  present  entirely  in 
the  dark  ;  and  the  verification  and  extension  of  Dr.  Hicks's  observations 
constitute  an  object  well  worthy  of  the  attention  of  Microscopists. 


^    Quart.  Joum.  Microsc.  Science,"  N.S.,  Vol.  ii.  (1862),  p.  96. 


340 


THE  MICKOSCOPE  AND  1T8  REVELATIONS. 


336.  What  has  commonly  been  regarded  as  the  ^fructification^  of 
Mosses — namely,  the  '  iirn  '  or  '  capsule  ^  filled  with  sporuleS;,  which  is 
borne  at  the  top  of  a  long  footstalk  that  springs  from  the  centre  of  a 
cluster  of  leaves  (Fig.  219,  a) — is  not  the  real  fructification,  but  its  pro- 
duct; for  Mosses,  like  Liverworts,  possess  both  antheridia  and  arcliegonia, 
although  they  are  by  no  means  conspicuous.  These  organs  are  some- 
times found  in  the  same  envelope  (or  perigone),  sometimes  on  different 
parts  of  the  same  plant,  sometimes  only  on  different  individuals;  but  in 

Tig*,  220. 


Antheridia  and  Antherozoids  of  Polytrichnm  commune:— a.,  group  oPantheridia,  mingled  with 
hairs  and  sterile  filaments  (paraphyses) :  of  the  three  antheridia,  the  central  one  is  in  the  act  of 
discharging  its  contents ;  that  on  the  left  is  not  yet  mature;  while  that  on  the  right  has  already 
emptied  itself,  so  that  the  cellular  structure  of  its  walls  becomes  apparent; — b,  cellular  contents 
of  an  antheridium,  previously  to  the  development  of  the  antherozoids;— c,  the  same,  showing  the 
first  appearance  of  the  antherozoids;— d,  the  same,  mature  and  discharging  the  antherozoids. 

either  case  they  are  usually  situated  close  to  the  axis,  among  the  bases  of 
the  leaves. — The  ^antheridia'  are  globular,  oval,  or  elongated  bodies 
(Fig.  220,  a),  composed  of  aggregations  of  cells,  of  which  the  exterior 
form  a  sort  of  capsule,  whilst  the  interior  are  sperm-cells,'  each  of  which,  ^ 
as  it  comes  to  maturity,  develops  within  itself  an  ^  antherozoid '  (b,  c, 
d);  and  the  antherozoids,  set  free  by  the  rupture  of  the  cells  within  which 


MICROSCOPIC  STRUCTURE  OF  HIGHER  CRYPTOCAMIA. 


341 


they  are  formed,  make  their  escape  by  a  passage  that  opens  for  them  at 
the  summit  of  the  aiitheridium.  The  antheridia  are  generally  surrounded 
by  a  cluster  of  hair-like  filaments,  composed  of  cells  joined  together  (Fig. 
220,  a),  which  are  called parapliyses;  these  seem  to  be  *  sterile'  or  unde- 
veloped antheridia.  The  ^  archegonia '  bear  a  general  resemblance  to 
those  ot  Mar c7ia7itia  (Fig.  214);  and  the  fertilization  of  their  contained 
^oospheres'  or  ^  germ-cells  Ms  accomplished  in  the  manner  already  de- 
scribed.   The  fertilized  '  embryo-cell '  becomes  gradually  deyeloped  by 


Mouth  of  capsule  of  Funaria,  showing  the  Double  Peristome  of 

Peristome  in  situ.  Fontinalis  antipyretica. 


cell-division  into  a  conical  body  elevated  upon  a  stalk;  and  this  at  length 
tears  across  the  walls  of  a  flask-shaped  archegonium  by  a  circular  fissure, 
carrying  the  higher  part  upwards  on  its  summit  as  a  calyptra  or  ^  hood  ' 


Double  Peristome  of  Bryum        Double  Penstome  of  Cinclidium 
intermedium.  arcticum. 

(Fig.  219,  B,  c),  while  the  lower  part  remains  to  form  a  kind  of  collar 
round  the  base  of  the  stalk. 

337.  The  Urn  or  ^  spore-capsule,'  which  is  thus  the  immediate  product 
of  the  generative  act,  is  closed  at  its  summit  by  an  operculum  or  lid  (Fig. 
219,  B,  0,  o),  which  falls  off  when  the  contents  of  the  capsule  are  mature, 
so  as  to  give  them  free  exit;  and  the  mouth  thus  laid  open  is  surrounded 
by  a  beautiful  toothed  fringe,  which  is  turned  the  peristome.  The  fringe, 
as  seen  in  its  original  undisturbed  position  (Fig.  221),  is  a  beautiful  ob- 


342 


THE  MICROSCOPE  AND  ITS  REVELATIONS. 


ject  for  the  Binocular  Microscope;  it  is  very  ^  hygometric/ executing, 
when  breathed-on,  a  curious  movement,  which  is  probably  concerned  in 
the  dispersion  of  the  spores.  In  Figs.  222-224,  are  shown  three  different 
forms  of  peristome,  spread  out  and  detached,  illustrating  the  varieties 
which  it  exhibits  in  different  genera  of  Moses; — varieties  whose  existence 
and  readiness  of  recognition  render  them  characters  of  extreme  value  to 
the  systematic  Botanist,  whilst  they  furnish  objects  of  great  interest  and 
beauty  for  the  Microscopist.  The  peristome  seems  always  to  be  originally 
double,  one  layer  springing  from  the  outer,  and  the  other  from  the  inner, 
of  two  layers  of  cells  which  may  be  always  distinguished  in  the  immature 
capsule  (Fig.  219,  c,  p);  but  one  or  other  of  these  is  frequently  wanting 
at  the  time  of  maturity,  and  sometimes  both  are  obliterated,  so  that  there 
is  no  peristome  at  all.  The  number  of  the  ^  teeth '  is  always  a  multiple 
of  4,  varying  from  4  to  64:  sometimes  they  are  prolonged  into  straight  or 
twisted  hairs. — The  spores,  or  gonidial  cells,  are  contained  in  the  upper 
part  of  the  capsule,  where  they  are  clustered  round  a  central  pillar,  which 
is  termed  the  cohwiella.  In  the  young  capsule  the  whole  mass  is  nearly 
solid  (Fig.  219,  c),  the  space  (1)  in  which  the  spores  are  developed  being 
very  small;  but  this  gradually  augments,  the  walls  becoming  more  con- 
densed; and  at  the  time  of  maturity  the  interior  of  the  capsule  is  almost 
entirely  occupied  by  the  spores.  These  are  formed  in  groups  of  four,  by 
the  duplicative  subdivision  of  the  ^mother-celP  which  first  differentiate 
themselves  from  those  forming  the  capsule  itself.  Thus  the  ^  spore-cap- 
sule in  Liverworts  and  Mosses,  being  the  immediate  product  of  the  act  of 
fertilization  (which  constitutes  the  point  of  departure  of  each  ^new  gene- 
ration^), is  to  be  considered  as  the  progeiiy  of  the  plant  that  bears  it; 
which,  supplying  the  nutriment  at  whose  expense  it  develops  itself,  acts 
as  its  '  nurse. ^ 

338.  The  development  of  the  spore  into  a  new  plant  commences  with 
the  rupture  of  its  firm,  yellowish-brown  outer-coat,  and  the  protrusion  of 
its  green  cell-wall  proper;  from  the  projecting  extremity  of  which  new 
ceils  are  put  forth  by^  a  process  of  out-growth,  which  form  a  sort  of  Con- 
fervoid  filament  (as  in  Fig.  231,  c).  At  certain  points  of  this  filament, 
its  component  cells  multiply  by  subdivision,  so  as  to  form  rounded  clus- 
ters, from  every  one  of  which  an  independent  plant  may  arise;  so  that 
several  individuals  may  be  evolved  from  a  single  spore.  And  as  a 
numerous  aggregate  of  spores  is  developed,  as  we  have  seen,  from  a  sin- 
gle germ-cell,  the  rapid  extension  of  the  Mosses  is  thus  secured,  although 
no  separate  individual  ever  attains  more  than  a  very  limited  size. 

339.  The  tribe  of  Spliagyiacem  or  '  Bog-Mosses,'  is  now  separated  by 
Muscologists  from  true  Mosses,  on  account  of  the  marked  differences  by 
which  they  are  distinguished;  the  three  groups,  HepaticcB,  Bryacece  (or 
ordinary  Mosses),  and  SphagnaceWy  being  ranked  as  together  forming  the 
Muscal  Alliance.  The  stem  of  the  SphagyiacecB  is  more  distinctly  differ- 
entiated than  that  of  the  Bryacem  into  the  central  or  medullary,  the  outer 
or  cortical,  and  the  intermediate  or  woody  portions;  and  a  very  rapid 
passage  of  fluid  takes  place  through  its  elongated  cells,  especially  in  the 
medullary  and  cortical  layers,  so  that  if  one  of  the  plants  be  placed  dry 
in  a  flask  of  water,  with  its  capitulum  of  leaves  bent  downwards,  the 
water  will  speadily  drop  from  this  until  the  flask  is  emptied.  The  leaf- 
cells  of  the  SphagnacecB  exhibit  a  very  curious  departure  from  the  ordi- 
nary type:  for  instead  of  being  small  and  polygonal,  they  are  large  and 
elongated  (Fig.  225);  they  contain  no  chlorophyll,  but  have  spiral  fibres 
loosely  coiled  in  their  interior;  and  their  membranous  walls  have  large 


MICKOSCOPIC  STRUCTURE  OF  HIGHER  CRYPTOGAMIA. 


34:3 


rounded  apertures,  by  which  their  cavities  freely  communicate  with  one 
another,  as  is  sometimes  curiously  evidenced  by  the  passage  of  Wheel- Ani- 
malcules that  make  their  habitation  m  these  chambers.  Between  these 
coarsely-spiral  cells  are  some  thick-walled  narrow  elongated  cells,  contain- 
ing chlorophyll;  these,  which  give  to  the  leaf  its  firmness,  do  not,  in  the 
very  young  leaf  (as  Prof.  Huxley  first  pointed  out')  differ  much  in 
appearance  from  the  others,  the  peculiarities  of  both  being  evolved  by 
a  gradual  process  of  differentiation.  The  antheridia  or  male  organs  of 
SphagnacecB  resemble  those  of  Liverworts,  rather  than  those  of  Mosses,  in 
their  form  and  arrangement;  they  are  grouped  in  catkins  at  the  tips  of 
lateral  branches,  each  of  the  imbricated  perigonal  leaves  inclosing  a  sin- 
gle globose  antheridium  on  a  slender  footstalk;  and  they  are  surrounded 
by  very  long  branched  paraphyses  of  cobweb-like  tenuity.  The  female 
organs,  or  archegonia,  which  do  not  differ  in  structure  from  those  of 
Mosses,  are  grouped  together  in  a  sheath  of  deep  green  leaves  at  the  end 


one  of  the  short  lateral  branchlets  at  the  sides  of  the  capitulum  or  sum- 
mit-crown of  leaves.  The  two  sets  of  organs  are  always  distributed  on 
different  branches,  and  in  some  instances  on  different  plants.  The  '  cap- 
sule,' which  is  formed  as  the  product  of  the  impregnation  of  the  germ-cell, 
is  very  uniform  in  all  the  species;  being  almost  spherical,  with  a  slightly 
convex  lid,  without  beak  or  point,  and  showing  no  trace  of  a  peristome; 
and  the  spores  it  contains  are  produced  in  groups  of  four  (as  in  Mosses) 
around  a  hemispherical  '  columella.'  Besides  the  ordinary  capsules,  how- 
ever, the  Sphagnacem  develop  a  smaller  set  of  sporogonia,  in  which 
'microspores'  are  formed  by  a  further  division  of  the  mother-cells;  the 
significance  of  these  is  unknown.  The  ordinary  spores,  when  germinat- 
ing, do  not  produce  the  branched  confervoid  filament  of  true  Mosses;  but 
if  growing  on  wet  peat,  evolve  themselves  into  a  lobed  foliaceous  '  pro- 

*  See  his  important  Article  on  '  The  Cell-Theory  '  in  the  British  and  Foreign 
dico-Chirurgical  Review,"  Vol.  xii.  (Oct.,  1853),  pp.  306,  307. 


Portion  of  the  leaf  of  Sphagnum ; 
showing:  the  large  cells,  a,  a,  a, 
with  spiral  fibres  and  communica- 
ting apertures;  and  the  intervening 
bands,  5,  6,  6,  composed  of  small 
elongated  cells. 


Obliqtie  section  of  footstalk  of 
Fern-leaf-  showing  bundle  of  Sca- 
larif  orm  Ducts. 


344: 


THE  MICROSCOPE  AND  ITS  REVELATIONS. 


thallium/  resembling  the  frond  of  Liverworts;  whilst,  if  they  develop  in 
water,  a  single  long  filament  is  formed,  of  which  the  lower  end  gives  off 
root-fibres,  while  the  upper  enlarges  into  a  nodule  from  which  the  young 
plant  is  evolved.  In  either  case,  the  prothallium  and  its  temporary 
roots  wither  away  as  soon  as  the  young  plant  begins  to  branch. — From 
their  extraordinary  power  of  imbibing  and  holding  water,  the  Sphagna- 
CGCB  are  of  great  importance  in  the  economy  of  Nature;  clothing  with 
vegetation  many  acreas  which  would  otherwise  be  sterile,  and  serving  as 
reservoirs  for  storing  up  moisture  for  the  use  of  higher  forms  of  vegeta- 
tion.' 

340.  Filices, — In  the  general  structure  of  Ferns  we  find  a  much 
nearer  approximation  to  Flowering  plants;  but  this  does  not  extend  to  their 
Eeproductive  apparatus,  which  is  formed  upon  a  type  essentially  the  same 
as  that  of  Mosses,  though  evolved  at  a  very  different  period  of  life.  As 
the  tissues  of  which  their  fabrics  are  composed  are  essentially  the  same  as 


FIG.  227*  ■  aSfi22S» 


Leaflet  of  Polypodium,  with  Sori.        Portion  of  Frond  of  Hoemionitis,  with  Sori. 


those  to  be  descibed  in  the  next  chapter,  it  will  not  be  requisite  here  to 
dwell  upon  them.  The  Stem  (where  it  exists)  is  for  the  most  part 
made  up  of  cellular  parenchyma,  which  is  separated  into  a  cortical  and  a 
medullary  portion  by  the  interposition  of  a  circular  series  of  fibro-vascu- 
lar  bundles  containing  true  Woody  tissue  and  Ducts.  These  bundles 
form  a  kind  of  irregular  network,  from  which  prolongations  are  given  off 
that  pass  into  the  leaf-stalks,  and  thence  into  the  midrib  and  its  lateral 
branches;  and  it  is  their  peculiar  arrangement  in  the  leaf-stalks,  which 
gives  to  the  transverse  section  of  these  the  figured  marking  commonly 
known  as  ''King  Charles  in  the  oak.^'  A  thin  section,  especially  if 
somewhat  oblique  (Fig.  226),  displays 'extremely  well  the  peculiar  charac- 
ter of  the  ducts  of  the  Fern;  which  are  termed  'scalariform,^  from  the 


^  See  Dr.  Braithwaite's  Papers  on  the  Sphagnacece  in  the  Monthly  Microsco- 
pical Journal,"  Vol.  vi.,  et  seq. 


MICROSCOPIC  STRUCTURE  OF  HIGHER  CRYPTOGAMIA. 


345 


resemblance  of  the  regular  markings  on  their  walls  to  the  rungs  of  a 
ladder. 

341.  What  is  usually  considered  the  fructification  ot  the 'Ferns  affords 
a  most  beautiful  and  readily-prepared  class  of  opaque  objects  for  the 
lower  powers  of  the  Microscope;  nothing  more  being  necessary  than  to 
lay  a  fragment  of  the  frond  that  bears  it  upon  the  glass  Stage-plate,  or 
to  hold  it  in  the  Stage-forceps,  and  to  throw  an  adequate  light  upon  it 
by  the  Side-condenser.  It  usually  presents  itself  in  the  form  of  isolated 
spots  on  the  under  surface  of  the  frond,  termed  sori,  as  in  the  common 
Polypodium  (Fig.  227),  and  in  the  Aspidium  (Fig.  229):  but  sometimes 
these  ^  sori '  are  elongated  into  bands,  as  in  the  common  Scolopendrum 
(hart's  tongue);  and  these  may  coalesce  with  each  other,  so  as  almost  to 
coyer  the  surface  of  the  frond  with  a  network,  as  in  Hcemionitis  (Fig. 
228);  or  they  may  form  merely  a  single  band  along  its  borders,  as  in  the 
common  Pteris  (brake-fern).  The  sori  are  sometimes  ^ naked'  on  the 
under  surface  of  the  fronds;  but  they  are  frequently  covered  with  a  deli- 
cate membrane  termed  the  indusium,  which  may  either  form  a  sort  of 
cap  upon  the  summit  of  each  sorus,  as  in  Aspidium  (Fig.  229),  or  a  long 
fold,  as  in  Scolopendrum  and  Pteris  ;  or  a  sort  of  cup,  as  in  Deparia  (Fig. 


Fig.  220  Tig.  250. 


Sorus  and  Indusium  of  Aspidium,         Sorus  and  cup-shaped  Indusium  of 

Deparia  prolifera. 


230).  Each  of  these  sori,  when  sufficiently  magnified,  is  found  to  be 
made  up  of  a  multitude  of  thecce  or  spore-capsules  (Figs.  229,  230), 
which  are  sometimes  closely  attached  to  the  surface  of  the  frond,  but  more 
commonly  spring  from  it  by  a  pedicle  or  footstalk.  The  wall  of  the 
theca  is  composed  of  flattened  cells,  applied  to  each  other  by  their  edges: 
but  there  is  generally  one  row  of  these  thicker  and  larger  than  the  rest, 
which  springs  from  the  pedicle,  and  is  continued  over  the  summit  of  the 
capsule,  so  as  to  form  a  projecting  ring,  which  is  known  as  the  annulus 
(Fig.  230).  This  ring  has  an  elasticity  superior  to  that  of  all  the  rest 
of  the  capsular  wall,  causing  it  to  split  across  when  mature,  so  that  the 
contained  spores  may  escape;  and  in  many  instances  the  two  halves  of 
the  capsule  are  carried  widely  apart  from  each  other,  the  fissures  extend- 
ing to  such  a  depth  as  to  separate  them  completely. — In  Osmurida  (the 
so-called  'flowering-fern')  and  Opliioglossum  (adder's  tongue),  the  thecae 
have  no  annulus. — It  will  frequently  happen  that  specimens  of  Fern- 
fructification  gathered  for  the  Microscope  will  be  found  to  have  all  the 
capsules  burst  and  the  spores  dispersed,  whilst  in  others  less  advanced 
the  capsules  may  all  be  closed;  others,  however,  may  often  be  met  with 
in  which  some  of  the  capsules  are  closed,  and  others  are  open;  and  if 


THE  MICROSCOPE  AND  ITS  REVELATIONS. 


these  be  watched  with  sufficient  attention,  the  rupture  of  some  of  the 
tliecsB  and  the  dispersion  of  the  spores  may  be  observed  to  take  place 
whilst  the  specimen  is  under  observations  in  the  field  of  the  Microscope. 
In  sori,  whose  capsules  hav*e  all  burst,  the  annuli  connecting  their  two 
halves  are  the  most  conspicuous  objects,  looking,  when  a  strong  light  is 
thrown  upon  them,  like  strongly-banded  worms  of  a  bright  brown  hue. 
This  is  particularly  the  case  in  Scolopendrum,  whose  elongated  sori  are 
remarkably  beautiful  objects  for  the  Microscope  in  all  their  stages;  until 
quite  mature,  however,  they  need  to  be  brought  into  view  by  turning 
back  the  two  indusial  folds  that  cover  them.  The  commonest  Ferns,  in- 
deed, which  are  found  in  almost  every  hedge,  furnish  objects  of  no  less 
beauty  than  those  yielded  by  the  rarest  exotics;  and  it  is  in  every  respect 
a  most  valuable  training  to  the  young,  to  teach  them  how  much  may  be 
found  to  interest,  when  looked  for  with  intelligent  eyes,  even  in  the 
most  familiar,  and  therefore  disregarded,  specimens  of  Nature's  handi- 
work. 

342.  The  ^  spores  ^  (Fig.  231,  a)  set  free  by  the  bursting  of  the  thecas^ 


Development  of  Prothallium  of  Pteris  serrulata:—A,  Spore  set  free  from  the  theca:— b,  Spore 
beginning  to  germinate,  putting  forth  the  tubular  prolongation  a,  from  the  principal  cell  6,'— c,  first- 
formed  linear  series  of  cells;— d,  Prothallium  taking  the  form  of  a  leaf -like  expansion;  a,  first,  and 
b,  second  radical  fibre;  c,  d,  the  two  iooes,  and  e,  the  indentation  between  them;  f,  /,  first-formed 
part  of  the  prothallium;     external  coat  of  the  original  spore;  h.  antheridia. 

usually  have  a  somewhat  angular  form  and  are  invested  by  a  yellowish  or 
brownish  outer  coat,  which  is  marked  very  much  in  the  manner  of  pollen- 
grains  (Fig.  277)  with  points,  streaks,  ridges,  or  reticulations.  When 
placed  upon  a  damp  surface,  and  exposed  to  a  sufficiency  of  light  and 
warmth,  the  spore  begins  to  'germinate;'  the  first  indication  of  its  vege- 
tative activity  being  a  slight  enlargement,  which  is  manifested  in  the 
rounding-off  of  its  angles.  This  is  followed  by  the  putting-forth  of  a 
tubular  prolongation  (b,  a)  of  the  internal  cell-wall  through  an  aperture 
in  the  outer  spore-coat;  and  moisture  being  absorbed  through  this,  the 
cell  becomes  so  distended  as  to  burst  the  external  unyielding  integument, 
and  soon  begins  to  elongate  itself  in  a  direction  opposite  to  that  of  the 
root-fibre.  A  production  of  new  cells  by  subdivision  then  takes  place 
from  its  growing  extremity:  this  at  first  proceeds  in  a  single  series,  so  as 


MICROSCOPIC  STRUCTURE  OF  HIGHER  CRXTTOGAMIA. 


847 


to  form  a  kind  of  confervoid  filament  (c);  but  the  multiplication  of  cells 
by  subdivision  soon  takes  place  transversely  as  well  as  longitudinally,  so 
that  a  flattened  leaf-like  expansion  (d)  is  produced,  so  closely  resembling 
that  of  a  young  Marcliantia  as  to  be  readily  mistaken  for  it.  This  expan- 
sion, which  is  termed  the  pro- 

thallium  varies  in  its  configura-  •^0^^222. 
tion,  in  different  species;  but  its 
essential  structure  always  remains 
the  same.  From  its  under  sur- 
face are  developed  not  merely  the 
root-fibres  {a,  b)  which  serve  at ' 
the  same  time  to  fix  it  in  the  soil 
and  to  supply  it  with  moisture, 
but  also  the  antheridia  and  arch- 
egonia  which  constitute  the  true 

representatives  of  the  essential  Development  of  the  Anthendia  and  Antherozoids 
■navfa  nf  fhp  TT'lnwPv  nf  Viiahpr^^  Pteris  serru1ata:—A,  projection  of  one  of  the 
pans  01  tne  riOWei  OI  iJlgnei  ^ells  of  prothallium,  showing  the  antheridial  cell  6, 
Plants.  Some  of  the  former  may  with  its  sperm-cells  e,  within  the  cavity  of  the  origi- 
i-,.,-        'iji.  nal  cell  a;— B,  Antheridium  completely  developed: 

be  distinguished  at  an  early  period  a,  wall  of  antheridialcell;  e,  sperm-cells,  each  inl 
nf  flip  rlpvplnnmpnf  nf  flip  iirnfVipl-  closing  an antherozoid; — c,  Antherozoid  more  high- 
01  tne  aeveiOpmentOI  tneprotnai  jnagmfled,  showing  its  large  extremity  a,  its 
hum  (A,  h) ;  and  at  the  time  OI  its  small  extremity  6,  and  its  cilia  d,  d. 


complete  evolution  these  bodies 
are  seen  in  considerable  numbers, 
especially  about  the  origins  of  the 
root-fibres.  Each  has  its  origin 
in  a  peculiar  protrusion  that  takes 
place  from  one  of  the  cells  of  the 
prothallium  (Fig.  232,  A,  a):  this 
is  at  first  entirely  filled  with  chlor- 
ophyll-gvanules;  but  soon  a  pecu- 
liar free  cell  (5)  is  seen  in  its  in- 
terior, filled  with  mucilage  and 
colorless  granules.  This  cell 
gradually  becomes  filled  with,  an- 
other brood  of  VCUnS^  cells  (e),  ArchegoniumofPfeWsserrwZafa;— a,  asseenfrom 
1   '  •1       11     '      -i  above;  a,  a,  a,  cells  surrounding  the  base  of  the  cav- 

and  increases  considerably  m  ltSity;6,  c,cZ;  successive  layers  of  cells,  the  highest  in- 
rlimPTiQinnci  on  fi<^  fill  fViP  -nrn- closing  a  quadrangular  orifice :—B,  side  view,  show- 
aimensions,  SO  as  to  nil  tne  prO-.^^  a;  a.  cavity  Sontaining  the  germ-cell,  J;  b,  b, 
lectlOn  which  incloses  it:  this  part  walls  of  the  archegonium,  made  up  of  the  four  lay- 

n-F  f>iP  nrio-inril  pnvifv  icj  nr>w  pni"^^^^^  ^'  ^'  ^'  "^^^^^  having  an  opening,/,  on 

01  tne  original  cavity  is  now  cut       summit:  c,c,  antherozoids  within  the  cavity; 

off  from  that  of  the  cell  of  which  large  extremity;  h,  thread-like  portion;  small 
no  1      I         1  J.1         J.1        extremity  m  contact  With  the  germ-cell,  and  dilated. 

it  was  an  offshoot,  and  the  anther- 

idium  henceforth  ranks  as  a  distinct  and  independent  organ.  Each  of  the 
sperm-cells  (b,  e)  included  within  the  antheridial  cell,  is  seen,  as  it 
approaches  maturity,  to  contain  a  spirally-coiled  filament;  and  when  set 
free  by  the  bursting  of  the  antheridium,  the  sperm-cells  themselves  burst, 
and  give  exit  to  their  antherozoids  (c),  which  execute  rapid  movements 
of  rotation  on  their  axes,  partly  dependent  on  the  six  long  cilia  with 
which  they  are  furnished. 

343.  The  archegonia  are  fewer  in  number,  and  are  found  upon  a 
different  part  of  the  prothallium.  Each  of  them  originates  in  a  sin- 
gle cell  of  its  superficial  layer,  which  undergoes  subdivision  by  a  horizon- 
tal partition.  Of  the  two  cells  thus  produced,  the  upper  gives  origin, 
by  successive  subdivisions,  to  the  '  neck  ^  of  the  archegonium,  which, 
when  fully  developed  (Fig.  233),  is  composed  of  twelve  or  more  cells. 


348 


THE  MICROSCOPE  AND  ITS  REVELATIONS. 


built  up  in  layers  of  four  cells  each,  one  upon  another,  so  as  to  form  a 
kind  of  chimney  or  shaft,  having  a  central  passage  that  leads  down  to  a 
cavity  at  its  base.  The  lower  of  the  two  first-formed  cells  becomes  the 
'  central  cell  ^  of  the  archegonium;  and  this  again  undergoing  horizontal 
subdivision,  the  lower  half  becomes  the  oosphere  or  germ-cell,  whilst  the 
upper  extends  itself  into  the  '  neck,^  and  forms  a  canal  filled  with  muci- 
*  laginous  protoplasm,  through  which  the  antherozoids  make  their  way  to 
the  oosphere  lying  at  its  bottom  (Fig.  233  B,  a).  The  oosphere,  when 
fertilized  by  the  penetration  of  the  antherozoids,  becomes  the  ^embryo- 
cell  ^  of  a  new  plant,  the  development  of  which  speedily  commences/ — In 
the  aberrant  group  of  Opliioglosseo  (Adders'  tongue  ferns),  the  develop- 
ment of  the  prothallium  takes  place  underground,  in  the  form  of  a  small 
roundish  tuber,  composed  of  parenchymatous  tissue  containing  no  chlo- 
rophyll, and  producing  antheridia  and  archegonia  on  its  upper  surface. 

344.  The  early  development  of  the  Embryo-cell  takes  place  according 
to  the  usual  method  of  repeated  binary  subdivision,  producing  a  homo- 
geneous globular  mass  of  cells.  Soon,  however,  rudiments  of  special 
organs  begin  to  make  their  appearance;  the  embryo  grows  at  the  expense 
of  the  nutriment  prepared  for  it  by  the  prothallium;  and  it  bursts  forth 
from  the  cavity  of  the  archegonium,  which  organ  in  the  mean  time  is 
becoming  atrophied.  In  the  very  beginning  of  its  development,  the  ten- 
dency is  seen  in  the  cells  of  one  extremity  to  grow  upward  so  as  to  evolve 
the  stem  and  leaves,  and  in  those  of  other  extremity  to  grow  downward 
to  form  the  root;  and  when  these  organs  have  been  sufficiently  developed 
to  absorb  and  prepare  .the  nutriment  which  the  young  Fern  requires,  the 
prothallium  decays  away.  Thus,  then,  the  ^  spore  ^  of  the  Fern  must 
be  considered  as  a  generative  gonidium  or  detached  flower-bud,  capable 
of  developing  itself  into  a  prothallium  that  may  be  likened  to  a  receptacle 
bearing  the  sexual  apparatus.  But  this  prothallium  serves  the  further 
purpose  of  ^ nursing^  the  embryoes  originated  by  the  generative  act; 
which  embryoes  finally  develop  themselves, — not,  as  in  Mosses,  into  mere 


^  The  study  of  the  development  of  the  spores  of  Ferns,  and  of  the  act  of  ferti- 
lization and  of  its  products,  may  be  conveniently  prosecuted  as  follows: — Let  a 
frond  of  a  Fern  whose  fructification  is  mature  be  laid  upon  a  piece  of  fine  paper, 
with  its  spore-bearing  surface  downwards;  in  the  course  of  a  day  or  two  this  paper 
will  be  found  to  be  covered  with  a  very  fine  brownish  dust,  which  consists  of  the 
discharged  spores.  This  must  be  carefully  collected,  and  should  be  spread  upon 
the  surface  of  a  smoothed  fragment  of  porous  sandstone,  the  stone  being  placed 
in  a  saucer,  the  bottom  of  which  is  covered  with  water;  and  a  glass  tumbler  being 
inverted  over  it,  the  requisite  supply  of  moisture  is  insured,  and  the  spores  will 
germinate  luxuriantly.  Some  of  the  prothallia  soon  advance  beyond  the  rest; 
and  at  the  time  when  the  advanced  ones  have  long  ceased  to  produce  anthe- 
ridia, and  bear  abundance  of  archegonia,  those  which  have  remained  behind  in 
their  growth  are  begining  to  be  covered  with  antheridia.  If  the  crop  be  now  kept 
with  little  moisture  for  several  weeks,  and  then  suddenly  watered,  a  large  num- 
ber of  antheridia  and  archegonia  simultaneously  open;  and  in  a  few  hours  after- 
wards, the  surface  of  the  larger  prothallia  will  be  found  almost  covered  with 
moving  antherozoids.  Such  prothallia  as  exhibit  freshly -opened  archegonia  are 
now  to  be  held  by  one  lobe  between  the  forefinger  and  thumb  of  the  left  hand,  so 
that  the  upper  surface  of  the  prothallium  lies  upon  the  thumb;  and  the  thinnest 
possible  sections  are  then  to  be  made  with  a  thin  narrow-bladed  knife,  perpen- 
dicularly to  its  surface.  Of  these  sections,  which,  after  much  practice,  may  be 
made  no  more  than  l-15th  of  a  line  in  thickness,  some  will  probably  lay  open  the 
canals  of  the  archegonia;  and  within  these,  when  examined  with  a  power  of  200 
or  i500  diameters,  antherozoids  may  be  occasionally  distinguished.  The  prothal- 
lium of  the  common  Osmicnda  regalis  will  be  found  to  afford  peculiar  facilities 
for  observation  of  the  development  of  the  antheridia,  which  are  produced  at  its 
margin.   (See  Rev.  F.  Howlett  in    Intellectual  Observer,"  Vol.  vii.,  p.  32.) 


MICROSCOPIO  STRUCTUiiE  OF  HIGHER  CRYPTOGAMIA. 


349 


spore-capsules, — but,  as  in  Phanerogamia,  into  entire  plants,  complete  in 
everything  but  the  true  generative  organs,  which  evolve  themselves  from 
the  detached  spores. 

345.  The  little  group  of  Equisetacem  (Horse-tails)  which  seem  nearly 
allied  to  the  Ferns  in  the  type  of  their  generative  apparatus,  though  that 
of  their  vegetative  portion  is  very  different,  affords  certain  objects  of  con- 
siderable interest  to  the  Microscopist.  The  whole  of  their  structure  is 
penetrated  to  such  an  extraordinary  degree  by  silex,  that  even  when  its 
organic  portion  has  been  destroyed  by  prolonged  maceration  in  dilute  nitric 
acid,  a  consistent  skeleton  still  remains.  This  mineral,  in  fact,  consti- 
tutes in  some  species  not  less  than  13  per  cent  of  the  whole  solid  matter, 
and  50  per  cent  of  the  inorganic  ash;  and  it  especially  abounds  in  the 
epidermis,  which  is  used  by  cabinet-makers  for  smoothing  the  surface  of 
wood.  Some  of  the  siliceous  particles  are  distributed  in  two  lines,  parallel 
to  the  axis;  others,  however,  are  grouped  into  oval  forms,  connected  with 
each  other,  like  the  jewels  of  a  necklace,  by  a  chain  of  particles  forming 
a  sort  of  curvilinear  quadrangle;  and  these  (which  are,  in  fact,  the  parti- 
cles occupying  the  cells  of  the  stomata)  are  arranged  in  pairs.  Their 
form  and  arrangement  are  peculiarly  well  seen  under  Polarized  light,  for 
which  the  prepared  epidermis  is  an  extremely  beautiful  object;  and  it  is 
asserted  by  Sir  D.  Brewster  (whose  authority  upon  this  point  has  been 
generally  followed)  that  each  siliceous  particle  has  a  regular  axis  of 
double  refraction.    According  to  Prof.  Bailey,  however,  the  effect  of  this 


and  similar  objects  (such  as  the  epidermis  of  Grasses)  upon  Polarized 
light,  it  is  not  produced  by  the  siliceous  particles,  but  by  the  organized 
tissues;  since,  when  the  latter  have  been  entirely  got  rid  of,  the  residual 
silex  shows  no  doubly-refracting  power/ — What  is  usually  designated  as 
the  fructification  of  the  Equisetaceae  forms  a  cone  or  spike  at  the  extrem- 
ity of  certain  of  the  stem-like  branches  (tha  real  stem  being  a  horizontal 
rhizoma);  and  consists  of  a  cluster  of  shield-like  disks,  each  of  which 
carries  a  circle  of  tlieccB  or  spore-capsules,  that  open  by  longitudinal  slits 
to  set  free  the  spores.  Each  of  these  spores  has,  attached  to  it,  two  pairs 
of  elastic  filament  (Pig.  234),  that  are  originally  formed  spiral  fibres  on 
the  interior  of  the  wall  of  the  primary  cell  within  which  it  is  generated, 
and  are  set  free  by  its  rupture;  these  are  at  first  coiled  up  around  the 
spore,  in  the  manner  represented  at  A,  though  more  closely  applied  to  the 
surface;  but,  on  the  liberation  of  the  spore,  they  extend  themselves  in  the 
manner  shown  at  B, — the  slightest  application  of  moisture,  however, 
serving  to  make  them  close  together  (the  assistance  which  they  afford  in 
the  dispersion  of  the  spores  being  no  longer  required)  when  the  spores 


Spores  of  Equisetum  with  their  Elastic  Filaments. 


^  See    Silliman's  American  Journal  of  Science,"  May,  1856. 


350 


THE  MICROSCOPE  AND  ITS  REVELATIONS. 


have  alighted  on  a  damp  surface.  If  a  number  of  these  spores  be  spread 
out  on  a  slip  of  glass  under  the  field  of  view,  and,  whilst  the  observer 
watches  them,  a  bystander  breathes  gently  upon  the  glass,  all  the  fila- 
ments will  be  instantaneously  put  in  motion,  thus  presenting  an  extremely 
curious  spectacle;  and  will  almost  as  suddenly  return  to  their  previous 
condition  when  the  effect  of  the  moisture  has  passed  off.  If  one  of  the 
theccB  which  has  opened,  but  has  not  discharged  its  spores,  be  mounted  in 
a  cell  with  a  movable  cover,  this  curious  action  may  be  exhibited  over  and 
over  again.  These  spores,  like  those  of  Ferns,  evolve  themselves  into  a 
prothallium;  and  this  develops  antheridia  and  archegonia,  the  former  at 
the  extremities  of  the  lobes,  and  the  latter  in  the  angles  between  them. 

346.  Nearly  allied  to  Ferns,  also,  is  a  curious  little  group  of  small 
aquatic  plants,  the  Rhizocarpece  (or  pepper- worts),  which  either  float  on 
the  surface,  or  creep  along  shallow  bottoms.  These  all  agree  in  having 
two  kinds  of  spores,  produced  in  separate  capsules;  the  larger,  or  '  mega- 
spores,^  giving  origin  to  prothallia  which  produce  archegonia  only;  and 
the  smaller,  or  ^microspores,'  undergoing  progressive  subdivision,  usually 
without  the  formation  of  a  distinct  prothallium,  each  of  the  cells  thus 
formed  giving  origin  to  an  antherozoid.  In  this,  as  we  shall  presently 
see  (§  349),  there  is  a  distinct  foreshadowing  of  the  mode  in  which  the 
generative  process  is  performed  in  Flowering  Plants;  the  ^microspore' 
obviously  corresponding  to  the  pollen-grain,  while  the  ^megaspore'  may 
be  considered  to  represent  the  primitive  cell  of  the  ovule. 

347.  Another  alliance  of  Ferns  is  to  the  LycopodiacecB  (Club-mosses); 
a  group  which  at  the  present  time  attains  a  great  development  in  warm 
climates,  and  which,  it  would  seem,  constituted  a  large  part  of  the 
arborescent  vegetation  of  the  Carboniferous  epoch. — In  the  Lycopodiem 
proper,  the  sporangia  are  all  of  one  kind,  and  all  the  spores  are  of  the 
same  size;  each,  as  in  Ophioglossum  (§  343),  giving  origin  to  a  subterran- 
eous prothallium,  that  develops  both  antheridia  and  archegonia.  The 
plant  which  originates  from  the  fertilized  '  germ-ccU '  of  the  archegonium, 
only  attains  in  colder  climates  a  Moss-like  growth,  with  a  creeping  stem 
usually  branching  dichotomously,  and  imbricated  leaves;  but  is  distin- 
guished from  the  true  mosses,  not  only  by  its  higher  general  organization 
(which  is  on  a  level  with  that  of  Ferns),  but  by  the  character  of  its  fructi- 
fication, which  is  a  club-shaped  ^  spike,'  bearing  small  imbricated  leaves, 
in  the  axils  of  which  lie  the  sporangia.  The  spores  developed  within 
these  are  remarkable  for  the  large  quantity  of  resinous  matter  they  con- 
tain, giving  them  an  inflammability  that  causes  their  being  used  in 
theatres  to  produce  ^artificial  lightning.' — But  in  the  allied  groups  of 
SelaginellecB  and  IsoeteWy  there  are  (as  in  the  Rliizocarpem)  two  kinds  of 
spores  produced  in  separate  sporangia;  one  set  producing  ^  megaspores,' 
from  which  archegonia-bearing  prothallia  are  developed;  and  the  other 
producing  ^microspores,'  which,  by  repeated  subdivision,  give  origin  to 
antherozoids  without  the  formation  of  prothallia.  It  is  a  very  interesting 
indication  of  a  tendency  towards  the  Phanerogamic  type  of  sexual  gene- 
ration, that  the  prothallium  in  this  group  is  chiefly  developed  within  the 
spore-case,  forming  a  kind  of  'endosperm'  (§  349),  only  the  small  part 
which  projects  from  the  ruptured  apex  of  the  spore  producing  one  or 
more  archegonia. — The  arborescent  Lepidodendra  and  SigillaricB  of  the 
Coal-measures  seem  to  have  formed  connecting  links  between  the  Vascu- 
lar Crytogams  and  the  Phanerogams,  alike  in  the  structure  of  their  Stems, 
and  in  their  Fructification.  For  the  LepidostroU  or  cone-like  'fruit' 
of  these  trees,  represent  the  club-shaped  spikes  of  the  Lycopodiacece;  and 


MICROSCOPIC  STRUCTURE  OF  HIGHER  CRYPTOGAMIA. 


351 


seem  to  have  borne  ^  megaspores '  in  the  sporangia  of  its  basal  portion, 
and  '  microspores'  in  those  of  its  upper  part.  Some  of  the  best  seams 
of  Coal  appear  to  have  been  chiefly  formed  by  the  accumulation  of  these 
^megaspores/ 


348.  Thus,  in  our  ascent  from  the  lower  to  the  higher  Cryptogams, 
we  have  seen  a  gradual  change  in  the  general  plan  of  structure,  bringing 
their  superior  types  into  a  close  approximation  to  the  Flowering  Plant, 
which  is  undoubtedly  the  highest  form  of  vegetation.  But  we  have 
everywhere  encountered  a  mode  of  Generation,  which,  whilst  essentially  the 
same  throughout  the  series,  is  no  less  essentially  distinct  from  that  of  the 
Phanerogam;  the  fertilizing  material  of  the  ^sperm-cells'  being  embodied, 
as  it  were,  in  self-moving  filaments,  which  find  their  way  to  '  germ-cells' 
by  their  own  independent  movements;  and  the  *  embryo-cell '  being  des- 
titute of  that  store  of  prepared  nutriment,  which  surrounds  it  in  the  true 
Seed,  and  supplies  the  material  for  its  early  development.  In  the  lower 
Cryptogamia,  we  have  seen  that  the  fertilized  oospore  is  thrown  at  once 
upon  the  world  (so  to  speak)  to  get  its  own  living;  but  in  Ferns  and 
their  allies,  the  ^embryo-cell'  is  nurtured  for  a  while  by  the  prothallium 
of  the  parent  plant.  While  the  true  reproduction  of  the  species  is  effected 
by  the  proper  Generative  act,  the  imiltij^lication  of  the  individual  is  ac- 
complished by  the  production  and  dispersion  of  '  gonidial '  spores;  and  this 
production,  as  we  have  seen,  takes  place  at  very  different  periods  of  exist- 
ence in  the  several  groups,  dividing  the  life  of  each  into  two  separate  epochs, 
in  which  it  presents  itself  under  two  very  distinct  phases  that  contrast 
remarkably  with  each  other.  Thus,  the  frond  of  the  Marchantia  evolved 
from  the  spore,  and  bearing  the  antheridia  and  archegonia,  is  that  which 
seems  naturally  to  constitute  the  Plant;  but  that  which  represents  this 
phase  in  the  Ferns  is  the  minute  Marchantia-like  prothallium.  In  Ferns, 
on  the  other  hand,  the  product  into  which  the  fertilized  ^embryo-cell' 
evolves  itself,  is  that  which  is  commonly  regarded  as  the  Plant:  and  this 
is  represented  in  the  Liverworts  and  Mosses  by  the  spore-capsule  alone.  ^ — 
We  shall  encounter  a  similar  diversity  (which  has  received  the  inappro- 
priate designation  of  ^alternation  of  generations')  in  some  of  the  lower 
forms  of  the  Animal  Kingdom. 


'  For  more  detailed  information  on  the  Structure  and  Classification  of  the  Cryp- 
togamia generally,  the  reader  is  referred  to  Prof.  Sachs'  *' Text-book  of  Botany," 
(Bennett's  translation),  and  to  Prof.  Hofmeister's  large  ^^Handbuch  der  Physi- 
ologischen  Botanik . " 


352 


THE  MICHOSCOPE  AND  ITS  KEVELATI02«IS. 


CHAPTER  IX. 

OF   THE  MICROSCOPIC   STRUCTURE  OF   PHANEROGAMIC  PLANTS. 

349.  BETWEEiq"  the  two  great  divisions  of  the  Vegetable  kingdom 
which  are  known  as  Cryptogamia  and  Phanerogamia,  the  separation  is 
by  no  means  so  abrupt  as  it  formerly  seemed  to  be.  For,  as  has  been 
already  shown,  though  the  Cryptogamia  were  formerly  regarded  as  alto- 
gether non-sexual,  a  true  Generative  process,  requiring  the  concurrence, 
of  male  and  female  elements,  is  traceable  throughout  the  series.  And  in 
the  higher  type  of  that  series,  we  have  seen  a  foreshadowing  of  those 
provisions  for  the  nurture  of  the  fertilized  embryo,  which  constitute  the 
distinctive  characters  of  the  Phanerogamia.  On  the  other  hand,  although 
we  are  accustomed  to  speak  of  Phanerogamia  as  ^flowering-plants,' yet 
not  only  are  the  conspicuous  parts  of  the  flower  often  wanting,  but  in  the 
important  group  of  Gymnospemis  (including  the  Coniferm  and  CycadecB)^ 
the  essential  parts  of  the  Generative  apparatus  are  reduced  to  a  condition 
of  extreme  simplicity,  closely  approximating  to  that  of  the  higher  Cryp- 
togams. There  are,  however,  certain  fundamental  differences  between 
the  modes  in  which  the  act  of  fertilization  is  performed  in  the  two 
groups.  For  (1)  whilst  in  all  the  higher  Cryptogams,  it  is  in  the  condi- 
tion of  free-moving  *  antherozoids '  that  the"  contents  of  the  sperm-cell 
find  their  way  to  the  germ-cell,  these  are  conveyed  to  it,  throughout  the 
Phanerogamic  series,  by  an  extension  of  the  lining  membrane  of  the 
sperm-cell  or  pollen-grain  into'a  tube,  which  penetrates  to  the  germ-cell 
contained  in  the  interior  of  the  body  called  the  ^  ovule.'  Again  (2), 
while  the  ^germ-cell '  or  oosphere  in  the  higher  Cryptogams  is  contained 
in  a  structure  that  originated  in  a  spore  detached  from  the  parent-plant, 
it  is  not  only  formed  and  fertilized  in  all  Phanerogams  whilst  still  borne 
on  the  parent  fabric,  but  continues  for  some  time  to  draw  from  it  the 
nutriment  it  requires  for  its  development  into  the  ^embryo.'  And  at  the 
time  of  its  detachment  from  the  parent,  the  matured  '  seed '  contains, 
not  merely  an  ^embryo'  already  advanced  a  considerable  stage,  but  a 
store  of  nutriment  to  serve  for  its  further  development  during  germina- 
tion. As  there  is  nothing  parallel  to  this  among  Cryptogams,  it  may  be 
said  that  reproduction  by  seeds,  not  the  possession  of  flowers,  is  the  dis- 
tinctive character  of  Phanerogams.  The  ovules,  which  when  fertilized 
and  matured  become  seeds,  are  developed  from  specially  modified  leaves, 
which  remain  open  in  Gymno^erms,  but  which,  in  all  other  Phanero- 
gams, fold  together  so  as  to  inclose  the  ovules  within  an  ^  ovary.'  Each 
ovule  consists  of  a  '  nucleus '  surrounded  by  ^  integuments '  which  remain 
unclosed  at  its  anterior  end,  leaving  open  a  short  canal  termed  the  '  mi- 
cropyle.'  One  cell  of  the  nucleus  undergoes  great  enlargement,  and  be- 
comes the  emiryO'SaCy  whose  cavity  is  filled,  in  the  first  instance,  with  a 


MICROSCOPIC  STRUCTURE  OF  PHANEROGAMIC  PLANTS. 


353 


mucilaginous  fluid  containing  protoplasm.  At  the  end  of  of  the  embryo- 
sac  nearest  the  micropyle,  a  germ-cell  or  ^oosphere^  is  developed;  in 
Phanerogams  generally  by  free  cell-formation  (§  226),  but  in  Gymno- 
sperms  indirectly  as  the  product  of  the  formation  of  a  ^  corpusculum/ 
which  represents  the  archegonium  of  Selaginella  (§  347).  By  a  further 
process  of  free  cell-formation,  the  remainder  of  the  embryo-sac  comes  to 
be  filled  with  cells,  constituting  what  is  termed  the  ^ endosperm;^  and 
this  serves,  like  the  prothallium  of  Ferns,  to  imbibe  and  prepare  nutri- 
ment which  is  afterwards  appropriated  by  the  embryo.  In  many  seeds 
(as  those  of  the  Leguminosce)  the  whole  nutritive  material  of  the  endo- 
sperm has  been  absorbed  into  the  ^cotyledons'  (or  seed-lobes)  of  the 
embryo,  by  the  time  that  the  seed  is  fully  matured  and  independent  of 
the  parent;  but  in  other  cases  it  remams  as  a  ^separate  albumen.'  In 
either  case  it  is  taken  into  the  substance  of  the  Embryo  during  its  germi- 
nation. 

350.  Elementary  Tissues, — No  marked  change  shows  itself  in  general 
organization,  as  we  pass  from  the  Cryptogamic  to  the  Phanerogamic 
Series  of  Plants.  For  a  large  proportion  of  the  fabric  of  even  the  most 
elaborately  formed  Tree  (including  the  parts  most  actively  concerned  in 
living  action)  is  made  up  of  components  of  the  very  same  kind  with  those 
which  constitute  the  entire  organisms  of  the  simplest  Cryptogams.  For 
although  the  Stems,  Branches,  and  Roots  of  trees  and  shrubs  are  princi- 
pally composed  of  tuoody  tissue,  such  as  we  do  not  meet  with  in  any  but 
the  highest  Cryptogamia,  yet  the  special  office  of  this  is  to  afford  mechani- 
cal support:  when  it  is  once  formed,  it  takes  no  further  share  in  the  vital 
economy,  than  to  serve  for  the  conveyance  of  fluid  from  the  roots  upwards 
through  the  stem  and  branches,  to  the  leaves;  and  even  in  these  organs, 
not  only  the  pith  and  the  bark,  with  the  ^medullary  rays,'  which  serve 
to  connect  them,  but  that  ^  cambium-layer'  intervening  between  the  bark 
and  the  wood  (§  372),  in  which  the  periodical  formation  of  the  new  layers 
both  of  bark  and  wood  takes  place,  are  composed  of  Cellular  substance. 
This  tissue  is  found,  in  fact,  wherever  ^7'0^^;^A  is  taking  place;  as,  for  ex- 
ample, in  the  ^spongioles'  or  growing-points  of  the  root-fibres,  in  the 
leaf-buds  and  leaves,  and  in  the  flower-buds  and  sexual  parts  of  the 
flower:  it  is  only  when  these  organs  attain  an  advanced  stage  of  develop- 
ment, that  ivoody  structure  is  found  in  them, — its  function  (as  in  the 
stem)  being  merely  to  give  support  to  their  softer  textures;  and  the  small 
proportion  of  their  substance  which  it  forms,  being  at  once  seen  in  those 
beautiful  ^  skeletons,' which,  by  a  little  skill  and  perseverance,  may  be 
made  of  leaves,  flowers,  and  certain  fruits.  All  the  softer  and  more 
pulpy  tissue  of  these  organs  is  composed  of  cells^  more  or  less  compactly 
aggregated  together,  and  having  forms  that  approximate  more  or  less 
closely  to  the  globular  or  ovoidal,  which  may  be  considered  as  their  origi- 
nal type. 

351.  As  a  general  rule,  the  rounded  shape  is  preserved  only  when  the 
cells  are  but  loosely  aggregated,  as  in  the  parenchymatous  (or  pulpy) 
substance  of  leaves  (Fig.  235),  and  it  is  then  only  that  the  distinctness  of 
their  walls  becomes  evident.  When  the  tissue  becomes  more  solid,  the 
sides  of  the  vesicles  are  pressed  against  each  other,  so  as  to  flatten  them 
and  to  bring  them  into  close  apposition;  and  they  then  adhere  to  one  an- 
other in  such  a  manner,  that  the  partitions  appear,  except  when  carefully 
examined,  to  be  single  instead  of  double  as  they  really  are.  Frequently 
it  happens  that  the  pressure  is  exerted  more  in  one  direction  than  in  an- 
other, so  that  the  form  presented  by  the  outline  of  the  cell  varies  accord- 

23 


354 


THE  MICROSCOPE  AND  ITS  REVELATIONS. 


ing  to  the  direction  in  which  the  section  is  made.  This  is  well  shown  in 
the  pith  of  the  young  shoots  of  Elder,  Lilac,  or  other  rapidly  growing 
trees;  the  cells  of  which,  when  cut  transversely,  generally  exhibit  circular 
outlines;  whilst,  when  the  section  is  made  vertically,  their  borders  are 
straight,  so  as  to  make  them  appear  like  cubes  or  elongated  prisms,  as  in 
Fig.  235.  Avery  good  example  of  such  a  cellular  parenchyma  is  to  be 
found  in  the  substance  known  as  Rice-paper;  which  is  made  by  cutting 
the  herbaceous  stem  of  a  Chinese  plant  termed  Aralia  papyrifera'  verti- 
cally round  and  round  with  a  long 
shai  p  knife,  so  that  its  tissues  may 
be  (as  it  were)  unrolled  in  a  sheet. 
The  shape  of  its  cells  when  thus 
prepared,  is  irregularly  prismatic, 
as  shown  in  Fig.  236,  B;  but  if 
the  stem  be  cut  transversely,  their 
outlines  are  seen  to  be  circular  or 
nearly  so  (a).  When,  as  often  hap- 
pens, the  cells  have  a  very  elongated 
form,  this  elongation  is  in  the  direc- 
tion of  their  growth,  which  is  that, 
of  course,  wherein  there  is  least 
resistance.  Hence  their  greatest 
length  is  nearly  always  in  the 
direction  of  the  axis;  but  there  is 
one  remarkable  exception,- — that, 
namely,  which  is  afforded  by  the 
'  medullary  rays  ^  of  Exogenous 
stems  (§  370),  whose  cells  are  great- 
ly elongated  in  the  horizontal  direc- 
tion (Fig.  359,  a),  their  growth  being  from  the  centre  of  the  stem  towards  its 
circumference.  Ibis  obvious  that  fluids  will  be  more  readily  transmitted 
in  the  direction  of  greatest  elongation,  being  that  in  which  they  will  have 
to  pass  through  tlie  least  number  of  partitions;  and  whilst  their  ordinary 
course  is  in  the  direction  of  the  length  of  the  Roots,  Stems,  or  Branches, 
they  will  be  enabled  by  meaDS  of  the  medullary  rays  to  find  tlieir  way  in 
the  transverse  A\vQQ,i\on. — One  of  the  most  curious  varieties  of  form  which 
Vegetable  cells  present,  is  the  stellate  cell,  represented  in  Fig.  237,  form- 
ing the  spongy  parenchymatous  substance  in  the  stems  of  many  aquatic 
plants,  of  the  Rush  for  example,  which  are  furnished  with  air-spaces.  In 
other  instances,  these  air  spaces  are  large  cavities  which  are  altogether 
left  void  of  tissue:  such  is  the  case  in  the  Nuphar  lutea  (yellow  water- 
lily),  the  footstalks  of  whose  leaves  contain  large  air-chambers,  the  walls 
of  which  are  built  up  of  very  regular  cubical  cells,  whilst  some  curiously- 
formed  large  stellate  cells  project  into  the  cavity  which  they  bound  (Fig. 
238). — The  dimensions  of  the  component  vesicles  of  Cellular  tissue  are 
extremely  variable;  for  although  their  diameter  is  very  commonly  between 
l-300th  and  l-500th  of  an  inch,  they  occasionally  measure  as  much  as 
l-30th  of  an  inch  across,  whilst  in  other  instances  they  are  not  more  than 
1.300th. 

352.  The  component  cells  of  Cellular  tissue  are  usually  held  together 
by  an  intercellular  substance,  which  may  be  considered  analogous  to  the 


Section  of  Leaf  of  Agave,  treated  with  dilute 
nitric  acid,  showing  the  primordial  utricle  con- 
tracted in  the  interior  of  the  cells:— a,  Epidermic 
cells;  6,  boundary-cells  of  the  stoma;  c,  cells  of 
parenchyma;     their  primordial  utricles. 


'  The  JSschyiiomene,  which  is  sometimes  named  as  the  source  of  this  article, 
is  an  Indian  plant  employed  for  a  similar  purpose. 


MICROSCOPIC  STRUCTURE  OF  PHANEROGAMIC  PLANTS.  355 


^ gelatinous^  layer  that  intervenes  between  the  cells  of  the  Algae  (§  ^29). 
This,  in  an  early  stage  of  their  development,  is  often  very  abundant,  oc- 
cupying more  space  than  the  cells  themselves,  as  is  seen  in  Fig.  239,  A; 
and  the  cell-cavities  are  not  separated  from  it  by  the  interposition  of  a 
distinct  membrane.  As  the  cells  enlarge  and  increase  by  duplicative  sub- 


Sections  of  Cellular  Parenchyma  of  Aralia,  or  Rice-papcF  plant:— A,  transversely  to  the  axis 
of  the  stem;  b,  in  the  direction  of  the  axis. 

division  (b),  the  intervening  substance  diminishes  in  relative  amount;  and 
as  the  cells  advance  towards  their  mature  condition  (c),  it  merely  shows 
itself  as  a  thin  layer  between  them.  There  are  many  forms  of  fully 
developed  cellular  parenchyma,  in  which,  in  consequence  of  the  loose 
aggregation  of  their  component  cells,  these  may  be  readily  isolated,  so  as 


Section  of  Cellular  parenchyma  Cubical  parenchyma,  with  stellate 

of  Rush.  cells,  from  petiole  of  Nuphar  lutea. 


to  be  prepared  for  separate  examination  without  the  use  of  re-agents 
which  alter  their  condition:  this  is  the  case  with  the  pulp  of  ripe  fruits, 
such  as  the  Strawberry  or  Currant  (the  Snowberry  is  a  particularly 
favorable  subject  for  this  kind  of  examination),  and  with  the  parenchyma- 
of  many  fleshy  leaves,  such  as  those  of  the  Carnation  {Diaiitlius  caryo- 


356 


THE  MICROSCOPE  AND  ITS  REVELATIONS. 


phyllus)  or  the  London  Pride  {Saxifraga  crassifolia).  Such  cells  usually 
contain  evident  nuclei,  which  are  turned  brownish-yellow  by  iodine, 
whilst  their  membrane  is  only  turned  pale-yellow;  and  in  this  way  the 
nucleus  may  be  brought  into  view,  when,  as  often  happens,  it  is  not 
.previously  distinguishable.  If  a  drop  of  the  iodized  solution  of  chloride 
of  zinc  be  subsequently  added,  the  cell-membrane  becomes  of  a  beautiful 
blue  color,  whilst  the  nucleus  and  the  granular  protoplasm  that  surrounds 
it  retain  their  brownish-yellow  tint.  The  use  of  dilute  nitric  or  sulphuric 
acid,  of  alcohol,  of  syi'up,  or  of  several  other  reagents,  serves  to  bring 
into  view  the  primordial  utricle  (§  223);  its  contents  being  made  to 
coagulate  and  shrink,  so  that  it  detaches  itself  from  the  cellulose  wall 
with  which  it  is  ordinarily  in  contact,  and  shrivels  up  within  its  cavity, 
as  shown  in  Fig.  235.  It  would  be  a  mistake,  however,  to  regard  this  as 
a  distinct  membrane;  for  it  is  nothing  else  than  the  peripheral  layer  of 
protoplasm,  naturally  somewhat  more  dense  than  that  which  it  includes, 
but  deriving  its  special  consistence  from  the  operations  of  reagents. 


ria.239. 

A  .1) 


Successive  stagres  of  Cell-formation  in  the  development  of  the  Leaves  of  Anacharis  alsinas- 
trum: — a,  growing  point  of  the  branch,  consisting  of  a  protoplasmic  mass  with  young  ceils,  the 
projections  at  its  base  being  the  rudiments  of  leaves;  b,  portion  of  one  of  these  incipient  leaves  in 
a  more  advanced  condition:  o,  the  same  in  a  still  later  stage  of  development. 

353.  It  is  probable  that  all  Cells,  at  some  stage  or  other  of  their 
growth,  exhibit,  in  a  greater  or  less  degree  of  intensity,  that  curious 
movement  of  cyclosis,  which  has  been  already  described  as  occurring  in 
the  Characece  (§  258),  and  which  consists  in  the  steady  flow  of  one  or  of 
several  currents  of  protoplasm  over  the  inner  wall  of  the  cell;  this  being 
rendered  apparent  by  the  movement  of  the  particles  which  the  current 
carries  along  with  it.  The  ^best  examj)les  of  it  are  found  among  sub- 
merged plants,  in  the  cells  of  which  it  continues  for  a  much  longer 
period  than  it  usually  does  elsewhere;  and  among  these  are  two,  the 
Vallisneria  spiralis  and  the  Anacharis  alsinastrum,  which  are  peculiarly 
fitted  for  the  exhibition  of  this  interesting  phenomenon. — The  Vallisne- 
ria is  an  aquatic  plant  that  grows  abundantly  in  the  rivers  of  the  south 
of  Europe,  but  is  not  a  native  of  this  country;  it  may,  however,  be  readily 
grown  in  a  tall  glass  jar  having  at  the  bottom  a  couple  of  inches  of  mould, 
which,  after  the  roots  have  been  inserted  into  it,  should  be  closely  pressed 
down,  the  jar  being  then  filled  with  water,  of  which  a  portion  should  be 


MICROSCOPIC  STRDCTURE  OF  PHANEROGAMIC  PLANTS.  357 

occasionally  changed/  The  jar  should  be  freely  exposed  to  light,  and 
should  be  kept  in  as  warm  but  equable  a  temperature  as  possible.  The 
long  grass-like  leaves  of  this  plant  are  too  thick  to  allow  the  transmission 
of  sufficient  light  through  them  for  the  purpose  of  this  observation;  and 
it  is  requisite  to  make  a  thin  slice  or  shaving  with  a  sharp  knife.  If  this 
be  taken  from  the  surface,  so  that  the  section  chiefly  consists  of  the  super- 
ficial layer  of  cells,  these  will  be  found  to  be  small,  and  the  particles  of 
chlorophyll,  though  in  great  abundance,  will  rarely  be  seen  in  motion. 
This  layer  should  therefore  be  sliced  off  (or,  perhaps  still  better,  scraped 
away)  so  as  to  bring  into  view  the  deeper  layer,  which  consists  of  larger 
cells,  some  of  them  greatly  elongated,  with  particles  of  chlorophyll  in 
smaller  number,  but  carried  along  in  active  rotation  by  the  current  of 
protoplasm;  and  it  will  often  be  noticed  that  the  directions  of  the  rota- 
tion in  contiguous  cells  are  opposite.  If  the  movement  (as  is  generally 
the  case)  be  checked  by  the  shock  of  the  operation,  it  will  be  revived 
again  by  gentle  warmth;  and  it  may  continue  under  favorable  circum- 
stances, in  the  separated  fragment,  for  a  period  of  weeks,  or  even  of 
months.  Hence,  when  it  is  desired  to  exhibit  the  phenomenon,  the  pre- 
ferable method  is  to  prepare  the  sections  a  little  time  before  they  are 
likely  to  be  wanted,  and  to  carry  them  in  a  small  vial  of  water  in  the 
waistcoat  pocket,  so  that  they  may  receive  the  gentle  and  continuous 
warmth  of  the  body.  In  summer,  when  the  plant  is  in  its  most  vigorous 
state  of  growth,  the  section  may  be  taken  from  any  one  of  the  leaves;  but 
in  winter,  it  is  preferable  to  select  those  which  are  a  little  yellow.  An 
Objective  of  l-4th  inch  focus  will  serve  for  the  observation  of  this  inter- 
esting phenomenon,  and  very  little  more  can  be  seen  with  a  l-8th  inch; 
but  the  l-25th  inch  constructed  by  Messrs.  Powell  and  Lealand  enables 
;:he  borders  of  the  protoplasmic  current,  which  carries  along  the  particles 
of  chlorophyll,  to  be  distinctly  defined;  and  this  beautiful  phenomenon 
may  be  most  luxuriously  watched  under  their  patent  Binocular  (§  81). 

354.  The  Anacharis  alsinastriim  is  a  water- weed,  which,  having  been 
accidentally  introduced  into  this  country  several  years  ago,  has  since 
spread  itself  with  such  rapidity  through  our  canals  and  rivers,  as  in  many 
instances  seriously  to  impede  their  navigation.  It  does  not  require  to 
root  itself  in  the  bottom,  but  floats  in  any  part  of  the  water  it  inhabits; 
and  it  is  so  tenacious  of  life,  that  even  small  fragments  are  sufficient  for 
the  origination  of  new  plants.  The  leaves  have  no  distinct  cuticle,  but 
are  for  the  most  part  composed  of  two  layers  of  cells,  and  these  are  elon- 
gated and  colorless  in  the  centre,  forming  a  kind  of  midrib;  towards  the 
margins  of  the  leaves,  however,  there  is  but  a  single  layer.  Hence  no 
preparation  whatever  is  required  for  the  exhibition  of  this  interesting 
phenomenon;  all  that  is  necessary  being  to  take  a  leaf  from  the  stem  (one 
of  the  older  yellowish  leaves  being  preferable),  and  to  j^lace  it  with  a 
drop  of  water,  either  in  the  Aquatic-box,  or  on  a  slip  of  glass  beneath  a 
thin-glass  cover.  A  higher  magnifying  power  is  required,  however,  than 
that  which  suffices  for  the  examination  of  the  cyclosis  in  Chara  or  in  Val- 
lisneria;  the  l-8th  inch  Object-glass  being  here  preferable  to  the  l-4th, 
and  the  assistance  of  the  Achromatic  Condenser  being  desirable.  With 


'  Mr.  Quekett  found  it  the  most  convenient  method  of  changing  the  water  in 
the  jars  in  which  Chara,  Vallisneria,  etc.,  are  growing,  to  place  them  occasion- 
ally under  a  water-tap,  and  allow  a  very  gentle  stream  to  fall  into  them  for  some 
hours;  for  by  the  prolonged  overflow  thus  occasioned,  all  the  impure  water,  with 
the  Conferva  that  is  apt  to  grow  on  the  sides  of  the  vessel,  may  be  readily  got  rid 
of. 


358 


THE  MICROSCOPE  AND  ITS  REVELATIONS. 


this  amplification,  the  phenomenon  may  be  best  studied  in  the  single 
layer  of  marginal  cells;  although,  when  a  lower  power  is  used,  it  is  most 
evident  in  the  elongated  cells  forming  the  central  portion  of  the  leaf. 
The  number  of  chlorophyll-granules  in  each  cell  varies  from  three  or  four 
to  upwards  of  fifty;  they  are  somewhat  irregular  in  shape,  some  being 
nearly  circular  flattened  discs,  whilst  others  are  oval;  and  they  are  usu- 
ally from  l-3000th  to  l-5000th  of  an  inch  in  diameter.  When  the  rota- 
tion is  active,  the  greater  number  of  these  granules  travel  round  the  mar- 
gin of  the  cells,  a  few,  however,  remaining  fixed  in  the  centre:  their  rate 
of  movement,  though  only  1-40 th  of  an  inch  per  minute,  being  sufficient 
to  carry  them  several  times  iwnd  the  cell  within  that  period.   As  in  the 

case  of  the  Vallisneria,  the  motion  may 
frequently  be  observed  to  take  place  in 
opposite  directions  in  contiguous  cells. 
The  thickness  of  the  layer  of  protoplasm 
in  which  the  granules  are  carried  round, 
is  estimated  by  Mr.  Wenham  at  no 
more  than  1-20, 000th  of  an  inch. 
When  high  powers  and  careful  illumi- 
nation are  employed,  delicate  ripples 
may  be  seen  in  the  protoplasmic  cur- 
rents.* 

355.  Cyclosis,  however,  is  by  no 
means  restricted  to  submerged  plants; 
for,  i  t  has  been  witnessed  by  numerous 
observers  in  so  great  a  variety  of  other 
species,  that  it  may  fairly  be  i)resumed 
to  be  universal.  It  is  especially  observ- 
able in  the  hairs  of  the  Epidermic  sur- 
face; and  according  to  Mr.  Wenham,* 
who  has  given  much  attention  to  this 
subject,  the  difficulty  is  to  find  the 
exceptions,  for  hairs  taken  alike  from 
the  loftiest  Elm  of  the  forest  to  the 
humblest  weed  that  we  trample  beneath 
our  feet,  plainly  exhibit  this  circula- 
tion.'^ Such  hairs  are  furnished  by 
various  parts  of  plants ;  and  what  is 
y  "-^^^^^(C^^^Cj  chiefly  necessary  is,  tliat  the  part  from 

'         ^  which  the  hair  is  gathered  should  be  in 

\,  .  ^.  .  „  .  ,      .    a  state  of  visrorous  growth.    The  hairs 

Rotation  of  fluid  m  Hairs  of  TVadescan^ia    i      ^^  ^       i  j.     i    ^   u    j.      •         ^  -^.i 

Virginica:  -a,  portion  of  cuticle  with  hair  should  be  detached  by  tearmg-oli  with 

attached;  a,  6,  c,  successive  cells  of  the  hair;  ^  nf  finp  nninfpd  fnrr»pr>«?   fhp  Tior- 

d, cells  of  the  cuticles;  e.  Stoma :-b,  joints  ^.  P^^^  ^J-  nne-poiniea  lOlCeps,  tne  por- 
of  a  beaded  hair,  showing  several  currents:  tion   of   the   Cuticle   irom  W^hich  they 

a,  Nucleus.  Spring;  care  being  taken  not  to  grasp 

the  hair  itself,  whereby  such  an  injury  would  be  done  to  it  as  to 
check  the  movement  within  it.  The  hair  should  then  be  placed  with  a 
drop  of  water  under  thin  glass;  and  it  will  generally  be  found  advantage- 


^  '  Quart.  Journ.  of  Microsc.  Science,"  Vol.  iii.  (1855),  p.  277. 

^  '  On  the  Sap-Circulation  in  Plants,'  in  Quart.  Journ.  of  Microsc.  Science,'' 
Yol.  iv.  (1856\  p.  44.— It  is  unfortunate  that  Mr.  Wenham  should  have  used  the 
term  '  circulation'  to  designate  this  phenomenon,  which  has  nothing  in  common 
with  that  movement  of  nutritive  fluid  through  tubes  or  channels,  to  which  the 
term  is  properly  applicable. 


MICROSCOPIC  STRUCTURE  OF  PHANEROGAMIC  PLANTS.  359 

ous  to  use  a  l-8th  inch  Objective,  with  an  Achromatic  Condenser  having 
a  series  of  diaphragms.  The  nature  of  the  movement  in  the  hairs  of  dif- 
ferent species  is  far  from  being  uniform.  In  some  instances,  the  currents 
pass  in  single  lines  along  the  entire  length  of  the  cells,  as  in  the  hairs 
from  the  filaments  of  the  Tradescantia  virginica,  or  Virginian  Spider- 
wort  (Fig.  240,  a);  in  others  there  are  several  such  currents  which  retain 
their  distinctness,  as  in  the  jointed  hairs  of  the  calyx  of  the  same  plant 
(b);  in  others,  again,  the  streams  coalesce  into  a  network,  the  reticula- 
tions of  which  change  their  position  at  short  intervals,  as  in  the  hairs  of 
Glaucium  luteiim;  whilst  there  are  cases  in  which  the  current  flows  in  a 
sluggish  uniformly-moving  sheet  or  layer.  Where  several  distinct  cur- 
rents exist  in  one  cell,  they  are  all  found  to  have  one  common  point  of 
departure  and  return,  namely,  the  nucleus  (b,  a);  from  which  it  seems 
fairly  to  be  inferred  that  this  body  is  the  centre  of  the  vital  activity  of 
the  cell. — Mr.  Wenham  states  that  in  all  cases  in  which  the  cyclosis  is 
seen  in  the  hairs  of  a  plant,  the  cells  of  the  cuticle  also  display  it,  pro- 
vided that  their  walls  are  not  so  opaque  or  so  strongly  marked  as  to  pre- 
vent the  movement  from  being  distinguished.  The  cuticle  may  be  most 
readily  torn  off  from  the  stalk  or  the  midrib  of  the  leaf;  and  must  then 
be  examined  as  speedily  as  possible,  since  it  loses  its  vitality  when  thus 
detached,  much  sooner  than  do  the  hairs.  Even  when  no  obvious  move- 
ment of  particles  is  to  be  seen,  the  existence  of  a  cyclosis  may  be  con- 
cluded from  the  peculiar  arrangement  of  the  molecules  of  the  protoplasm, 
which  are  remarkable  for  their  high  refractive  power,  and  which  when 
arranged  in  a  ^moving-train,^  appear  as  bright  lines  across  the  cell;  and 
these  lines,  on  being  carefully  watched,  are  seen  to  alter  their  relative 
positions. — The  leaf  of  the  common  Plantago  (Plantain  or  Dock)  fur- 
nishes an  excellent  example  of  cyclosis;  the  movement  being  distinguish- 
able at  the  same  time  both  in  the  cells  and  in  the  hairs  of  the  cuticle  torn 
from  its  stalk  or  midrib.  It  is  a  curious  circumstance  that  when  a  plant 
which  exhibits  the  cyclosis  is  kept  in  a  cold  dark  place  for  one  or  two 
days,  not  only  is  the  movement  suspended,  but  the  moving  particles  col- 
lect together  in  little  heaps,  which  are  broken-up  again  by  the  separate 
motion  of  their  particles,  when  the  stimulus  of  light  and  warmth  occa- 
sions a  renewal  of  the  activity.  It  is  well  to  collect  the  s]3ecimens  about 
midday,  that  being  the  time  when  the  rotation  is  most  active,  and  the 
movement  is  usually  quickened  by  artificial  warmth,  which,  indeed,  is  a 
necessary  condition  in  some  instances  to  its  being  seen  at  all.  The  most 
convenient  method  of  applying  this  warmth,  while  the  object  is  on  the 
stage  of  the  Microscope,  is  to  blow  a  stream  of  air  upon  the  thin-glass 
cover,  through  a  glass  or  metal  tube  previously  heated  in  a  spirit-lamp. 

356.  The  walls  of  the  cells  of  plants  are  frequently  thickened  by  inter- 
nal deposits,  which  may  present  very  different  appearances  according  to 
the  manner  in  which  they  are  arranged.  In  its  simplest  condition,  such 
a  deposit  forms  a  thin  uniform  layer  over  the  whole  internal  surface  of 
the  cellulose- wall,  scarcely  detracting  at  all  from  its  transparence,  and 
chiefly  distinguishable  by  the  '  dotted '  appearance  which  the  membrane 
then  presents  (Fig.  236,  a).  These  dots,  however,  are  not  pores,  as 
their  aspect  might  naturally  suggest,  but  are  merely  points  at  which  the 
deposit  is  wanting,  so  that  the  original  cell- wall  there  remains  unthick- 
ened.  A  more  complete  consolidation  of  Cellular  tissue  is  effected  by 
deposits  of  sclerogen  (a  substance  which,  when  separated  from  the  resi- 
nous and  other  matters  that  are  commonly  associated  with  it,  is  found 
to  be  allied  in  chemical  composition  to  cellulose)  in  successive  layers,  one 


360 


THE  MICROSCOPE  AND  ITS  REVELATIOKS. 


Fig.  241. 


within  another  (Pig.  241,  a),  which  present  themselves  as  concentric 
rings  when  the  cells  containing  them  are  cut  through;  and  these  laj^ers 
are  sometimes  so  thick  and  numerous  as  almost  to  obliterate  the  origi- 
nal cavity  of  the  cell.  By  a  continuance  of  the  same  arrangement  as  that 
which  shows  itself  in  the  single  layer  of  the  dotted  cell — each  deposit 
being  deficient  at  certain  points  and  these  points  corresponding  with 

each  other  in  the  successive  layers — 
a  series  of  passages  is  left,  by  which 
the  cavity  of  the  cell  is  extended  at 
some  points  to  its  membranous  wall; 
and  it  commonly  happens  that  the 
points  at  which  the  deposit  is  want- 
ing on  the  walls  of  the  contiguous 
cells,  are  coincident,  so  that  the  mem- 
branous partition  is  the  only  obstacle 
to  the  communication  between  their 
cavities  (Figs.  241-243).  It  is  of 
such  tissue  that  the  '  stones '  of  stone- 
fruit,  the  gritty  substance  which 
surrounds  the  seeds  and  forms  little 
hard  points  in  the  fleshy  substance 
of  the  Pear,  the  shell  of  the  Cocoa- 
nut,  and  the  albumen  of  the  seed 
of  Phytelephas  (known  as  '  vegetable 
ivory'),  are  made  up;  and  we  see  the  use  of  this  very  curious  arrange- 
ment, in  permitting  the  cells,  even  after  they  have  attained  a  consider- 
able degree  of  consolidation,  still  to  remain  permeable  to  the  fluid  re- 
quired for  the  hutrition  of  the  parts  which  such  tissue  incloses  and 
protects. 


Tissue  of  the  Testa  or  Seed -coat  of  a  Star- 
Anise  .'—A,  as  seen  iu  section;  b,  as  seen  on  the 
surface. 


Tig.  24^ 


Section  of  Cherry-stone,  cutting  Section  of  Coquilla-nut,  in  the 

the  cells  transversely,  direction  of  the  long  diameters 

of  the  cells. 

357.  The  deposit  sometimes  assumes,  however,  the  form  of  definite 
fibres^  which  lie  coiled-up  in  the  interior  of  cells,  so  as  to  form  a  single, 
a  double,  or  even  a  triple  or  quadruple  spire  (Fig.  244).  ^noh.  sjnral 
cells  are  found  most  abundantly  in  the  leaves  of  certain  Orchideous 


MICROSCOPIC  STRUCTURE  OF  PHANEROGAMIC  PLANTS.  361 


plants,  immediately  beneatli  the  cuticle,  where  they  are  brought  into 
view  by  Tcrtical  sections;  and  they  may  be  obtained  in  an  isolated  state  by 
macerating  the  leaf  and  jieeling  off  the  cuticle  so  as  to  expose  the  layer 
beneath,  which  is  then  easily  separated  into  its  components.  In  an  Or- 
chideous  plant,  named  Saccolahiiim  gtittatum,  the  spiral  cells  are  unusually 
long,  and  have  spires  winding  in  opposite  directions;  so  that,  by  their 
mutual  intersection,  a  series  of  diamond-shaped  markings  is  produced. 
Spiral  cell  are  often  found  upon  the  surface  of  the  testa  or  outer  coat  of 
seeds:  and  in  the  Collonvia grandifioray  the  Salvia  verienaca  (Wild  Clary), 
and  some  other  plants,  the  membrane  of  these  cells  if  so  weak,  and  the 
elasticity  of  their  fibres  so  great,  that,  when  the  membrane  is  softened  by 
the  action  of  water,  the  fibres  suddenly  uncoil  and  elongate  themselves 
(Fig.  245),  springing  out,  as  it  were,  from  the  surface  of  the  seed,  to 
which  they  give  a  peculiar  flocculent  appearance.    This  very  curious  phe- 

FiG.  243. 


Spiral  cells  of  leaf  of  Oncidium.  Spiral  fibres  of  Seed-coat  of  Collomia, 


nomenon,  which  is  not  unfrequently  spoken  of  by  persons  ignorant  of  its 
true  nature  as  the  '  germination  ^  of  the  seed,  may  be  best  observed  in  the 
following  manner: — A  very  thin  transverse  slice  of  the  seed  should  first 
be  cut,  and  laid  upon  the  lower  glass  of  the  Aquatic  box;  the  cover 
should  then  be  pressed  down,  and  the  box  placed  upon  the  stage,  so  that 
the  Microscope  may  be  exactly  focussed  to  the  object,  the  power  em- 
ployed being  the  1-inch,  2-3ds-inch,  or  the  |-inch.  The  cover  of  the 
aquatic-box  being  then  removed,  a  small  drop  of  water  should  be  placed 
on  that  part  of  its  internal  surface  with  which  the  slice  of  the  seed  had 
been  in  contact;  and  the  cover  being  replaced,  the  object  should  be  im- 
mediately looked  at.  It  is  important  that  the  slice  of  the  seed  should  be 
very  thin,  for  two  reasons:  first,  that  the  view  of  the  spires  may  not  be 
confused  by  their  aggregation  in  too  great  numbers;  and  second,  that  the 
drop  of  water  should  be  held  in  its  place  by  capillary  attraction,  instead 
of  running  down  and  leaving  the  object,  as  it  will  do  if  the  glasses  be  too 
widely  separated. 

358.  In  some  part  or  other  oi  most  Plants,  we  meet  with  cells  con- 
taining granules  of  starch,  which  especially  abounds  in  the  tubers  of  the 
Potato,  and  in  the  seeds  of  Cereals  and  Legumes.  Starch  grains  are  ori- 
ginally formed  in  the  interior  of  Chlorophyll-corpuscles;  but  as  they  in- 


362 


THE  MICROSCOPE  AND  ITS  REVELATIONS. 


crease  in  size,  the  chlorophyll  thins  itself  out  as  a  mere  covering  film, 
and  at  last  disappears  altogether.  So  long  as  the  starch-grains  remain 
imbedded  in  the  protoplasm-layer,  they  continue  to  grow;  but  when  they 
accumulate  so  as  to  occupy  the  cell-cavity,  their  growth  stops.  They 
are  sometimes  minute  and  very  numerous,  and  so  closely  packed  as  to 
fill  the  cell-cavity  (Fig.  246);  in  other  instances  they  are  of  much  larger 
dimensions,  so  that  only  a  small  number  of  them  can  be  included  in 
any  one  cell;  while  in  other  cases,  again,  they  are  both  few  and  minute,  so 
that  they  form  but  a  small  proportion  of  the  cell- con  tents.  Their  nature 
is  at  once  detected  by  the  addition  of  a  solution  of  Iodine,  which  gives 
them  a  beautiful  blue  color.  Each  granule,  when  highly  magnified,  ex- 
hibits a  peculiar  spot,  termed  the  liilum;  round  which  are  seen  a  set  of 
circular  lines,  that  are  for  the  most  part  concentric  (or  nearly  so)  with  it. 
When  viewed  by  Polarized  light,  each  grain  exhibits  a  dark  cross,  the  point 
of  intersection  being  at  the  hilum  (Fig.  247);  and  when  a  Selenite-plate 
is  interposed,  the  cross  becomes  beautifully  colored.  Opinions  have  been 
very  much  divided  regarding  the  internal  structure  of  the  starch-grain; 
but  the  doctrine  of  Nageli,^  that  it  is  composed  of  successive  layers  which 


increase  by  ^intussusception,'  is  the  one  now  generally  adopted.  These 
layers  differ  in  their  proportion  of  water;  the  outermost  layer,  which  is 
the  most  solid,  having  within  it  a  watery  layer;  this,  again,  being  suc- 
ceeded by  a  firm  layer,  which  is  followed  by  a  watery  layer;  and  so  on, — 
the  proportion  of  water  increasing  towards  the  centre  in  both  kinds  of 
layers,  and  attaining  its  maximum  in  the  innermost  part  of  the  grain 
where  the  formation  of  new  layers  takes  place,  causing  the  distention  of 
the  older  ones. — Although  the  dimensions  of  the  Starch-grains  produced 
by  any  one  species  of  plant  are  by  no  means  constant,  yet  there  is  a  cer- 
tain average  for  each,  from  which  none  of  them  depart  very  widely;  and 
by  reference  to  this  average,  the  starch-grains  of  different  plants  that 
yield  this  product  in  abundance  may  be  microscopically  distinguished 
from  one  another,  a  circumstance  of  considerable  importance  in  com- 
merce. The  largest  starch-grains  in  common  use  are  those  of  the  plant 
(a  species  of  Canna)  known  as  Tous  les  mois;  the  average  diameter  of 
those  of  the  Potato  is  about  the  same  as  the  diameter  of  the  smallest  of 
the  Tous  les  mois;  and  the  size  of  the  ordinary  starch-grains  of  Wheat 
and  of  Sago  is  about  the  same  as  that  of  the  smallest  grains  of  Potato- 
starch;  while  the  granules  of  J?z6J6-starch  are  so  very  minute  as  to  be  at 
once  distinguishable  from  any  of  the  preceding. 

^  See  his  papers  in  Sitzungsberichte  der  Kon.  Bayer.  Akad.  der  Wissen- 
schaften,"  1862  and  1863;  and  Sachs'  Handbook  of  Botany"  (Bennett's  Transla- 
tion),  pp.  56-62. 


Jig.  247* 


Cells  of  Poeony,  filled  with  starch. 


Granules  of  Starchy  as  seen  under 
Polarized  light. 


MICROSCOPIC  STRUCTURE  OF  PHANEROGAMIC  PLANTS.  363 


359.  Deposits  of  Mineral  matter  in  a  crystalline  condition,  known  as 
raioliides^  are  not  unfrequently  found  in  vegetable  cells;  where  they  are 
at  once  brought  into  view  by  the  use  of  Polarized  light.  Their  designa- 
tion (derived  from  paq)i^,  a  needle)  is  very  appropriate  to  one  of  the 
most  common  states  in  which  these  bodies  present  themselves,  that, 
namely,  of  bundles  of  needle-like  crystals,  lying  side-by-side  in  the  cavity 
of  the  cell;  such  bundles  are  well  seen  in  the  cells  lying  immediately 
beneath  the  cuticle  of  the  bulb  of  the  medicinal  Squill.  It  does  not  apply, 
however,  to  other  forms  Avhicli  are  scarcely  less  abundant;  thus,  instead 
of  bundles  of  minute  needles,  single  large  crystals,  octohedral  or  prismatic, 
are  frequently  met  with;  and  the  prismatic  crystals  are  often  aggregated 
in  beautiful  stellate  groups.  One  of  the  most  common  materials  of 
raphidesis  Oxalate  of  Lime,  which  is  generally  found  in  the  stellate  orm; 
and  no  plant  yields  these  stellate  raphides  so  abundantly  as  the  common 
Rhubarb,  the  best  specimens  of  the  dry  medicinal  root  containing  as  much 
as  35  per  cent  of  them.  In  the  cuticle  of  the  bulb  of  the  Onion  the 
same  material  occurs  under  the  octohedral  or  the  prismatic  form.  In 
other  instanceSjthe  Calcareous  base  is  combined  with  Tartaric,  Citric,  or 
Malic  acid;  and  the  acicular  raphides  are  said  to  consist  usually  of 
Phosphate  of  Lime.  Some  Eaphides  are  as  long  as  l-40th  of  an  inch,  while 
others  measure  no  more  than  1-lOOth.  They  occur  in  ail  parts  of  plants, 
— the  Wood,  Pith,  Bark,  Boot,  Leaves,  Stipules,  Sepals,  Petals,  Fruits, 
and  even  in  the  Pollen.  They  are  always  situated  in  cells,  and  not,  as 
some  have  stated,  in  intercellular  passages;  the  cell-membrane,  however,  is 
often  so  much  thinned  away  as  to  be  scarcely  distinguishable.  Certain 
plants  of  the  Cactus  tribe,  when  aged,  have  their  tissues  so  loaded  with 
raphides  as  to  become  quite  brittle;  so  that  when  some  large  specimens  of 
C,  senilis,  said  to  be  a  thousand  years  old,  were  sent  to  Kew  Gardens 
from  South  America,  some  years  since,  it  was  found  necessary  for  their 
preservation  during  transport  to  pack  them  in  cotton,  like  jewelry. 
Raphides  are  probably  to  be  considered  as  non-essential  results  of  the 
Vegetative  processes;  being  for  the  most  part  produced  by  the  nnion  of 
organic  acids  gencated  in  the  plant,  with  mineral  bases  imbibed  by  it 
from  the  soil.  The  late  Mr.  E.  Quekett  succeeded  in  artificially  pro- 
ducing raphides  within  the  cells  of  Rice-paper  (§  351),  by  first  filling 
these  with  Lime-water  by  means  of  the  air-pump,  and  then  placing  the 
paper  in  weak  solutions  of  Phosphoric  and  Oxalic  acids.  The  artificial 
raphides  of  Phosphate  of  Lime  were  rhombohedral;  while  those  of 
Oxalate  of  Lime  were  stellate,  exactly  resembling  the  natural  raphides  of 
the  Rhubarb.^ 

360.  A  large  proportion  of  the  denser  parts  of  the  fabric  of  the  higher 
Plants  is  made  up  of  the  substance  which  is  known  as  ligneous  tissue  or 
woody  fibre.  This,  however,  can  only  be  regarded  as  a  very  simple  vari- 
ety of  cellular  tissue;  for  it  is  composed  of  peculiarly  elongated  cells 
(Fig.  259),  usually  pointed  at  their  two  extremities  so  as  to  become 

^  The  materials  of  the  above  paragraph  are  derived  from  the  excellent  section  on 
*  this  subject  ia  Prof.  Quekett's  Lectures  on  Histology."  Besides  the  Vegetable 
structures  therein  named  as  affording  good  illustrations  of  different  kinds  of 
Raphides,  may  be  mentioned  the  parenchyma  of  the  leaf  of  Agave^  Aloe,  Cycas, 
Encephalartos,  etc. ;  the  cuticle  of  the  bulb  of  the  Hyacinth,  Tulip,  and  Garlic 
(and  probably  of  other  bulbs);  the  bark  of  the  Apple,  Cascarilla,  Cinchona,  Lime, 
Locust,  and  many  other  trees;  the  pith  of  Eleagnus,  and  the  testa  of  the  seeds  of 
Anagallis  and  the  Elm.  — The  Raphides  characteristic  of  the  different  Natural 
Orders  of  Plants  were  carefully  studied  by  Mr.  Gulliver;  who  gave  an  account  of 
them  in  successive  Papers  in    Ann.  Nat.  Hist.,"  1861  et  seq. 


364: 


THE  MICROSCOPE  AND  ITS  REVELATIONS. 


spindle-shaped,  whose  walls  have  a  special  tendency  to  undergo  consoli- 
dation by  the  internal  deposit  of  sclerogen.  It  is  obvious  that  a  tissue 
consisting  of  elongated  cells,  adherent  together  by  their  entire  length, 
and  strengthened  by  internal  deposit,  must  possess  much  greater  tenacity 
than  any  tissue  in  which  the  cells  depart  but  little  from  the  primitive 
spherical  form;  and  we  accordingly  find  Woody  fibre  present  wherever  it 
is  requisite  that  the  fabric  should  posses  not  merely  density,  but  the 
power  of  resistance  to  tension.  In  the  higher  classes  of  the  Vegetable 
Kingdom  it  constitutes  the  chief  part  of  the  stem  and  branches,  where 
these  have  a  firm  and  durable  character;  and  even  in  more  temporary 
structures,  such  as  the  herbaceous  stems  of  annual  Plants,  and  the  leaves 
and  flowers  of  almost  every  tribe,  this  tissue  forms  a  more  or  less  impor- 
tant constituent,  being  especially  found  in  the  neighborhood  of  the  spiral 
vessels  and  ducts,  to  which  it  affords  protection  and  support.  Hence  the 
bundles  of  fasciculi  composed  of  these  elements,  which  form  the  ^  veins' 
of  leaves,  and  which  give  ^  stringiness '  to  various  esculent  vegetable  sub- 
stances, are  commonly  known  under  the  name  of  fibro-vascidar  tissue. 
In  their  young  and  unconsolidated  state,  the  ligneous  cells  seem  to  con- 
duct fluids  with  great  facility  in  the  direction  of  their  length;  and  in  the 
Coniferous  tribe,  whose  stems  and  branches  are  destitute  of  ducts,  they 
afford  the  sole  channel  for  the  ascent  ot  the  sap.  But  after  their  walls 
have  become  thickened  by  internal  deposit,  they  are  no  longer  subservient 
to  this  function:  nor,  indeed,  do  they  then  appear  to  fulfil  any  other 
purpose  in  the  Vegetable  economy  than  that  of  affording  mechanical  sup- 
port. It  is  this  which  constitutes  the  difference  between  the  alburnum  or 
^sap-wood/  and  the  duramen  or  ^heartwood,^  of  Exogenous  stems 
(§  369). 

3G1.  A  peculiar  set  of  markings  seen  on  the  Woody  fibres  of  the  Con- 
iferce,  and  of  some  other  tribes,  is  represented  in  Fig.  248;  in  each  of 
these  spots  the  inner  circle  appears  to  mark  a  deficiency  of  the  lining 
deposit,  as  in  the  pitted  cells  of  other  plants:  whilst  the  outer  circle 
indicates  the  boundary  of  a  lenticular  cavity  which  intervenes  between 
the  adjacent  cells  at  this  point.  There  are  varieties  in  this  arrangement 
so  characteristic  of  different  tribes,  that  it  is  sometimes  possible  to 
determine,  by  the  microscopic  inspection  of  a  minute  fragment,  even  of 
a  Fossil  wood,  the  tribe  to  which  it  belonged.  The  woody  fibre  thus 
marked  is  often  designated  as  gla7iclular, 

3G2.  All  the  more  perfect  forms  of  Phanerogamia  contain,  in  some 
part  of  their  fabric,  the  peculiar  structures  which  are  known  as  spiral 
vessels,'^  These  have  the  elongated  shape  of  woody  fibres;  but  the 
internal  deposit,  as  in  the  spiral  cells  (§  357),  takes  the  form  of  a  spiral 
fibre  winding  from  end  to  end,  remaining  distinct  from  the  cell-wall, 
and  retaining  its  elasticity;  this  fibre  may  be  single,  double,  or  even 
quadruple, — this  last  character  presenting  itself  in  the  very  large  elon- 
gated fibre-cells  of  the  Nepenthes  (Chinese  Pitcher-plant).  Such  cells 
are  especially  found  in  the  delicate  membrane  (medullary  sheath)  sur- 
rounding the  pith  of  Exogens,  and  in  the  midst  of  the  woody  bundles 
occurring  in  the  stem  of  Endogens;  thence  they  proceed  in  each  case  to 
the  leaf-stalks,  through  which  they  are  distributed  to  the  leaves.  By 
careful  dissection  under  the  Microscope,  they  may  be  separated  entire; 

^  So  long,  however,  as  they  retain  their  original  cellular  character,  and  do  not 
coalesce  with  each  other,  these  fusiform  spiral  cells  cannot  be  regarded  as  having 
any  more  claim  to  the  designation  of  vessels,  than  have  the  elongated  cells  of  the 
ligneous  tissue. 


MICROSCOPIC  STRUCTURE  OF  PHANEROGAMIC  PLANTS.  365 


but  their  structure  may  be  more  easily  displayed  by  cutting  round,  but 
not  through^  the  leaf-stalk  of  the  Strawberry,  Geranium,  etc.,  and  then 
drawing  the  parts  asunder.  The  membrane  composing  the  tubes  of  the 
vessels  will  thus  be  broken  across;  but  the  fibres  within,  being  elastic, 
will  be  drawn  out  and  unrolled.  Spiral  vessels  are  sometimes  found  to 
convey  liquid,  whilst  in  other  cases  they  contain  air  only;  the  conditions 
of  this  difference  are  not  yet  certainly  known. 

3G3.  Although  fluid  generally  finds  its  way  with  tolerable  facility 
through  the  various  forms  of  cellular  tissue,  especially  in  the  direction  of 
the  greatest  length  of  their  cells,  a  more  direct  means  of  connection 
between  distant  parts  is  required  for  its  active  transmission.  This  is 
afforded  by  ducts,  which  consist  merely  of  cells  laid  end  to  end,  the  par- 
titions between  them  being  more  or  less  obliterated.  The  origin  of  these 
Ducts  in  cells  is  occasionally  very  evident,  both  in  the  contraction  of 


Fia.  249. 


Section  of  Coniferous  Wood  Longitudinal  section  of  stem  of  Italian  Heed  : — a, 

in  the  direction  of  the  Fibres,  Cells  of  the  Pith;  6,  Fibro-vascular  bundle,  contain- 

showing their  ' glandular '  dots  :  ing  1,  Annular  duct;  2,  Spiral  duct;  3,  Pitted  duct, 

—a,  a,  a.  Medullary  Rays  cross-  with  Woody  fibre;  c,  Cells  of  the  integument, 
ing  the  fibres. 


their  calibre  at  regular  intervals,  and  in  the  persistence  of  remains  of 
their  partitions  (Fig.  263,  5,  b)  ;  but  in  most  cases  it  can  only  be  ascer- 
tained by  studying  the  history  of  their  development,  neither  of  these 
indications  being  traceable.  The  component  cells  appear  to  have  been 
sometimes  simply  membranous,  but  more  commonly  to  have  been  of  the 
fibrous  type  (§  357).  Some  of  the  ducts  formed  from  the  latter  (Fig. 
249,  2)  are  so  like  continuous  spiral  vessels  as  to  be  scarcely  distinguish- 
able from  them,  save  in  the  want  of  elasticity  in  their  spiral  fibre,  w^hich 
causes  it  to  break  when  the  attempt  is  made  to  draw  it  out.  This  rup- 
ture would  seem  to  have  taken  place,  in  some  instances,  from  the  natural 
elongation  of  the  cells  by  growth;  the  fibre  being  broken  uj)  into  rings, 
which  lie  sometimes  close  together,  but  more  commonly  at  considerable 
intervals;  such  a  duct  is  said  to  be  annular  (Fig.  249,  1).  Intermediate 


366 


THE  MICROSCOPE   AND  ITS  REVELATIONS. 


forms  between  the  spiral  and  annular  ducts,  wliicli  show  the  derivation 
of  the  latter  from  the  former,  are  very  frequently  to  be  met  with.  The 
spires  are  sometimes  broken  up  still  more  completely,  and  the  fragments 
of  the  fibre  extend  in  various  directions,  so  as  to  meet  and  form  an  irreg- 
ular network  lining  the  duct,  which  is  then  said  to  be  reticulated.  The 
continuance  of  the  deposit,  however,  gradually  contracts  the  meshes, 
leaving  the  walls  of  the  duct  marked  only  by  pores  like  those  of  porous  ^ 
cells  (§  356);  and  such  canals,  designated  as  pitted  diwct'^,  are  especially 
met  with  in  parts  of  most  solid  structure  and  least  rapid  growth  (Fig. 
249,  3).  The  '  scalarif orm '  ducts  of  Ferns  (§  340)  are  for  the  most  part 
of  the  spiral  type;  but  spiral  ducts  are  frequently  to  be  met  with  also  in 
the  rapidly  growing  leaf-stalks  of  Flowering-plants,  such  as  the  Rhubarb. 
Not  unfrequently,  however,  we  find  all  forms  of  ducts  in  the  same 
bundle,  as  seen  in  Fig.  249.  The  size  of  these  ducts  is  occasionally  so 
great  as  to  enable  their  openings  to  be  distinguished  by  the  unaided  eye; 
they  are  usually  largest  in  stems  whose  size  is  small  in  proportion  to  the 
surface  of  leaves  which  they  support,  such  as  the  common  Cane,  or  the 
Vine;  and,  generally  speaking,  they  are  larger  in  woods  of  dense  texture, 
such  as  Oak  or  Mahogany,  than  in  those  of  which  the  fibres,  remaining 
unconsolidated,  can  serve  for  the  conveyance  of  fluid.  They  are  entirely 
absent  in  the  Coniferce, 

364.  The  Vegetable  tissues  whose  principal  forms  have  been  now 
described,  but  among  which  an  immense  variety  of  detail  is  found,  may 
be  either  studied  as  they  present  themselves  in  thin  sections  of  the  vari- 
ous parts  of  the  plant  under  examination,  or  in  the  isolated  conditions  in 
which  they  are  obtained  by  dissection. — The  former  process  is  the  most 
easy,  and  yields  a  large  amount  of  information;  but  still  it  cannot  be 
considered  that  the  characters  of  any  tissue  have  been  properly  determined 
until  it  has  been  dissected  out.  Sections  of  some  of  the  hardest  Vegetable 
substances,  such  as  ^vegetable  ivory,^  the  '  stones^  of  fruit,  the  ^  shell  'of 
the  Cocoa-nut,  etc.  (§  356},  can  scarcely  be  obtained  except  by  slicing 
and  grinding  (§  192);  and  these  may  be  mounted  either  in  Canada 
balsam  or  in  Glycerine  jelly.  In  cases,  however,  in  which  the  tissues  are 
of  only  moderate  firmness,  the  section  may  be  most  readily  and  effectu- 
ally made  with  the  '  Microtome  ^  (§  184);  and  there  are  few  parts  of  the 
Vegetable  fabric  which  may  not  be  advantageously  examined  by  this 
means,  any  very  soft  or  thin  portions  being  placed  in  it  between  two  pieces 
of  cork,  elder-pith,  or  carrot.  In  certain  cases,  however,  in  which  even 
this  compression  would  be  injurious,  the  sections  must  be  made  with  a 
sharp  knife,  the  substance  being  laid  on  the  nail  or  a  slip  of  glass. — In 
dissecting  the  Vegetable  Tissues,  scarcely  any  other  instrument  will  be 
found  really  necessary,  than  a  pair  of  needles  (in  handles),  one  of  them 
ground  to  a  cutting  edge.  The  adhesion  between  the  component  cells, 
fibres,  etc.,  is  often  suliiciently  weakened  by  a  few  hours'  maceration  to 
alloAv  of  their  readily  coming  apart,  when  they  are  torn  asunder  by  the 
needle  points  beneath  the  simple  lens  of  a  Dissecting-microscope.  But  if 
this  should  not  prove  to  be  the  case,  it  is  desirable  to  emj)loy  some  other 
method  for  the  sake  of  facilitating  their  isolation.  None  is  so  effectual 
as  the  boiling  of  a  thin  slice  of  the  substance  under  examination,  either 
in  dilute  nitric  acid,  or  in  a  mixture  of  nitric  acid  and  chlorate  of  potass. 
This  last  method  (which  was  devised  by  Schultz)  i*s  the  most  rapid  and 
effectual,  requiring  only  a  few  minutes  for  its  performance;  but  as 
oxygen  is  liberated  with  such  freedom  as  to  give  an  almost  explosive 
character  to  the  mixture,  it  should  be  put  in  practice  with  extreme 


MICROSCOPIC  STRUCTURE  OF  PHANEROGAMIC  PLANTS.  367 


caution.  After  being  thus  treated,  the  tissue  should  be  boiled  in  alcohol, 
and  then  in  water;  and  it  will  then  be  found  very  easy  to  tear  apart  the 
individual  cells,  ducts,  etc.,  of  which  it  may  be  composed.  These  may 
be  preserved  by  mounting  in  weak  spirit. 

365.  Stem  and  Root. — It  is  in  the  stems  and  roots  that  we  find  the 
greatest  variety  of  tissues  in  combination,  and  the  most  regular  plans  of 
structure;  and  sections  of  these  viewed  under  a  low  magnifying  power 
are  objects  of  peculiar  beauty,  independently  of  the  scientific  information 
which  they  afford.  The  Axis  (under  which  term  is  included  the  stem 
with  its  branches,  and  the  roots  with  its  ramifications)  always  has  for  the 
basis  of  its  structure  a  dense  cellular  parenchyma;  though  this,  in  the 
advanced  stage  of  development,  may  constitute  but  a  small  portion  of  it. 
In  the  midst  of  the  parenchyma  we  generally  find  '  fibro-vascular ' 
bundles,  consisting  of  woody  fibre,  with  ducts  of  various  kinds,  and 
(very  commonly)  spiral  vessels.  It  is  in  the  mode  of  arrangement  of 
these  bundles,  that  the  fundamental  difference  exists  between  the  stems 

1^  f25_a. 


Transverse  Section  of  Stem  of  young  Palm,  Portion  of  Transverse  Section  of  Stem  of 

Wanghie  Cane, 


which  are  commonly  designated  as  endogenous  (growing  from  within), 
and  those  which  are  more  correctly  termed  exogenous  (growing  on  the 
outside):  for  in  the  former  the  bundles  are  dispersed  throughout  the 
whole  diameter  of  the  axis  without  any  peculiar  plan,  the  intervals 
I  between  them  being  filled  up  by  cellular  parenchyma;  whilst  in  the  latter 
they  are  arranged  side  by  side  in  such  a  manner  as  to  form  a  hollow 
cylinder  of  wood,  which  includes  within  it  the  portion  of  the  cellular 
substance  known  as  pith,  whilst  it  is  itself  inclosed  in  an  envelope  of  the 
same  substance  that  forms  the  bai^Jc.  These  two  plans  of  Axis-formation 
respectively  characteristic  of  those  two  great  groups  into  which  Phanero- 
gams are  subdivided — namely,  the  Monocotyledons  and  the  Dicotyledons 
— will  now  be  more  particularly  described. 

366.  When  a  transverse  section  (Fig.  250)  ot  a  monocotyledonoics  Stem. 
is  examined  microscopically,  it  is  found  to  exhibit  a  number  of  fibro- 
vascular  bundles,  disposed  without  any  regularity  in  the  midst  of  the 
mass  of  cellular  tissue,  which  forms  (as  it  were)  the  matrix  or  basis  of 
the  fabric.    Each  bundle  contains  two,  three,  or  more  large  ducts,  which 


368 


THE  MICROSCOPE  AND  ITS  REVELATIONS. 


are  at  once  distinguished  bj  the  size  of  their  openings;  and  these  are 
surrounded  by  woody  fibre  and  spiral  vessels,  the  transverse  diameter  of 
which  is  so  extremely  small,  that  the  portion  of  the  bundles  which  they 
form  is  at  once  distinguished  in  transverse  section  by  the  close7iess  of  its 
texture  (Fig.  251).  The  bundles  are  least  numerous  in  the  centre  of  the 
stem,  and  become  gradually  more  api^roximated  towards  its  circumference 
but  it  frequently  happens  that  the  portion  of  the  area  in  which  they  are 
most  compactly  arranged  is  not  absolutely  at  its  exterior,  this  portion 
being  itself  surrounded  by  an  investment  composed  of  cellular  tissue  only; 
and  sometimes  we  find  the  central  portion,  also,  completely  destitute  of 
fibro-vascular  bundles;  so  that  a  sort  of  indication  of  the  distinction  be- 
tween Pith,  Wood,  and  Bark  is  here  presented.  This  distinction,  however, 
is  very  imperfect;  for  Ave  do  not  find  either  the  central  or  the  peripheral 

f)ortions  ever  separable,  like  pith  and  bark,  from  the  intermediate  woody 
ayer.  In  its  young  state,  the  centre  of  the  stem  is  always  filled-up  with 
cells;  but  these  not  unfrequently  disappear  after  a  time,  except  at  the 
nodes,  leaving  the  stem  hollow,  as  we  see  in  the  whole  tribe  of  Grasses, 


Diagram  of  the  first  formation  of  Transverse  Section  of  Stem  of  Clematis:— a,  pith ;  5,  6,  &» 
an  Exogenous  Stem  ;  —a,  Pith  ;  b  6,  woody  bundels;  c,  c,  c,  medullary  x'ays. 

Bark;  c  c,  plates  of  cellular  tissue 
(Medullar  Rays)  left  between  the 
Woody  Bundles  d  d , 

When  a  vertical  section  is  made  of  a  woody  stem  (as  that  of  a  Palm)  of 
sufficient  length  to  trace  the  whole  extent  of  the  fibro-vascular  bundles, 
it  is  found  that  whilst  they  pass  at  their  upper  extremity  into  the  leaves, 
they  pass  at  the  lower  end  toward  the  surface  of  the  stem,  and  assist,  by 
their  interlacement  with  the  outer  bundles,  in  forming  that  extremely 
tough  investment  which  the  lower  ends  of  these  stems  present.  New 
fibro-vascular  bundles  are  being  continually  formed  in  the  upper  part  of 
the  stem,  in  continuity  with  the  leaves  which  are  successively  put  forth  at 
its  summit;  but  while  these  take  part  in  the  elongation  of  the  stem,  they 
contribute  but  little  to  the  increase  of  its  diameter.  For  those  which  are 
most  recently  formed  only  pass  into  the  centre  of  the  stem  during  the 
higher  part  of  their  course,  and  usually  make  their  way  again  to  its  ex- 
terior at  no  great  distance  below;  and  when  once  formed,  they  receive 
no  further  additions.  It  was  from  the  idea  formerly  entertained  that 
these  successively-formed  bundles  descend  in  the  interior  of  the  stem 
through  its  entire  length  until  they  reach  the  roots,  and  that  the  stem  is 
thus  continually  receiving  additions  to  its  interior,  that  the  term  endoge- 
nous was  givea  to  this  type  of  stem -structure;  but  from  the  fact  just 
stated  regarding  the  course  of  the  fibro-vascular  bundles,  it  is  obvious 
that  such  a  doctrine  cannot  be  any  longer  admitted. 


MICUOSCOPIO  STRUCTURE  OF  PHANKROGAMIO  PLANTS.  369 


367.  In  the  Stems  of  dicotyledonous  Phanerogams,  on  the  other  hand, 
we  find  a  method  of  arrangement  of  the  several  parts,  which  must  be  re- 
garded as  the  highest  form  of  the  development  of  the  axis,  being  that  in 
which  the  greatest  differeyitiation  exists.  A  distinct  division  is  always 
seen  in  a  transverse  section  (Fig.  252)  between  three  concentric  areae, — 
the  pithy  the  wood,  and  the  bark;  the  first  {a)  being  central,  the  last  {b) 
peripheral,  and  these  having  the  wood  interposed  between  them,  its  circle 
being  made  up  of  wedged-shaped  bundles  {d,  d),  kept  apart  by  the  bands 
(Cy  c),  that  pass  between  the  pith  and  the  bark. — pith  (Fig.  253,  a) 
is  almost  invariably  composed  of  cellular  tissue  only,  which  usually,  pre- 
sents (in  transverse  section)  a  hexagonal  areolation.  When  newly  formed 
it  has  a  greenish  hue,  and  its  cells  are  filled  with  fluid,  but  it  gradually 
dries-up  and  loses  its  color;  and  not  unfrequently  its  component  cells  are 
torn  apart  by  the  rapid  growth  of  their  envelope,  so  that  irregular  cavities 
are  found  in  it;  or,  if  the  stem  should  increase  with  extreme  rapidity,  it 
becomes  hollow,  the  pith  being  reduced  to  fragments,  which  are  found 
adhering  to  its  interior  wall.    The  pith  is  immediately  surrounded  by  a 


Transvprso  Section  of  Stem  of  Rham-  Portion  of  the  same,  more 
nus  (Buckthorn),  showing  concentric  highly  magnified, 

layers  of  wood. 


delicate  membrane  consisting  almost  entirely  of  spiral  vessels,  which  is 
termed  the  medullary  sheath. 

368.  The  woody  portion  of  the  stem  (Fig.  253,  5,  S),  is  made  up  of 
woody  fibres,  usually  with  the  addition  of  ducts  of  various  kinds;  these, 
however,  are  absent  in  one  large  group,  the  ConifercB  or  Fir  tribe  with 
its  allies  (Figs.  258-260)  in  which  the  woody  fibres  are  of  unusually  large 
dameter,  and  have  the  peculiar  markings  already  described  (§361). 
In  any  stem  or  branch  of  more  than  one  year's  growth,  the  woody 
structure  presents  a  more  or  less  distinct  appearance  of  division  into 
concentric  rings,  the  number  of  which  varies  with  the  age  of  the  tree 
(Fig.  254).  The  composition  of  the  several  rings,  which  are  the  sec- 
tions of  so  many  cylindrical  layers,  is  uniformly  the  same,  however 
difiEerent  their  thickness;  but  the  arrangement  of  the  two  principal  ele- 
ments— namely,  the  woody  fibre  and  the  ducts — varies  different  species: 
the  ducts  being  sometimes  almost  uniformly  diffused  through  the  whole 
layer,  but  in  other  instances  being  confined  to  its  inner  part;  while  in 
other  cases,  again,  they  are  dispersed  with  a  certain  regular  irregularity 
(if  such  an  expression  may  be  allowed),  so  as  to  give  a  curiously  figured 
24 


370 


THE  MICROSCOPE  AND  ITS  KEVELATIONS. 


appearance  to  the  transverse  section  (Figs.  254,  255).  The  general  fact, 
however,  is,  that  the  ducts  predominate  towards  the  inner  side  of  the 
ring  (which  is  the  part  of  it  lirst  formed),  and  that  the  outer  portion  of 
each  layer  is  almost  exclusi  vely  composed  of  woody  tissue :  such  an  arrange- 
ment is  shown  in  Fig.  253.  This  alternation  of  ducts  and  woody  fibre 
frequently  serves  to  mark  the  succession  of  layers,  when,  as  it  is  not  un- 
common, there  is  no  very  distinct  line  of  separation  between  them. 

369.  The  number  of  layers  is  usually  considered  to  correspond  with 
that  of  the  years  during  which  the  stem  or  branch  has  been  growing;  and 
this  is,  no  doubt,  generally  true  in  regard  to  the  trees  of  temparate  cli- 
mates, which  thus  ordinarily  increase  by  'annual  layers.^  There  can  be 
no  doubt,  however,  that  such  is  not  the  universal  rule:  and  that  we  should 
be  more  correct  in  stating  that  each  layer  indicates  an  epoch  of  vegetation; 
which,  in  temperate  climates,  is  usually  (but  not  invariably)  a  year,  but 
which  is  commonly  much  less  in  the  case  of  trees  flourishing  in  tropical 
regions.  Thus  among  the  latter  it  is  very  common  to  find  the  leaves 
regularly  shed  and  replaced  ttoice  or  even  thrice  in  a  year,  or  five  times  in 
two  years;  and  for  every  crop  of  leaves  there  will  be  a  corresponding  layer 
of  wood.  It  sometimes  happens,  even  in  temperate  climates,  that  trees 
shed  their  leaves  prematurely  in  consequence  of  continued  drought,  and 
that,  if  rain  then  follow,  afresh  crop  of  leaves  appears  in  the  same  season; 
and  it  cannot  be  doubted  that  in  such  a  year  there  would  be  two  rings  of 


Portion  of  Transverse  Section  of  Stem  of  Hazel^  showing,  in  the  portion  a,  6,  c,  six  narrow  layers 
of  Wood. 

wood  produced,  which  would  probably  not  together  exceed  the  ordinary 
single  layer  in  thickness.  That  such  a  division  may  even  occur  as  a  conse- 
quence of  an  interruption  to  the  processes  of  vegetation,  produced  by  sea- 
sonal changes, — as  by  heat  and  drought  in  a  tree  that  flourishes  best  in  a 
cold  damp  atmosphere,  or  by  a  fall  of  temperature  in  a  tree  that  requires 
heat, — would  appear  from  the  frequency  with  which  a  double  or  even  a 
multiple  succession  is  found  in  transverse  sections  of  wood  to  occupy  the 
place  of  a  single  one.  Thus  in  a  section  of  Hazel  stem  (in  the  Author's 
possession),  of  which  a  portion  is  represented  in  Fig.  256,  between  two 
layers  of  the  ordinary  thickness  there  intervenes  a  band  whose  breadth  is 
altogether  less  than  that  of  either  of  them,  and  which  is  yet  composed  of 
no  fewer  than  six  layers,  four  of  them  {c)  being  very  narrow,  and  each 
of  the  other  two  {a,  b)  being  about  as  wide  as  these  four  together. — The 
inner  layers  of  wood,  being  not  only  the  oldest,  but  the  most  solidified 
by  matters  deposited  within  their  component  cells  and  vessels,  are  spoken 
of  collectively  under  the  designation  ^^^^rame7^  or  '  heart-wood.'  On  the 
other  hand,  it  is  through  the  cells  and  ducts  of  the  outer  and  newer  lay- 
ers that  the  sap  rises  from  the  roots  towards  the  leaves;  and  these  are 
consequently  designated  as  alburnum  or  ^sap-wood.'  The  line  of  demar- 
cation between  the  two  is  sometimes  very  distinct,  as  in  Lignum-vitae  and 
Cocos  wood;  and  as  a  new  layer  is  added  every  year  to  the  exterior  of  the 


MICROSCOPIC  STRUCTURE  OF  PHANEROGAMIC  PLANTS.  371 


alburnum,  an  additional  layer  of  the  innermost  part  of  the  alburnum  is 
every  year  consolidated  by  internal  deposit,  and  is  thus  added  to  the 
exterior  of  the  duramen.  More  generally,  however,  this  consolidation 
is  gradually  effected,  and  the  alburnum  and  duramen  are  not  separated 
by  any  abrupt  line  of  division. 

370.  The  medullary  rays  which  cross  the  successive  rings  of  wood 
connecting  the  cellular  substance  of  the  pith  with  that  of  the  bark,  and 
dividing  each  ring  of  wood  into  wedge-shaped  segments,  are  thin  plates 
of  cellular  tissue  (Fig.  253,  c,  c),  not  usually  extending  to  any  great  depth 
in  the  vertical  direction.    It  is  not  often,  however,  that  their  character 


EiG.  257* 


Portion  of  Transverse  Section  of  the  Stem  of  Cedar:— pith ;  6,  6,  6,  woody  layers ;  c,  bark. 


can  be  so  clearly  seen  in  a  transverse  section  as  in  the  diagram  just  re- 
ferred to;  for  they  are  usually  compressed  so  closely  as  to  appear  darker 
than  the  wedges  of  woody  tissue  between  which  they  intervene  (Figs.  255, 
257;;  and  their  real  nature  is  best  understood  by  a  comparison  of  longi- 
titdinal  sections  made  in  two  different  directions, — namely  radial  and 
tangential, — with  the  t  r  a  n  s  v  e  rse. 
Three  such  sections  of  a  fossil  Coni- 
ferous wood  in  the  Author's  possession 
are  shown  in  Figs.  258-260.  The 
stem  was  of  such  large  size,  that,  in 
so  small  a  part  of  the  area  of  its 
transverse  section  as  is  represented  in 
Fig.  258,  the  medullary  rays  seem 
to  run  parallel  to  each  other,  instead 
of  radiating  from  a  common  centre. 
They  are  very  narrow  ;  but  are  so 
closely  set  together,  that  only  two  or 
three  rows  of  woody  fibres  (no  ducts 

beins:  here  present)  intervene  between    ^         ^  ^  ^  ^.      ^  , 

o     .       K  '       T      ^-1.^  l^v^^U,.       Portion  of  Transverse  Section  of  large 

any   pair   OI   them.       in  tne  longltU-   stem  of  coniferous  wood  (fossil),  showing 

clinal  section  taken  in  a  radial  direction  l^^^^^^^^it^V^fl^^t^^^ 
(Fig.  259),  and  consequently  passing  Medullary  Rays, 
in  the  same  course  with  the  medul- 
lary rays,  these  are  seen  as  thin  plates  (a,  a,  a)  made-up  of  superposed 
cells  very  much  elongated,  and  crossing  in  a  horizontal  direction  the 
woody  fibres  which  lie  parallel  to  one  another  vertically.    And  in  the 
tangential  section  (Fig.  260),  which  passes  a  direction  at  right  angles  to 


872 


THE  MICROSCOPE  AND  ITS  REVELATIONS. 


that  of  the  medullary  rays,  and  therefore  cuts  them  across,  we  see  that 
each  of  the  plates  thus  formed  has  a  very  limited  depth  from  above 
downwards,  and  is  composed  of  no  more  than  one  thickness  of  horizontal 


cells. — A  section  of  the  stem  of  Mahogany  taken  in  the  same  direction  as 
the  last  (Fig.  261),  gives  a  very  good  view  of  the  cut  ends  of  the  medullary 
rays,  as  they  pass  between  the  woody  fibres;  and  they  are  seen  to  be  here 


Fig.  261  .—Vertical  Section  of  Mahogany. 

Fig.  2d-2. —Transverse  section  of  a  l^ossil  Wood;  snowing  the  Medullary  Rays,  a,  a,  a,  a,  a,  a, 
running  nearly  parallel  to  each  other,  and  the  openings  of  large  Ducts  in  the  midst  of  the  woody 
fibres. 

Fig.  263. — ^Vertical  (tangential)  section  of  the  same  wood ;  showing  the  Woody  fibres  separated 
by  the  Medullary  Rays,  and  by  the  large  Ducts,  b  b,  b  b. 

of  somewhat  greater  thickness,  being  composed  of  two  or  three  rows  of 
cells,  arranged  side  by  side. 


Tig.  25!) 


"Pig.  200. 


Portion  of  Vertical  Section  of  the 
same  wood,  taken  in  a  radial  direc- 
tion, showing  the  glandular  Woody 
fibres,  without  Ducts,  crossed  by  the 
Medullary  Rays,  a,  a. 


Portion  of  Vertical  Section 
of  the  same  wood,  taken  in  a 
tangential  direction,  so  as  to 
cut  across  the  Medullary 
Rays. 


MICROSCOPIC  STRUCTURE  OF  PHANEROGAMIC  PLANTS.  373 


371.  In  another  fossil  wood,  whose  transverse  section  is  shown  in  Fig. 
262,  and  its  tangential  section  in  Fig.  263,  the  medullary  rays  are  seen 
to  occupy  a  much  larger  part  of  the  substance  of  the  stem:  being  shown 
in  the  transverse  section  as  broad  bands  {a  a,  a  a)  intervening  between 
the  closely-set  woody  fibres,  among  which  some  large  ducts  are  scattered; 
whilst  in  the  tangential,  they  are  observed  to  be  not  only  deeper  than  the 
preceding  from  above  downwards,  but  also  to  have  a  much  greater  thick-' 
ness.  This  section  also  gives  an  excellent  view  of  the  ducts,  bh,  iiy 
which  are  here  plainly  seen  to  be  formed  by  the  coalescence  of  large 
cylindrical  cells,  lying  end-to-end. — In  another  fossil  wood  in  the  Author's 
possession,  the  medullary  Rays  constitute  a  still  larger  proportion  of  the 
stem;  for  in  the  transverse  section  (Fig.  261),  they  are  seen  as  very  broad 
bands  5),  alternating  with  plates  of  woody  structure  (a,  a),  whose 
thickness  is  often  less  than  their  own;  whilst  in  the  tangential  section 
(Fig.  265)  the  cut  extremities  of  the  medullary  rays  occupy  a  very  large 
part  of  the  area,  having  apparently  determined  the  sinuous  course  of  the 


Transverse  and  Vertical  Sections  of  a  Fossil  Wood;  showing  the  separation  of  the  Woody- 
plates,  aa^aa^  by  the  very  large  Medullary  Rays,  hb^bb, 

woody  fibres;  instead  of  looking  (as  in  Fig.  260)  as  if  they  had  forced 
their  way  between  the  woody  fibres,  which  there  hold  a  nearly  straight 
and  parallel  course  on  either  side  of  them. — The  medullary  rays  main- 
tain a  connection  between  the  external  and  the  internal  parts  of  the  cel- 
lular basis  of  the  stem,  which  have  been  separated  by  the  interposition  of 
the  wood. 

372.  The  larh  may  be  usually  found  to  consist  of  three  principal 
layers;  the  external,  or  epiphloeumy  also  termed  the  suherous  (or  corky) 
layer;  the  middle,  or  mesophlceum^  also  termed  the  cellular  envelope;  and 
the  internal,  or  eyidophloewn,  which  is  more  commonly  known  as  the  liher. 
The  two  outer  layers  are  entirely  cellular;  and  are  chiefly  distinguished 
by  the  form,  size,  and  direction  of  their  cells.  The  epiplilmim  is  gener- 
ally composed  of  one  or  more  layers  of  colorless  or  brownish  cells,  which 
usually  present  a  cubical  or  tabular  form,  and  are  arranged  with  their 
long  diameters  in  the  horizontal  direction;  it  is  this  which,  when  devel- 
oped to  an  unusual  thickness,  forms  cork,  a  substance  which  is  by  no 
means  the  product  of  one  kind  of  tree  exclusively,  but  exists  in  greater 
or  less  abundance  in  the  bark  of  every  exogenous  stem.  The  mesophlcemn 
consists  of  cells,  usually  of  green  color,  prismatic  in  their  form,  and  dis- 


374 


THE  MICROSCOPE  AND  ITS  REVELATIONS. 


posed  with  their  long  diameters  parallel  to  the  axis;  it  is  more  loosely 
arranged  than  the  preceding,  and  contains  intercellular  passages,  which 
often  form  a  network  of  canals  that  have  been  termed  laticiferous  yessels; 
and,  although  usually  less  developed  than  the  suberous  layers,  it  some- 
times constitutes  the  chief  thickness  of  the  bark.  The  liber  or  '  inner 
bark/  on  the  other  hand,  usually  contains  woody  fibre  in  addition  to  the 
cellular  tissue  and  laticiferous  canals  of  the  preceding;  and  thus  ap- 
proaches more  nearly  in  its  character  to  the  woody  layers,  with  which  it 
is  in  close  proximity  on  its  inner  surface.  The  '  liber  ^  may  generally  be 
found  to  be  made  up  of  a  succession  of  thin  layers,  equalling  in  number 
those  of  the  wood,  the  innermost  being  the  last  formed;  but  no  such  suc- 
cession can  be  distinctly  traced  either  in  the  cellular  envelope  or  in  the 
suberous  layer,  although  it  is  certain  that  they  too  augment  in  thickness 
by  additions  to  their  interior,  whilst  their  external  portions  are  frequently 
thrown-off  in  the  form  of  thickish  plates,  or  detach  themselves  in  smaller 
and  thinner  laminae. — The  bark  is  always  separated  from  the  wood  by  the 
camhi'Um-layer,  which  is  the  part  wherein  all  new  growth  takes  place; 
this  seems  ito  consist  of  mucilaginous  semi-fluid  matter;  but  it  is  really 
made-up  of  cells  of  a  very  delicate  texture,  which  gradually  undergo 
transformation,  whereby  they  are  for  the  most  part  converted  into  woody 
fibres,  ducts,  spiral  vessels,  etc.  These  materials  are  so  arranged  as  to 
augment  the  fibro-vascular  bundles  of  the  wood  on  their  external  surface, 
thus  forming  a  new  layer  of  '  alburnum,^  which  incloses  all  those  that 
preceded  it;  whilst  they  also  form  a  new  layer  of  ^  liber,'  on  the  interioi' 
of  all  those  which  preceded  it:  they  also  extend  the  medullary  raj'S, 
which  still  maintain  a  continuous  connection  between  the  pith  and  the 
bark;  and  a  portion  remains  unconverted,  so  as  always  to  keep  apart  the 
liber  and  the  alburnum. — This  type  of  stem-structure  is  termed  exogenous; 
a  designation  which  applies  very  correctly  to  the  mode  of  increase  of  the 
woody  layers,  although  (as  just  shown)  the  liber  is  formed  upon  a  truly 
endogenous  plan. 

373.  Numerous  departures  from  the  normal  type  are  found  in  partic- 
ular tribes  of  Dicotyledons.  Thus  in  some  the  wood  is  not  marked  by 
concentric  circles,  their  growth  not  being  interrupted  by  any  seasonal 
change.  In  other  cases,  again,  each  woody  zone  is  separated  from  the 
next  by  the  interposition  of  a  thick  layer  of  cellular  substance.  Some- 
times wood  is  formed  in  the  bark  (as  in  Calycantlms),  so  that  several 
woody  columns  are  produced,  which  are  quite  independent  of  the  princi- 
pal woody  axis,  but  cluster  around  it.  Occasionally  the  woody  stem  is 
divided  into  distinct  segments  by  the  peculiar  thickness  of  certain  of  the 
medullary  rays;  and  in  the  stem  of  which  Fig.  266  represents  a  trans- 
verse section,  these  cellular  plates  form  four  large  segments,  disposed  in 
the  manner  of  a  Maltese  cross,  and  alternating  with  the  four  woody  seg- 
ments, which  they  equal  in  size. 

374.  The  Exogenous  stem,  like  the  (so-called)  Endogenous,  consists, 
in  its  first-developed  state,  of  cellular  tissue  only;  but  after  the  leaves 
have  been  actively  performing  their  functions  for  a  short  time,  we  find  a 
circle  of  fibro-vascular  bundles,  as  represented  in  Fig.  252,  interposed 
between  the  central  (or  medullary)  and  the  peripheral  (or  cortical)  por- 
tions of  the  cellular  matrix;  these  fibro-vascular  bundles  being  themselves 
separated  from  each  other  by  plates  of  cellular  tissue,  which  still  remain 
to  connect  the  central  and  the  peripheral  portions  of  that  matrix.  This 
first  stage  in  the  formation  of  the  Exogenous  axis,  in  which  its  principal 
parts — the  pith,  wood,  bark,  and  medullary  rays — are  marked-out,  is  seen 


MICROSCOPIC  STRCCTURE  OF  PHANEROGAMIC  PLANTS.  375 


even  in  the  stems  of  herbaceous  Plants,  which  are  destined  to  die  down  at 
the  end  of  the  season  (Fig.  267)  ;  and  sections  of  these,  which  are  very 
easily  prepared,  are  most  interesting  Microscopic  objects.  In  such  stems, 
the  difference  between  the  Endogenous  and  the  Exogenous  type  is  man- 
ifested in  little  else  than  the  disposition  of  the  fibro-vascular  layers; 
which  are  scattered  through  nearly  the  whole  of  the  cellular  matrix 
(although  more  abundant  towards  its  exterior)  in  the  former  case;  but 
are  limited  to  a  circle  within  the  peripheral  portion  of  the  cellular  tissue 
in  the  latter.  It  is  in  the  further  development  which  takes  place  during 
succeeding  years  in  the  woody  stems  of  perennial  Exogens,  that  those 
characters  are  displayed,  which  separate  them  most  completely  from  the 
Ferns  and  their  allies,  whose  stems  contain  a  cylindrical  layer  of  fibro- 
vascular  bundles,  as  well  as  from  ^'so-called)  Endogens.    For  whilst  the 


fibro-vascular  layers  of  the  latter,  when  once  formed,  undergo  no  further 
increase,  those  of  Exogenous  stems  are  progressively  augmented  on  their 
outer  side  by  the  metamorphosis  of  the  cambium-layer;  so  that  each  of 
the  bundles  which  once  lay  as  a  mere  series  of  parallel  cords  beneath  the 
cellular  investment  of  a  first-year's  stem,  may  become  in  time  the  small 
end  of  a  wedge-shapi'd  mass  of  wood,  extending  continuously  from  the 
centre  to  the  exterior  of  a  trunk  of  several  feet  in  diameter,  and  becom- 
ing progressively  thicker  as  it  passes  upwards.  The  fibro-vascular  bundles 
of  Exogens  are  therefore  spoken  of  as  ^indefinite;'  whilst  those  of  Endo- 
gens and  Acrogens  (Ferns,  etc.)  are  said  to  be  definite'  or  ^ closed.^ 

375.  The  structure  ot  the  roots  of  Endogens  and  Exogens  is  essentially 
the  same  in  plan  with  that  of  their  respective  Stems.  Generally  speaking, 
however,  the  roots  of  Exogens  have  no  pith,  although  they  have  medullary 
rays;  and  the  succession  of  distinct  rings  is  less  apparent  in  them,  than 
it  is  in  the  stems  from  which  they  diverge.  In  the  delicate  radical 
filaments  which  proceed  from  the  larger  root-fibres,  a  central  bundle  of 
vessels  will  be  seen,  enveloped  in  a  sheath  of  cellular  substance;  and  this 
investment  also  covers-in  the  end  of  the  fibril,  which  is  usually  somewhat 
dilated,  and  composed  of  peculiarly  succulent  tissue,  forming  what  is 
termed  the  spongiole.    The  structure  of  the  radical  filaments  may  be  well 


Fig.  267. 


Fig.  2f)f^ 


Transverse  section  of  the  stem  of  a 
climbing. plant  {Aristo lochia^)  from 
New  Zealand. 


Portion  of  transverse 
section  of  Arctium  (Bur- 
dock), showing  one  of  the 
Fibro-vascular  bundles 
that  lies  beneath  the  cellu- 
lar integument. 


376 


THE  MICROSCOPE  AND  ITS  REVELATIONS. 


studied  in  the  common  DucJciveed,  every  floating  leaf  of  which  has  a  single 
fibril  hanging  down  from  its  lower  surface. 

376.  The  structure  of  Stems  and  Eoots  cannot  be  thorouglily  examined 
in  any  other  way,  than  by  making  sections  in  different  dh^ections  with  the 
Microtome.  The  general  instructions  already  given  (§  184)  leave  little  to 
be  added  respecting  this  special  class  of  objects;  the  chief  points  to  be 
attended  to  being  the  preparation  of  the  Stems,  etc.,  for  slicing,  the  sharp- 
ness of  the  knife  and  the  dexterity  with  which  it  is  handled,  and  the  method 
of  mounting  the  sections  when  made.  The  wood,  if  green,  should  first 
be  soaked  in  strong  alcohol  for  a  few  days,  to  get  rid  of  the  resinous 
matter;  and  it  should  then  be  macerated  in  water  for  some  days  longer, 
for  the  removal  of  its  gum,  before  being  submitted  to  the  cutting-process. 
If  the  wood  be  dry,  it  should  first  be  softened  by  soaking  for  a  sufficient 
length  of  time  in  water,  and  then  treated  with  spirit,  and  afterwards  with 
water,  like  green  wood.  Some  woods  are  so  little  affected  even  by  pro- 
longed maceration,  that  boiling  in  water  is  necessary  to  bring  them  to 
the  degree  of  softness  requisite  for  making  sections.  Xo  wood  that  has 
once  been  dry,  however,  yields  such  good  sections  as  that  which  is  cut  fresh. 
When  a  piece,  of  the  appropriate  length,  has  been  placed  in  the  grasp  of 
the  Section-instrument  (wedges  of  deal  or  other  soft  wood  being  forced-in 
with  it,  if  necessary  for  its  firm  fixation),  a  few  thick  slices  should  first  be 
taken  to  reduce  its  surface  to  an  exact  level;  the  surface  should  then  be 
wetted  with  spirit,  the  Micrometer-screw  moved  through  a  small  part  of  a 
revolution,  and  the  slice  taken  off  with  the  razor^  the  motion  given  to  which 
should  partake  both  of  draiviyig  ViVLdi  pushing.  A  little  practice  will  soon 
enable  the  operator  to  discover,  in  each  case,  Jioio  thin  he  may  venture  to 
cut  his  sections  without  a  breach  of  continuity;  and  the  Micrometer-screw 
should  be  turned  so  as  to  give  the  required  elevation.  If  the  surface  of 
the  wood  has  been  sufficiently  wetted,  the  section  will  not  curl-up  in  cut- 
ting, but  will  adhere  to  the  surface  of  the  razor,  from  which  it  is  best  de- 
tached by  dipping  the  razor  in  water  so  as  to  float  away  the  slice  of  wood, 
a  camel-hair  pencil  being  used  to  push  it  off,  if  necessary.  All  the  sections 
that  may  be  found  sufficiently  thin  and  perfect,  should  be  put  aside  in  a 
bottle  of  weak  spirit  until  they  be  mounted.  For  the  minute  examination 
of  their  structure,  they  may  be  mounted  either  in  weak  spirit  or  in  gly- 
cerine jelly.  Where  a  mere  general  view  only  is  needed,  dry-mounting 
answers  the  purpose  sufficiently  well;  and  there  are  many  stems,  such  as  the 
Clematis,  of  which  transverse  sections  rather  thicker  than  ordinary  make 
very  beautiful  opaque  objects,  when  mounted  dry  on  a  black  ground. 
Canada  Balsam  should  not  be  had  recourse  to,  except  in  the  case  of  very 
opaque  section,  as  it  usually  makes  the  structure  too  transparent.  Trans- 
verse section,  however,  when  slightly  charred  by  heating  between  two  plates 
of  glass  until  they  turn  brown,  may  be  mounted  with  advantage  in  Canada 
balsam  and  are  then  very  showy  specimens  for  the  Gas-Microscope.  The 
number  of  beautiful  and  interesting  objects  which  may  be  thus  obtained 
from  even  the  commonest  Trees,  Shrubs,  and  herbaceous  Plants,  at  the 
cost  of  a  very  small  amount  of  trouble,  can  scarcely  be  conceived  save  by 
those  who  have  specially  attended  to  these  wonderful  structures.  And  a  ; 
careful  study  of  sections  made  in  different  parts  of  the  stem,  especially  in 
the  neigborhood  of  the  ^growing  point,^  will  reveal  to  the  eye  of  the 
Physiologist  some  of  the  most  important  phenomena  of  Vegetation.  The 
judicious  use  of  the  stai7iing process  (§§  200-203)  not  only  improves  the 
appearance  of  such  sections,  but  adds  greatly  to  their  scientific  value. — 
Fossil  Woods,  when  well  preserved,  are  generally  <s^7^^?^^^^Z,  and  can  only  be 


MICROSCOPIC,  STRUCTURE  OF  PHANEROGAMIC  PLANTS 


377 


cut  and  polished  by  a  lapidary's  wheel.  Should  the  Microscopist  be  fortu- 
nate enough  to  meet  with  a  portion  of  a  calciUed  stem  in  which  the 
organic  structure  is  preserved,  he  should  proceed  with  it  after  the  manner 
of  other  hard  substances  which  need  to  be  reduced  by  grinding  (§§  182-194). 

377.  Epidermis  and  Leaves, — On  all  the  softer  parts  of  the  higher 
>  plants,  save  such  as  grow  under  water,  we  find  a  surface  layer,  differing 
in  its  texture  from  the  parenchyma  beneath,  and  constituting  a  distinct ' 
membrane,  known  as  the  Epidermis, This  membrane  is  composed  of 


Epiderm  of  Leaf  of  Yucca,  Epiderm  of  Leaf  of  Indian  Corn  (Zea  Mais). 

cells,  the  walls  of  which  are  flattened  above  and  below,  whilst  they  adhere 
closely  to  each  other  laterally,  so  as  to  form  a  continuous  stratum  (Figs. 
272,  274,  a,  a\  Their  shape  is  different  in  almost  every  tribe  of  plants; 
thus  in  the  epiderm  of  the  Yucca  (Fig.  268),  Indian  Corn  (I'ig.  269), 
Iris  (Fig.  273),  and  most  other  Monocotyledons,  the  cells  are  elongated. 


tt 


Portion  of  Epiderm  of  inferior  surface  of  Leaf  of  Apple,  with  layer  of  Parenchyma  in  imme- 
diate contact  with  it:— a,  a,  elongated  cells  overlying  the  veins  of  the  leaf;  6,  6,  ordinary  epiderm 
cells,  overlying  the  parenchyma;  c,c,  stomata;  green  cells  of  the  parenchyma,  forming  a  very 
open  network  near  the  lower  surface  of  the  leaf. 

^  This  term,  borrowed  from  Animal  structure,  is  singularly  inappropriate  in 
Botany,  since  it  properly  designates  a  layer  lying  upon  the  derm  or  true  skin:  and 
the  Writer  would  have  therefore  preferred  to  retain  the  old  term  *  Cuticle,'  were  it 
not  that  this  is  now  applied  by  the  highest  authorities  to  the  thin  pellicle  covering 
the  Epiderm  (§  381). 


378 


THE  MICROSCOPE  AND  ITS  REVELATIONS. 


and  present  an  approach  to  a  rectangular  contour;  their  margins  being 
straight  in  the  Yucca  and  Iris,  but  minutely  sinuous  or  crenated  in  the 
Indian  Corn.  In  most  Dicotyledons,  on  the  other  hand,  the  cells  of  the 
epiderm  depart  less  from  the  form  of  circular  disks;  but  their  margins 
usually  exhibit  large  irregular  sinuosities,  so  that  they  seem  to  fit  together 
like  the  pieces  of  a  dissected  map,  as  is  seen  in  the  epiderm  of  the  Apple 
(Fig.  270,  h,b).  Even  here,  however,  the  cells  of  that  portion  of  the  epi- 
derm {a,  a)  which  overlies  the  ^  veins  ^  of  the  leaf,  have  an  elongated 
form,  approaching  that  of  the  wood-cells  of  which  these  veins  are  chiefly 
composed;  and  it  seems  likely,  therefore,  that  the  elongation  of  the  ordi- 
nary epiderm-cell  of  Monocotyledons  has  reference  to  that  parallel  arrange- 
ment of  the  veins  which  their  leaves  almost  constantly  exhibit. 

378.  The  cells  of  the  epiderm  are  colorless,  or  nearly  so,  having  no 
chlorophyll  in  their  interior;  and  their  walls  are  generally  thickened  by 
secondary  deposit,  especially  on  the  side  nearest  the  atmosphere.  This 
deposit  is  of  a  waxy  nature,  and  consequently  renders  the  membrane  very 
impermeable  to  fluids,  so  as  to  protect  the  soft  tissue  of  the  leaf  from  dry- 
ing up.  In  most  European  plants  the  epiderm  contains  but  a  single  row 
of  cells,  which,  moreover,  are  usually  thin-sided;  whilst  in  the  generality 
of  tropical  species,  there  exist  two,  three,  or  even  four  layers  of  thick-sided 
cells;  this  last  number  being  seen  in  the  Oleander,  the  epiderm  of  which. 


TiGr27U 


Portion  of  Epiderm  of  upper  surface  of  Leaf  of  Rochea  falcata,  as  seen  at  a  from  its  inner 
side,  and  at  b  from  its  outer  side:— a,  a,  small  cells  forming  ianer  layer;  6,  6,  large  prominent  cells 
of  outer  layer;  c,  c,  stomata  disposed  between  the  latter. 

when  separated,  has  an  almost  leathery  firmness.  This  difference  in  con- 
formation is  obviously  adapted  to  the  conditions  of  growth  under  which 
these  plants  respectively  exist;  since  the  epiderm  of  a  plant  indigenous  to 
temperate  climates,  would  not  afford  a  sufficient  protection  to  the  inte- 
rior structure  against  the  rays  of  a  tropical  sun;  whilst  the  less  powerful 
heat  of  this  country  would  scarcely  overcome  the  resistance  presented  by 
the  dense  and  non-conducting  tegument  of  a  species  formed  to  exist  in 
tropical  climates. 

379.  A  very  curious  modification  of  the  epiderm  is  presented  by  the 
Rochea  falcata,  which  has  the  surface  of  its  ordinary  epiderm  (Figs.  271, 
x!72,  a,  a,  nearly  covered  with  a  layer  of  large  prominent  isolated  cells, 

b,  A  somewhat  similar  structure  is  found  in  the  Mesembryantliemum 
crystallinum,  commonly  known  as  the  Ice  plant;  a  designation  it  owes  to 
the  peculiar  appearance  of  its  surface,  which  looks  as  if  it  were  covered 
with  frozen  dewdrops.  In  other  instances,  the  epiderm  is  partially 
invested  by  a  layer  of  scales,  w4iich  are  nothing  else  than  flattened  cells, 
often  having  a  very  peculiar  form;  whilst  in  numerous  cases,  again,  we 
find  the  surface  beset  with  hairs,  which  occasionally  consist  of  single 


MICROSCOPIC  STRUCTURE  OF  PHANEROGAMIC  PLANTS. 


379 


elongated  cells,  but  are  more  commonly  made  up  of  a  linear  series, 
attached  end  to  end,  as  in  Fig.  240.  Sometimes  these  hairs  bear  little 
glandular  bodies  at  their  extremities,  by  the  secretion  of  which  a  peculiar 
viscidity  is  given  to  the  surface  of  the  leaf,  as  in  the  Sundew  (Drosera); 
in  other  instances,  the  hair  has  a  glandular  body  at  its  base,  with  whose 
secretion  it  is  moistened,  so  that  when  this  secretion  is  of  an  irritating 
quality,  as  in  the  Nettle,  it  constitutes 

a  ^ sting.'    A  great  variety  of  such  Fig. 272. 

organs  may  be  found,  by  a  microsco- 
pic examination  of  the  surface  of  the 
leaves  of  plants  having  any  kind  of 
superficial  investment  to  the  epiderm. 
Many  connecting  links  present  them- 
selves between  hairs  and  scales,  such 
as  the  stellate  hairs  of  the  Deutzia 
scaira,  which  a  good  deal  resemble 
these  within  the  air-chambers  of  the 
Yellow  Waterlily  (Fig.  238). 

380.  The  Epidermis  in  many  Portion  of  Vertical  Section  of  leaf  of  i?o^ 
plants,   especially  those  belonging  to   c/im  showing  the  smallcells      a,  of  the  in- 

1      x-v  i   -1       1       -i        n        n     •         ner  layer  of  Epidermis;  the  large  cells,  6,6, 

the  Grass  tribe,  has  its  cell-walls  im-   of  the  outer  layer;  cone  of  thestomata;  d, 

i-jrpo-nnfprl  wifh  Q^V^r   liVp  fhnt  of  fhp   ^'  ^^^^^  parenchyma;    l,  cavity  be- 

piegnatea  Wlin  Sltex^  IIKC  tnat  OI  tne  tween  the  parenchymatous  cells,  into  which 

Equisetum  (§  345);  so  that  when  the  the  stoma  opens, 
organic  matter  seems  to  have  been 

got  rid-of  by  heat  or  by  acids,  the  forms  of  the  cuticle-cells,  hairs,  sto- 
mata,  etc.,  are  still  marked-out  in  silex,  and  (unless  the  dissipation  of 
the  organic  matter  has  been  most  perfectly  accomplished)  are  most  beau- 
tifully displayed  by  Polarized  light.  Such  silicified  epiderms^  are  found 
in  the  husks  of  the  grains  yielded  by  these  plants:  and  there  is  none  in 
which  a  larger  proportion  of  mineral  matter  exists,  than  that  of  Rice, 
which  contains  some  curious  elongated  cells  with  toothed  margins.  ^  The 
hairs  with  which  the  palece  (chaff-scales)  of  most  Grasses  are  furnished, 
are  strengthened  by  the  like  siliceous  deposit;  and  in  the  Festuca praten- 
sis,  one  of  the  common  meadow-grasses,  the  paleae  are  also  beset  with 
longitudinal  rows  of  littls  cup-like  bodies  formed  of  silica.  The  epiderm 
and  scaly  hairs  of  Deutzia  scabra  also  contain  a  large  quantity  of  silex; 
and  are  remarkably  beautiful  objects  for  the  Polariscope. 

381.  Externally  to  the  epidermis  there  usually  exists  a  very  delicate 
transparent  'cuticle,'  showing  no  decided  traces  of  organization,  though 
occasionally  somewhat  granular  in  appearance,  and  marked  by  lines  that 
seem  to  be  impressions  of  the  junctions  of  'iie  cells  with  which  it  was  in 
contact.  When  detached  by  maceration,  it  not  only  comes  off  from  the 
surface  of  the  epiderm,  but  also  from  that  of  the  hairs,  etc.,  which  this 
may  bear.  This  membrane  is  obviously  formed  by  the  agency  of  the 
epidermic  cells;  and  it  seems  to  consist  of  the  external  layers  of  their 
thickened  cellulose  walls,  which  have  coalesced  with  each  other,  and  have 
separated  themselves  from  the  subjacent  layers. 

382.  In  nearly  all  plants  which  possess  a  distinct  epidermis,  this  is 
perforated  by  the  minute  openings  termed  stomata  (Figs.  270-272,  c,  c)) 
which  are  bordered  by  cells  of  a  peculiar  form,  distinct  from  those  of  the 
epidermis,  and  more  resembling  in  character  those  of  the  tissue  beneath. 
These  boundary-cells  are  usually  somewhat  kidney-shaped,  and  lie  in 
pairs  (Fig.  273,  h,  h),  with  an  oval  opening  between  them;  but  by  an  al- 
teration in  their  form,  the  opening  may  be  contracted  or  nearly  closed. 


380 


THE  MICROSCOPE  AND  ITS  REVELATIONS. 


In  the  epiderm  of  Yucca,  however,  the  opening  is  bounded  by  two  pairs 
of  cells,  and  is  somewhat  quadrangular  (Fig.  268);  and  a  like  doubling 
of  the  boundary-cells,  with  a  narrow  slit  between  them,  is  seen  in  the 
epiderm  of  the  hidian  corn  (Fig.  269).  In  the  stomata  of  no  Phanero- 
gam, however,  do  we  meet  with  any  conformation  at  all  to  be  compared 
in  complexity  with  that  which  has  been  described  in  the  humble  Mar- 
chantia  (§  332). — Stomata  are  usually  found  most  abundantly  (anu'^ 
sometimes  exclusively)  in  the  epiderm  of  the  loiver  surfaces  of  leaves, 
where  they  open  into  the  air-chambers  that  are  left  in  the  parenchyma 
which  lies  next  the  inferior  epiderm;  in  leaves  which  float  on  the  surface 
of  water,  however,  they  are  found  in  the  epiderm  of  the  upper  surface 
only;  whilst  in  leaves  that  habitually  live  entirely  submerged,  as  there  is 


Fig.  273. 


Portion  of  Epidermis  of  Leaf  of  Iris  ger- 
manica,  torn  from  its  surface,  and  carrying 
away  with  it  a  portion  of  the  parenchymat- 
ous layer  in  immediate  contact  with  it:— a, 
a,  elongated  cells  of  the  cuticle;  6,  6,  cells 
of  the  stomata*  c.  c,  cells  of  the  parenchy- 
ma d,  impressions  on  the  epidermic  cells 
formed  hy  their  contact;  *i,  e,  cavities  in  the 
parenchyma,  corresponding  to  the  stomata. 


Vertical  section  of  Epidermis  and  of  portion  of 
subjacent  parenchyma  of  leaf  of  Iris  germanica 
taken  in  a  transverse  direction  :~a,  a,  cells  of 
epiderm;  5,  6,  cells  at  the  sides  of  the  stomata;  c, 
c,  small  green  cells  placed  within  these;  d,  d, 
openings  of  the  stomata;  e,  e,  cavities  In  the 
parenchyma  into  which  the  stomata  open;  /,  /, 
cells  of  the  parenchyma. 


no  distinct  epiderm,  so  there  are  no  stomata.  In  the  erect  leaves  of 
Grasses,  the  Iris  tribe,  etc.,  they  are  found  equally  (or  nearly  so)  on  both 
surfaces.  As  a  general  fact,  they  are  least  numerous  in  succulent  plants, 
whose  moisture,  obtained  in  a  scanty  supply,  is  destined  to  be  retained 
in  the  system;  whilst  they  abound  most  in  those  which  exhale  fluid  most 
readily,  and  therefore  absorb  it  most  quickly.  It  has  been  estimated  that 
no  fewer  than  160,000  are  contained  in  every  square  inch  of  the  under 
surface  of  the  leaves  of  Hydra7igea  and  of  several  other  plants;  the  great- 
est number  seeming  always  to  be  present  where  the  upper  surface  of  the 
leaves  is  entirely  destitute  of  these  organs.  In  Iris  gerinmiica,  each  sur- 
face has  nearly  12,000  stomata  in  every  square  inch,  and  in  Yucca,  each 
surface  has  40,000. — In  Oleander,  Banhsia,  and  some  other  plants,  the 
stomata  do  not  open  directly  upon  the  lower  surface  of  the  epiderm,  but 
lie  in  the  deepest  part  of  little  pits  or  depressions,  which  are  excavated 
in  it  and  lined  with  hairs;  the  mouths  of  these  pits,  with  the  hairs  that 
line  them,  are  well  brought  into  view  by  taking  a  thin  slice  from  the  sur- 
face of  the  epiderm  with  a  sharp  knife;  but  the  form  of  the  cavities  and 
the  position  of  the  stomata  can  only  be  well  made-out  in  vertical  sections 
of  the  leaves. 

383.  The  internal  structure  of  leaves  is  best  brought  into  view  by 
making  vertical  sections,  that  shall  traverse  the  two  layers  of  epiderm 


MICROSCOPIC  STRUCTURE  OF  PHANEROGAMIC  PLANTS. 


381 


and  the  intermediate  cellular  parenchyma;  portions  of  such  sections  are 
shown  in  Figs.  272,  274,  and  275.  In  close  apposition  witli  the  cells  of 
the  upper  epiderm  (Fig.  274,  a,  a),  which  may  or  may  not  be  perforated 
with  stomata  {c,  c,  d,  d)  we  find  a  layer  of  soft  thin-walled  cells,  con- 
taining a  large  quantity  of  chlorophyll;  these  generally  press  so  closely 
one  against  another,  that  their  sides  become  mutually  flattened,  and  no 
spaces  are  left,  save  where  there  is  a  definite  air-chamber  into  which  the 
stoma  opens  (Fig.  274,  e);  and  the  compactness  of  this  superficial  layer  is 
well  seen,  when,  as  often  happens,  it  adheres  so  closely  to  the  epiderm, 
as  to  be  carried  away  with  this  when  it  is  torn  otf  (Fig.  273,  c,  c). 
Beneath  this  first  layer  of  leaf-cells,  there  are  usually  several  others  rather 
less  compactly  arranged;  and  the  tissue  gradually  becomes  more  and  more 
lax,  its  cell  not  being  in  close  apposition,  and  large  intercellular  passages 
being  left  amongst  them,  until  we  reach  the  lower  epiderm,  which  the 
parenchyma  only  touches  at  certain  points,  its  lowest  layer  forming  a  set 
of  network  (Fig.  270,  d,  d)  with  large  interspaces,  into  which  the  sto- 
mata open.  It  is  to  this  arrangement  that  the  darker  shade  of  green 
almost  invariably  presented  by  the  superior  surfaces  of  leaves  is  princi- 


fiG.,275. 


Portion  of  vertical  longitudional  section  of  leaf  of  Iris^  extending  from  one  of  its  flattened  sides 
to  the  other:— a,  a,  elongated  cells  of  Epiderm;  6,  6,  stomata  cut  through  longitudinally;  c,  c, 
green  cells  of  parenchyma;  d,  d,  colorless  tissue,  occupying  interior  of  leaf. 

pally  due;  the  color  of  the  component  cells  of  the  parenchyma  not  being 
deeper  in  one  part  of  the  leaf  than  in  another. — In  those  plants,  however, 
whose  leaves  are  erect  instead  of  being  horizontal,  so  that  their  two  surfaces 
are  equally  exposed  to  light,  the  parenchyma  is  arranged  on  both  sides  in 
the  same  manner,  and  their  epiderms  are  furnished  with  an  equal  num- 
ber of  stomata.  This  is  the  case,  for  example,  with  the  leaves  of  the 
common  garden  Iris  (Fig.  275);  in  which,  moreover,  we  find  a  central 
portion  {d,  d)  formed  by  thick- walled  colorless  tissue,  very  different  either 
from  ordinary  leaf-cells  or  from  woody  fibre.  The  explanation  of  its 
presence  is  to  be  found  in  the  peculiar  conformation  of  the  leaves;  for  if 
we  pull  one  of  them  from  its  origin,  we  shall  find  that  what  appears  to  be 
the  flat  expanded  blade  really  exposes  but  half  its  surface;  the  blade 
being  doubled  together  longitudinally,  so  that  what  may  be  considered 
its  under  surface  is  entirely  concealed.  The  two  halves  are  adherent  to- 
gether at  their  upper  part,  but  at  their  lower  they  are  commonly  sep- 
arated by  a  new  leaf  which  comes-up  between  them;  and  it  is  from  this 
arrangement,  which  resembles  the  position  of  the  legs  of  a  man  on  horse- 
back, that  the  leaves  of  the  Iris  tribe  are  said  to  be  equitant,  Now  by 
tracing  the  middle  layer  of  colorless  cells,  d,  d,  down  to  that  lower  por- 
tion of  the  leaf  where  its  two  halves  diverge  from  one  another,  we  find  that 
it  there  becomes  continuous  with  the  epiderm,  to  the  cells  of  which  (Fig. 


382 


THE  MICROSCOPE   AND  ITS  REVELATIONS. 


275,  a)  these  bear  a  strong  resemblance  in  every  respect  save  the  greater 
proportion  of  their  breadth  to  their  length. — Another  interesting  variety 
in  leaf-structure  is  presented  by  the  Water-Lily  and  other  Plants  whose 
leaves  float  on  the  surface;  for  here  the  usual  arrangement  is  entirely  re- 
versed, the  closely-set  layers  of  green  leaf -cells  being  found  in  contact 
with  the  lower  surface,  whilst  all  the  upper  part  of  the  leaf  is  occupied 
.'by  a  loose  spongy  parenchyma,  containing  a  very  large  number  of  air-  ' 
spaces  that  give  buoyancy  to  the  leaf;  and  these  spaces  communicate 
with  the  external  air  through  the  numerous  stomata,  which,  contrary  to 
the  general  rule  (§  382),  are  here  found  in  the  upper  epiderm  alone/ 

384.  The  examination  of  the  foregoing  structures  is  attended  with 
very  little  difficulty.  Many  epiderms  may  be  torn  off,  by  the  exercise 
of  a  little  dexterity,  from  the  surfaces  of  the  leaves  they  invest,  without 
any  preparation;  this  is  especially  the  case  with  Monocotyledons  generally, 
the  veins  of  whose  leaves  run  parallel,  and  with  such  Dicotyledons  as 
have  very  little  woody  structure  in  their  leaves;  in  those,  on  the  other 
hand,  whose  leaves  are  furnished  with  reticulated  veins  to  which  the  epi- 
derm adheres  (as  is  the  case  in  by  far  the  larger  proportion),  this  can 
only  be  detached  by  first  macerating  the  leaf  for  a  few  days  in  water;  and 
if  their  texture  should  be  particularly  firm,  the  addition  of  a  few  drops 
of  nitric  acid  to  the  water  will  render  their  cuticles  more  easily  separable. 
Epiderms  may  be  advantageously  mounted  either  in  weak  spirit,  or  in 
glycerine-jelly. — Very  good  sections  of  most  leaves  may  be  made  by  a 
sharp  knife,  handled  by  a  careful  manipulator;  but  it  is  generally  prefer- 
able to  use  the  Microtome,  placing  the  leaf  between  two  pieces  either  of 
very  soft  cork  or  of  elder-pith  or  carrot,  or  imbedding  it  in  parafiine  (§  189). 
In  order  to  study  the  structure  of  leaves  with  the  fulness  that  is  needed 
for  scientific  research,  numerous  sections  should  be  made  in  different 
directions;  and  slices  taken  parallel  to  the  surfaces,  at  different  distances 
from  them,  should  also  be  examined.  There  is  no  known  medium  in 
which  such  sections  can  be  preserved  altogether  without  change;  but 
some  one  of  the  methods  formerly  described  (§  306)  will  generally  be 
found  to  answer  sufficiently  well. 

385.  Flowers, — Many  small  fiowers,  when  looked-at  entire  with  a  low 
magnifying  power,  are  very  striking  Microscopic  objects;  and  the  interest 
of  the  young  in  such  observations  can  scarcely  be  better  excited,  than  by 
directing  their  attention  to  the  new  view  they  thiis  acquire  of  the  ^  com- 
posite' nature  of  the  humble  down-trodden  Daisy,  or  to  the  beauty  of 
the  minute  blossoms  of  many  of  those  U^nhelliferous  Plants  which  are 
commonly  regarded  only  as  rank  weeds.  The  Scientific  Microscopist, 
however,  looks  more  to  the  organization  of  the  separate  parts  of  the  flower; 
and  among  these  he  finds  abundant  sources  of  gratification,  not  merely 
to  his  love  of  knowledge,  but  also  to  his  taste  for  the  beautiful.  The 
general  structure  of  the  sepals  wadi petals,  which  constitute  the  ^  perianth^ 
or  floral  envelope,  closely  corresponds  with  that  of  leaves;  the  chief  dif- 
ference lying  in  the  peculiar  change  of  hue  which  the  chlorophyll  almost 
invariably  undergoes  in  the  latter  class  of  organs,  and  very  frequently  in 
the  former  also.  There  are  some  petals,  however,  whose  cells  exhibit 
very  interesting  peculiarities,  either  of  form  or  marking,  in  addition  to 
their  distinctive  coloration;^  such  are  those  of  the  Geranium  (Pelargo- 

^  See  the  classical  Memoir  by  Ad.  Brongniart  on  the  Structure  of  Leaves,  in 
Ann.  des  Sci.  Nat.,"  Tom.  xxi.  (1830)  pp.  430-458. 
2  See  especially  Mr.  Tuffen  West  '  On  some  Conditions  of  the  Cell- Wall  in  the 
Petals  of  t lowers,'  in    Quart.  Journ.  of  Microsc,  Science,*'  Vol.  vii.  (1859),  p.  22. 


MICROSCOPIC  STRUCTURE  OF  PHANEROGAMIC  PLANTS. 


383 


nium),  of  which  a  small  portion  is  represented  in  Fig.  276.  The  dif- 
ferent portions  of  this  petal — when  it  has  been  dried  after  stripping  it  of 
its  epiderm,  immersed  for  an  hour  or  two  in  oil  of  turpentine,  and  then 
mounted  in  Canada  balsam — exhibit  a  most  beautiful  variety  of  yivid 
coloration,  which  is  seen  to  exist  chiefly  in  the  thickened  partitions  of 
the  cells;  whilst  the  surface  of  each  cell  presents  a  very  curious  opaque 
spot  with  numerous  diverging  prolongations.  This  method  of  prepara- 
tion, however,  does  not  give  a  true  idea  of  the  structure  of  the  cells;  for 
each  of  them  has  a  peculiar  mammillary  protuberance,  the  base  of  which 
is  surrounded  by  hairs;  and  this  it  is 
which  gives  the  VQlvety  appearance 
to  the  surface  of  the  petal,  and 
which,  when  altered  by  drying  and 
compression,  occasions  the  peculiar 
spots  represented  in  Fig.  276.  Their 
real  character  may  be  brought  into 
view  by  Dr.  Inman's  method;  which 
consists  in  drying  the  petal  (when 
stripped  of  its  epiderm)  on  a  slip  of 
glass,  to  which  it  adheres,  and  then 
placing  on  it  a  little  Canada  balsam 
diluted  with  Turpentine,  which  is  to 
be  boiled  for  an  instant  over  the 
spirit-lamp,  after  which  it  is  to  be 
covered  with  a  thin  glass.  The  boil- 
ing '  blisters^  it,  but  does  not  remove  the  color;  and  on  examination  many 
of  the  cells  will  be  found  showing  the  mammilla  very  distinctly,  with  a 
score  of  hairs  surrounding  its  base,  each  of  these  slightly  curved,  and 
pointing  towards  the.  apex  of  the  mammilla. — The  petal  of  the  common 
Scarlet  Pimpernel  {AnagalUs  arveiisis),  that  of  the  common  Chick-weed 
{Stellaria  media),  together  with  many  others  of  a  small  and  delicate 
character,  are  also  very  beautiful  microscopic  objects;  and  the  two  just 
named  are  peculiarly  favorable  subjects  for  the  examination  of  the  spiral 
vessels  in  their  natural  position.  For  the  ^  veins  ^  which  traverse  these 
petals  are  entirely  made-up  of  spiral  vessels,  none  of  which  individually 
attain  any  great  length;  but  one  follows  or  takes  the  place  of  another,  the 
conical  commencement  of  each  somewhat  overlapping  the  like  termina- 
tion of  its  predecessor;  and  where  the  ^veins'  seem  to  branch,  this  does 
not  happen  by  the  bifurcation  of  a  spiral  vessel,  but  by  the  ^splicing-on^ 
(so  to  speak)  of  one  to  the  side  of  another,  or  of  two  new  vessels  diverg- 
ing from  one  another  to  the  end  of  that  which  formed  the  principal  vein. 

386.  The  anthers  m^di  pollen-grains,  also,  present  numerous  objects  of 
great  interest,  both  to  the  scientific  Botanist  and  to  the  amateur  Micro- 
scopist.  In  the  first  place,  they  afford  a  good  opportunity  of  study- 
ing that  form  of  ^free^  cell-development  which  seems  peculiar  to  the 
parts  concerned  in  the  reproductive  process,  and  which  consists  in  the 
development  of  a  new  cell-wall  round  an  isolated  mass  of  protoplasm 
forming  part  of  the  contents  of  a  ^parent-cell;'  so  that  the  new  cell  lies 
free  within  its  cavity,  instead  of  being  formed  by  its  subdivision,  as  in  the 
ordinary  method  of  multiplication  (§  226). — If  the  anther  be  examined 
•by  thin  sections  at  an  early  stage  of  its  development  within  the  young 
flower-bud,  it  will  be  found  to  be  made-up  of  ordinary  cellular  parenchyma 
in  which  no  peculiarity  anywhere  shows  itself :  but  a  gradual  ^differentia- 
tion '  speedily  takes  place,  consisting  in  the  development  of  aset  of  avery 


Cells  from  Petal  of  Geranium 
{Pelargonium.) 


384 


THE  MICROSCOPE  AND  ITS  REVELATIONS. 


large  cells  in  two  vertical  rows,  which  occupy  the  place  of  the  loculi  or 
*  pollen-chambers '  that  afterwards  present  themselves;  and  these  cells 
give  origin  to  the  pollen-grains,  whilst  the  ordinary  parenchyma  remains 
to  form  tlie  walls  of  the  pollen-chambers.  The  pollen-grains  are  formed 
within  ^  mother-cells/  the  endoplasm  of  each  breaking  up  into  four  seg- 
ments. These  become  invested  by  a  double  envelope,  a  firm  extine,  and 
a  thin  mtme  ;  and  they  are  set  free,  when  mature,  by  the  bursting  of  the 
pollen-chambers.  It  is  not  a  little  curious  that  the  layer  of  cells  which 
lines  the  pollen-chambers  should  exhibit,  in  a  considerable  proportion  of 
plants,  a  strong  resemblance  in  structure,  though  not  in  form,  to  the 
elaters  of  Marchantia  (Fig.  218).  For  they  have  in  their  interior  a 
fibrous  deposit;  which  sometimes  forms  a  continuous  spiral  (like  that  in 
Fig.  244),  as  in  Narcissus  and  Hyoscyamus;  but  it  is  often  broken-up,  as 
it  were,  into  rings,  as  in  the  Iris  and  Hyacinth;  in  many  instances  forms 
an  irregular  network,  as  in  the  Violet  and  Saxifrage;  in  other  cases,  again, 
forms  a  set  of  interrupted  arches,  the  fibres  being  deficient  on  one  side, 
as  in  the  Yellow  Water-lily,  Bryony,  Primrose,  etc.;  whilst  a  very  pecu- 
liar stellate  aspect  is  often  given  to  these  cells,  by  the  convergence  of  the 
interrupted  fibres  towards  one  point  of  the  cell-wall,  as  in  the  Cactus, 
Geranium,  Madder,  and  many  other  well-known  plants.  Various  inter- 
mediate modifications  exist;  and  the  particular  form  presented  often 
varies  in  different  parts  of  the  wall  of  one  and  the  same  anther.  It  seems 
probable  that,  as  in  Hepaticse,  the  elasticity  of  these  spiral  cells  may 
have  some  share  in  the  opening  of  the  pollen-chambers  and  in  the  dis- 
persion of  the  pollen-grains. 

387.  The  form  of  the  pollen-grains  seems  to  depend  in  part  upon  the 
mode  of  division  of  the  cavity  of  the  parent-cell  into  quarters;  generally 
speaking  it  approaches  the  spheroidal,  but  it  is  sometimes  elliptical,  and 
sometimes  tetrahedral.  It  varies  more,  however,  when  the  pollen  is  dry, 
than  when  it  is  moist;  for  the  effect  of  the  imbibition  of  fluid,  which 
usually  takes-place  when  the  pollen  is  placed  in  contact  with  it,  is  to 
soften-down  angularities,  and  to  bring  the  cell  nearer  to  the  typical 
sphere.  The  extine  or  outer  coat  of  the  pollen-grain  often  exhibits  very 
curious  markings,  which  seem  due  to  an  increased  thickening  at  some 
points,  and  a  thinning-away  at  others.  Sometimes  these  markings  give 
to  the  surface-layer  so  close  a  resemblance  to  a  stratum  of  cells  (Fig.  277, 
B,  c,  d),  that  only  a  very  careful  examination  can  detect  the  difference. 
The  roughening  of  the  surface  by  spines  or  knobby  protuberances,  as 
shown  at  a,  is  a  very  common  feature;  and  this  seems  to  enable  the  ])ol- 
len-grains  more  readily  to  hold  to  the  surface  whereon  they  may  be  cast. 
Besides  these  and  other  inequalities  of  the  surface,  most  pollen-grains 
have  what  appear  to  be  pores  or  slits  in  their  extine  (varying  in  number 
in  different  species),  through  which  the  intine  protrudes  itself  as  a  tube, 
when  the  bulk  of  its  contents  has  been  increased  by  imbibition;  it  seems 
probable,  however,  that  the  extine  is  not  absolutely  deficient  at  these 
points,  but  is  only  thinned- away.  Sometimes  the  pores  are  covered  by 
little  disk-like  pieces  or  lids,  which  fall-off  when  the  pollen-tube  is  pro- 
truded. This  action  takes  place  naturally  when  the  pollen-grains  fall 
upon  the  surface  of  the  stigma,  which  is  moistened  with  a  viscid  secre- 
tion; and  the  pollen-tubes,  at  first  mere  protrusions  of  the  inner  coat  of 
their  cell,  insinuating  themselves  between  the  loosely-packed  cells  of  the 
stigma,  grow  downwards  through  the  style,  sometimes  even  to  the  length 
of  several  inches,  until  they  reach  the  ovarium.  The  first  change — 
namely,  the  protrusion  of  the  inner  membrane  through  the  pores  of  the 


MICKOSCOPIC  STEUCTUEK  OF  PHANEROGAMIC  PLANTS. 


385 


Pollen-grains  of,— a,  Althcearosea;  b,  Cobcea 
scandens;  c,  Plassiflora  ccerulea;  d,  Ipomcea 
purpurea. 


exterior — may  be  made  to  take  place  artificially  by  moistening  tlie  pollen 
with  water,  thin  syrup,  or  dilute  acids  (different  kinds  of  pollen-grains 
requiring  different  modes  of  treatment);  but  the  subsequent  extension  by 
growth  will  only  take  place  under  the  natural  conditions.  By  treating 
some  pollen-grains,  as  those  of  Lilmm  Japonicum,  L,  ruhriim,  or  L, 
aiirahim,  with  the  viscid  liquid  abundantly  secreted  by  the  stigma,  not 
only  may  the  extrusion  and  length- 
ening of  the  pollen-tubes  be  watched,  I'lo.  277. 
but  the  grains  with  their  extruded 
tubes  may  be  preserved  almost  un- 
changed by  mounting  in  this  liquid. 

388.  The  darker  kinds  of  pollen 
may  be  generally  rendered  transpar- 
ent by  mounting  in  Canada  balsam; 
or,  if  it  be  desired  to  avoid  the  use 
of  heat,  in  the  Benzole  solution  of 
Canada  balsam  (§  205),  setting  aside 
the  slide  for  a  time  in  a  warm  place. 
For  the  less  opaque  pollens,  the 
Dammar  solution  (§  1G8,  d)  is  pre- 
ferable. The  more  delicate  pollens, 
however,  become  too  transparent  in 
either  of  these  media:  and  it  is  con- 
sequently preferable  to  mount  them 
either  dry  or  (if  they  will  bear  it  with- 
out rupturing)  in  fluid.  The  most 
interesting  forms  are  found,  for  the 
most  part,  in  plants  of  the  orders  Amarantacec^,  OicJioracem,  Cucurbit- 
acecB,  MalvacecB,  and  Passiflorem;  others  are  furnished  also  by  Convolvulus, 
Campanula,  CEnotliera,  Pelargonium  (Geranium),  Polygonum,  Sedum, 
and  many  other  plants.  It  is  frequently  preferable  to  lay-down  the  entire 
anther,  with  its  adherent  pollen-grains  (where  these  are  of  a  kind  that  hold 
to  it),  as  an  opaque  object;  this  may  be  done  with  great  advantage  in  the 
case  of  the  common  Mallow  {Malva  sylvestris)  or  of  the  Hollyhock  {AWma 
rosea);  the  anthers  being  picked  soon  after  they  have  opened,  whilst  a 
large  proportion  of  their  pollen  is  yet  undischarged;  and  being  laid  down 
as  flat  as  possible,  before  they  have  begun  to  wither,  between  two  pieces 
of  smooth  blotting-paper,  then  subjected  to  moderate  pressure,  and  finally 
mounted  upon  a  black  surface.  They  are  then,  when  properly  illumin- 
ated, most  beautiful  objects  for  objectives  of  2-3ds,  1.  or  2  in.  focus, 
especially  with  the  Binocular  Microscope.^ 

389.  The  structure  and  development  of  the  ovules  tnau  are  produced 
within  the  ovarium  at  the  base  of  the  pistil,  and  the  operation  in  which 
their  fertilization  essentially  consists,  are  subject  of  investigation  which 
have  a  peculiar  interest  for  scientific  Botanists,  but  which,  inconsequence 
of  the  special  difficulties  that  attend  the  inquiry,  are  not  commonly  re- 
garded as  within  the  province  of  ordinary  Microscopists. — Some  general 

^  It  sometiraes  happens  that  when  the  pollen  of  Pines  or  Firs  is  set  free,  large 
quantities  of  it  are  carried  by  the  wind  to  a  great  distance  from  the  woods  and 
plantations  in  which  it  has  been  produced,  and  are  deposited  as  a  fine  yellow  dust, 
so  strongly  resembling  Sulphur  as  to  be  easily  mistaken  for  it.  This  (supposed) 
general  diffusion  of  sulphur  (such  as  occurred  in  the  neighborhood  of  Windsor  in 
1879)  has  frightened  ignorant  rustics  into  the  belief  that  the  *  end  of  the  world '  was 
at  hand.  Its  true  nature  is  at  once  revealed  by  placing  a  few  grains  of  it  under 
the  Microscope. 

25 


386 


THE  MICROSCOPE  AND  ITS  REVELATIONS. 


instructions,  however,  may  prove  useful  to  such  as  would  like  to  inform 
themselves  as  to  the  mode  in  which  the  generative  function  is  performed 
in  Phanerogams.  In  tracing  the  origin  and  early  history  of  the  ovule, 
very  thin  sections  should  be  made  through  the  flower-bud,  both  vertically 
and  transversely;  but  when  the  ovule  is  large  and  distinct  enough  to  be 
separately  examined,  it  should  be  placed  on  the  thumb-nail  of  the  left 
hand,  and  very  thin  sections  made  with  a  sharp  razor;  the  ovule  should 
not  be  allowed  to  dry-up,  and  the  section  should  be  removed  from  the 
blade  of  the  razor  by  a  wetted  camel-hair  pencil.  The  tracing-downwards 
the  pollen-tubes  through  the  tissue  of  the  style,  may  be  accomplished  by 
sections  (which,  however,  will  seldom  follow  one  tube  continuously  for 
any  great  part  of  its  length),  or,  in  some  instances,  by  careful  dissection 
with  needles.  Plants  of  the  Orcliis  tribe  are  the  most  favorable  subjects 
for  this  kind  of  investigation;  which  is  best  carried-on  by  artificially  apply- 
ing the  pollen  to  the  stigma  of  several  flowers,  and  then  examining  one  or 
more  of  the  styles  daily.  If  the  style  of  flower  of  an  Epipactis  (says 
Schacht),  to  which  the  pollen  has  been  applied  about  eight  days  previously, 
be  examined  in  the  manner  above  mentioned,  the  observer  will  be  surprised 
at  the  extraordinary  number  of  pollen-tubes,  and  he  will  easily  be  able  to 
trace  them  in  large  strings,  even  as  far  as  the  ovules.  Viola  tr  icolor  (Hearts- 
ease) and  Ribes  nigrum  and  riibriim  (Black  and  Eed  Currant)  are  also  good 
plants  for  the  purpose;  in  the  case  of  the  former  plant,  withered  flowers 
may  be  taken,  and  branched  pollen-tubes  will  not  unfrequently  be  met 
with.^'  The  entrance  of  the  pollen-tube  into  the  micropyle  may  be  most 
easily  observed  in  Orcliideous  plants  and  in  Enplirasia;  it  being  only 
necessary  to  tear-open  with  a  needle  the  ovary  of  a  flower  which  is  just 
withering,  and  to  detach  from  the  placenta  the  ovules,  almost  every  one 
of  which  will  be  found  to  have  a  pollen-tube  sticking  in  its  micropyle. 
These  ovules,  however,  are  too  small  to  allow  of  sections  being  made, 
whereby  the  origin  of  the  embryo  may  be  discerned;  and  for  this  pur- 
pose, (Enothera  (Evening  Primrose)  has  been  had  recourse  to  by  Hotf- 
meister,  whilst  Schacht  recommends  Latlirma  squamaria,  Pedicitlaris 
palustris,  and  particularly  Pedicularis  sylvatica, 

390.  We  have  now,  in  the  last  place,  to  notice  the  chief  points  of 
interest  to  the  Microscopist  which  are  furnished  by  mature  seeds.  Many 
of  the  smaller  kinds  of  these  bodies  are  very  curious,  and  some  are  very 
beautiful  objects,  when  looked-at  in  their  natural  state  under  a  low  mag- 
nifying power.  Thus  the  seed  of  the  Poppy  (Fig.  278,  a)  presents  a 
regular  reticulation  upon  its  surface,  pits  for  the  most  part  hexagonal 
being  left  between  projecting  walls;  that  of  Caryopliyllum  (d)  is  regularly 
covered  with  curiously-jagged  divisions,  every  one  of  which  has  a  small 
bright  black  hemispherical  knob  in  its  middle,  that  of  Amarantlms  hypo- 
cliondriamis  has  its  surface  traced  with  extremely  delicate  markings  (b)  ; 
that  of  Antirrhinum  is  strangely  irregular  in  shape  (c),  and  looks  almost 
like  a  piece  of  furnace-slag;  and  that  of  many  Bignoniacem  is  remarkable 
for  the  beautiful  radiated  structure  of  the  translucent  membrane  which 
surrounds  it  (e).  This  structure  is  extremely  well  seen  in  the  seed  of 
the  Eccremocarpns  scaber,  a  half-hardy  climbing  plant  now  common  in 
our  gardens;  and  when  its  membranous  '  wing'  is  examinedunder  a  suffi- 
cient magnifying  power,  it  is  found  to  be  formed  by  an  extraordinary 
elongation  of  the  cells  of  the  seed-coat  at  the  margin  of  the  seed,  the 
side-walls  of  which  cells  (those,  namely,  which  lie  in  contact  with  one 
another)  are  thickened  so  as  to  form  radiating  ribs  for  the  support  of  the 
wing,  whilst  the  front  and  back  walls  (which  constitute  its  membranous 


MICROSCOPIC  STRUCTURE  OF  PHANEROGAMIC  PLANTS. 


387 


surface)  retain  their  original  transparence,  being  marked  only  with  an 
indication  of  spiral  deposit  in  their  interior.  In  the  seed  of  Dictyoloma 
Peruviana,  besides  the  principal  ^  wing  ^  prolonged  from  the  edge  of 
the  seed-coat,  there  is  a  series  of  successively  smaller  wings,  whose  mar- 
gins form  concentric  rings  over  either  surface  of  the  seed;  and  all  these 
wings  are  formed  of  radiating  fibres  only,  composed,  as  in  the  preceding 
case,  of  the  thickened  walls  of  adjacent  cells;  the  intervening  membr^ine, 
originally  formed  by  the  front  and  back  walls  of  these  cells,  having  dis- 
appeared, apparently  in  consequence  of  being  unsupported  by  any  second- 
ary deposit.^  Several  other  seeds,  as  those  of  Sp'/ienogyne  sj)eciosa  and 
Lophospermum  eruhescens,  possess  wing-like  appendages;  but  the  most 
remarkable  development  of  these  organs  is  said  by  Mr.  Quekett  to  exist 
in  a  seed  of  Calosantlies  hidica,  an  East  Indian  plant,  in  which  the  wing 
extends  more  than  an  inch  on  either  side  of  the  seed. — Some  seeds  are 
distinguished  by  a  peculiarity  of  form,  which  although  readily  discernible 
by  the  naked  eye,  becomes  much  more  striking  when  they  are  viewed 
under  a  very  low  magnifying  power:  this  is  the  case,  for  example,  with 
*the  seeds  of  the  Carrot,  whose  long  radiating  processes  make  it  bear, 
under  the  Microscope^  no  trifling  resemblance  to  some  kinds  of  star-fish; 


2  ly 


Seeds,  as  seen  under  a  low  magnifying  power:— a,  Poppy;  b,  Amaranthus  (Prince's  feather); 
Antirrhinum  majus  (Snap-dragon);  d,  Caryophyllum  (Clove-pink);  e,  Bignonia, 

and  with  those  of  Cyantlius  mi7ior,  which  bear  about  the  same  degree  of 
resemblance  to  shaving  brushes.  In  addition  to  the  preceding,  the  fol- 
lowing may  be  mentioned  as  seeds  easily  to  be  obtained,  and  as  worth 
mounting  for  opaque  objects: — Ajiagallis,  Anetlmm  graveolens,  Begoniay 
Carum  carui,  Coreopsis  tindoria,  Datura,  Delphinium,  Digitalis,  Ma- 
tine.  Erica,  Gentiana,  Gesnera,  Hyoscyamus,  Hypericum,  Lepidium, 
Limnocharis,  Linaria,  Lychnis,  Mesemlryanthemum,  Nicotiana,  Ori- 
gamme  omtes,  Orohanche,  Petunia,  Reseda,  Saxifraga,  Scrophularia, 
Sidum,  Semper vivum,  Silene,  Stellaria,  Symphytum  asperrimum,  and 
Verbena,  The  following  may  be  mounted  as  transparent  objects  in 
Canada  balsam: — Drosera,  Hydrangea,  Monotropa,  Orchis,  Parnassia, 


1  See  H.  B  Brady  in  Transactions  of  Microsc.  Society,''  N.S.,  Vol.  ix.  (1861), 
p.  65. 


388 


THE  MICROSCOPE  AND  ITS  RKYELATIONS. 


Pyrola,  Saxifraga.'^  The  seeds  of  Umbelliferous  plants  generally  are 
remarkable  for  the  peculiar  vittcB,  or  receptacles  for  essential  oil,  which 
are  found  in  their  coats.  Various  points  of  interest  respecting  the 
structure  of  the  testcB  or  envelopes  of  seeds,  such  as  the  fibre  cells  of  (7o5<35« 
and  Collomia,  the  stellate  cells  of  the  Star-Anisey  and  the  densely-conso- 
lidated tissue  of  the  ^ shells^  of  the  Coquilla-nut,  Cocoa-mit,  etc., — 
havijig  been  already  noticed,  we  cannot  here  stop  to  do  more  than  advert 
to  the  peculiarity  of  the  constitution  of  the  husk  of  the  Corn-grains,  In 
these,  as  in  other  Grasses,  the  ovary  itself  continues  to  envelop  the  seed, 
giving  a  covering  to  it  that  surrounds  its  own  testa,  and  closely  adheres 
to  it.  The  ^  bran  ^  detached  in  grinding  consists  not  only  of  these  two 
coats,  but  also  (as  the  Microscope  reveals)  of  an  outer  layer  of  the  grain 
itself,  formed  of  hexagonal  cells  disposed  with  great  regularity.  As  these 
are  filled  with  gluten,  the  removal  of  this  layer  takes  away  one  of  the 
most  nutritious  parts  of  the  grain;  and  it  is  most  desirable,  therefore, 
that  only  the  two  outer  indigestible  coats  should  be  detached  by  the 
^  decorticating '  process  devised  for  the  purpose.  The  hexagonal  cell-layer 
is  so  little  altered  by  a  high  temperature,  as  still  to  be  readily  distinguish-  • 
able  when  the  grain  has  been  ground  after  roasting, — thus  enabling  the 
Microscopist  to  detect  even  a  small  admixture  of  roasted  Corn  with  Cof- 
fee or  Chicory,  without  the  least  difficulty.^ 

^  These  lists  have  been  chiefly  derived  from  the    Micrographic  Dictionary." 

2  In  a  case  in  which  the  Author  was  called  upon  to  make  such  an  investigation, 
he  found  as  many  as  thirty  distinctly-recognizable  fragments  of  this  cellular  en- 
velope, in  a  single  grain  of  a  mixture  consisting  of  Chicory  with  only  5  per  cent, 
of  roasted  Corn. 


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